2125

2125

Batch leaching tests qualitative and quantitative x-ray powder diff raction (XRPD) analyses and geochemical modeling were used to investigate the leaching mechanisms of Cr(VI) from chromite ore processing residue (COPR) samples obtained from an urban area in Hudson County New Jersey Th e pH of the leaching solutions was adjusted to cover a wide range between 1 and 125 Th e concentration levels for total chromium (Cr) and Cr(VI) in the leaching solutions were virtually identical for pH values gt5 For pH values lt5 the concentration of total Cr exceeded that of Cr(VI) with the diff erence between the two attributed to Cr(III) Geochemical modeling results indicated that the solubility of Cr(VI) is controlled by Cr(VI)-hydrocalumite and Cr(VI)-ettringite at pH gt105 and by adsorption at pH lt8 However experimental results suggested that Cr(VI) solubility is controlled partially by Cr(VI)-hydrocalumite at pH gt105 and by hydrotalcites at pH gt8 in addition to adsorption of anionic chromate species onto inherently present metal oxides and hydroxides at pH lt8 As pH decreased to lt10 most of the Cr(VI) bearing minerals become unstable and their dissolution contributes to the increase in Cr(VI) concentration in the leachate solution At low pH ( lt15) Cr(III) solid phases and the oxides responsible for Cr(VI) adsorption dissolve and release Cr(III) and Cr(VI) into solution

Leaching Mechanisms of Cr(VI) from Chromite Ore Processing Residue

Mahmoud Wazne Santhi Chandra Jagupilla Deok Hyun Moon Christos Christodoulatos and Agamemnon KoutsospyrosStevens Institute of Technology

Millions of tonnes of COPR were deposited at numerous

urban areas in the United States the United Kingdom

(UK) and elsewhere in the world during the fi rst half of the

last century (Burke et al 1991 Weng et al 1994 James

1996 Farmer et al 1999 Graham et al 2006) Th e COPR

was benefi cially used as structural fi ll because of its favorable

structural quality as a granulated material Th e deposited COPR

was produced by the high lime process where the chromite ore

was roasted at approximately 1200degC to oxidize the chromium

in the ore from the trivalent to the hexavalent state Hexavalent

chromium [Cr(VI)] was then chemically combined with the

sodium in the added soda ash to form Na2CrO

4 (Allied Signal

1982) Lime was added during the roasting process to act as a

mechanical separator allowing oxygen to react with the chromite

and sodium carbonate Lime also served as a sequestering

agent combining with various ore impurities to form insoluble

compounds Th e sodium chromate formed during the roasting

process was extracted with hot water as a weak yellow liquor

solution Th e sodium chromate was then converted into sodium

dichromate by reaction with sulfuric acid After draining the

residue was discarded Th e disposed COPR contains unreacted

chromite ore and unextracted chromate Even though the high

lime process has ceased in the United States and the UK it is still

being used in China Russia Kazakhstan India and Pakistan

(Darrie 2001)

In the environment chromium exists mainly in two oxida-

tion states as hexavalent and trivalent chromium Th e Cr(VI) is

highly mobile severely toxic at moderate doses and classifi ed as

a respiratory carcinogen in humans In contrast Cr(III) is used

as a dietary element at low doses and in most environmental

systems is immobile (Higgins et al 1998) Th e COPR which

contains both Cr(III) and Cr(VI) is not as benign as initially

thought a yellow chromate solution is often observed to leach

from locations where COPR is deposited and elevated Cr(VI)

concentrations are measured in groundwater and water bodies

in the proximity of these sites (Farmer et al 2002) In conse-

quence COPR has become a major contamination source of the

highly mobile and toxic Cr(VI) in many urban areas

Leaching of Cr(VI) from soils and COPR has been studied fairly

extensively however COPR originating from various localities may

Abbreviations bgs below ground surface CAC calcium aluminum oxide chromium

hydrates COPR chromite ore processing residue DI deionized EDS energy dispersive

x-ray spectroscopy SEM scanning electron microscopy XRPD x-ray powder diff raction

Center for Environmental Systems Stevens Institute of Technology Castle Point on

Hudson Hoboken NJ 07030

Copyright copy 2008 by the American Society of Agronomy Crop Science

Society of America and Soil Science Society of America All rights

reserved No part of this periodical may be reproduced or transmitted

in any form or by any means electronic or mechanical including pho-

tocopying recording or any information storage and retrieval system

without permission in writing from the publisher

Published in J Environ Qual 372125ndash2134 (2008)

doi102134jeq20070443

Received 22 Aug 2007

Corresponding author (mwaznestevensedu)

copy ASA CSSA SSSA

677 S Segoe Rd Madison WI 53711 USA

TECHNICAL REPORTS HEAVY METALS IN THE ENVIRONMENT

2126 Journal of Environmental Quality bull Volume 37 bull NovemberndashDecember 2008

diff er in terms of leaching behavior and mineralogy due to varia-

tions in chromite ore composition extraction process and deposi-

tion practices Some ores could be rich in silica iron aluminum

or impurities which cause the resulting mineralogy to vary For

example the COPR at Glasgow has twice the amount of silicon as

the COPR at New Jersey Th is may have very strong implication

on the mineralogy of COPR as it may favor the formation of ce-

mentitious silicious phases Similarly ineffi cient extraction of sul-

fates during the sodium sulfate operation during processing or the

intrusion of sulfate due the oxidation of sulfur in certain kiln fuel

oil (Allied Signal) may change the resulting mineralogy of COPR

Th e introduction of sulfates to COPR may cause the formation of

ettringite a swell causing mineral in cement chemistry Reports of

catastrophic heave and swell are abound at the Jersey site however

no heave was reported at Glasgow (Geelhoed et al 2002 Der-

matas et al 2006a) Moreover often times COPR is mixed with

indigenous soils at some deposition sites whereas at other sites it

is deposited as pure COPR Even at pure COPR sites the man-

ner of deposition may have implications on COPR behavior For

example if COPR is deposited in homogenous layers its behavior

may diff er had it been deposited through conduits where the fi ner

material is expected to settle to the bottom of the heap and the

coarser material will collect on top similar to alluvial depositions at

deltas Th e mineralogical composition of the fi ne and coarse mate-

rials is expected to be diff erent (Dermatas et al 2006b) Moreover

due to its high alkalinity COPR absorbs signifi cant amounts of

atmospheric CO2 where it combines with COPR constituents to

form various carbonate phases Th e degree of atmospheric exposure

and manner of CO2 exposure may determine the amount of CO

2

absorbed For example CO32minus constitutes approximately 115 of

total mass of COPR at the Jersey site (total carbonate at Glasgow

is not reported) In addition hydrotalcites were clearly identifi ed

at the Jersey site but are not reported at Glasgow Th is has implica-

tions on COPR mineralogy and leaching behavior because hydro-

talcites which are magnesium aluminum carbonate hydrates are

reported to sequester chromates through anionic exchange (Alvar-

ez-Ayuso and Nugteren 2005) Consequently the leaching behav-

ior and the resultant mineralogy will diff er signifi cantly depending

on the parent ore extraction process and deposition practices

Moreover modeling techniques may diff er in terms of scope

and methodology For example Yalčin and Uumlnluuml (2006) reported

on a mechanistic model to simulate Cr(VI) leaching from COPR

obtained at an industrial plant in Turkey Th e model incorporates

a sequence of batch (dissolution) and fl ushing (leaching) opera-

tions It uses Runge Kutta method to solve the resultant coupled

diff erential equations and determine the parameters of the leaching

reactor Th e study does not address the geochemistry of COPR

leaching or mineralogy Conversely Geelhoed et al (2002) reports

on the leaching of Cr(VI) in COPR originating from a Glasgow

site Th e authors use leaching tests XRPD and equilibrium based

geochemical modeling to describe the leaching behavior of COPR

Th e concentration of leached Ca Al Si Mg and Cr(VI) are used

to calibrate for the solubility constants of some solid phases that

were used in the model calculations Th e mineral phases identi-

fi ed in the residues of the leaching tests and other phases that may

be present in COPR are used in their model Even though the

dissolution of COPR is kinetically controlled (dissolution time is

beyond experimental time frame) the authors used the ionic activ-

ity product instead of equilibrium solubility to account for the

discrepancy in the concentration of some elements between the

measured ones and the ones predicted by the model had they used

equilibrium constants as reported in the literature For example

they modifi ed the equilibrium constant of Cr(VI)-ettringite to

make the model fi t their experimental data down to pH 8 even

though they did not identify Cr(VI)-ettringite in any sample

through XRPD or scanning electron microscopy (SEM)ndashenergy

dispersive x-ray spectroscopy (EDS) Th e discrepancy between the

measured and predicted concentrations could be due to the slow

dissolution rate of some COPR minerals such as brownmillerite

which may encapsulate Cr(VI) minerals as reported by Kremser

et al (2005) Alternatively Cr(VI) could be encapsulated in other

phases that were not identifi ed in their study such as hydrotalcites

which are thermodynamically stable down to pH 8

In this study batch leaching tests complemented with

qualitative and quantitative XRPD analyses SEM-EDS and

geochemical modeling are used to investigate the leaching

behavior of Cr(VI) in COPR originating from a site in Hud-

son County New Jersey Th e experimental and modeling

results are then explained in terms of potential mineral phases

that could be controlling Cr(VI) leaching and the kinetics of

dissolution of thermodynamically unstable minerals present

on site Th e fi ndings of this study are relevant to assess the

potential of hexavalent chromium contamination emanating

from COPR sites in Hudson County New Jersey Th ey are

also relevant for the development of remediation strategies

and chemical treatment of Cr(VI) in COPR

Materials and Methods

MaterialsTh e COPR samples used in this investigation were ob-

tained from a study area in Jersey City NJ Th e samples were

collected from two stratigraphic layers (B1 and B2) during a

major bulk-sampling event from 14 borings across the site

Layer B1 represents the upper most unsaturated zone and layer

B2 lies directly below layer B1 Th e water table level fl uctuates

within layer B2 throughout the entire site A composite sample

B1B2 was prepared by mixing equal amounts of all samples

from layers B1 and B2 across the borings Th e samples were

thoroughly homogenized before any COPR material was used

All chemical reagents used were of ACS or higher-grade quality

and were obtained from Fisher Scientifi c (Suwanee GA) All

stock solutions were prepared using deionized (DI) water

Sample CharacterizationTh e elemental composition of the COPR samples was deter-

mined by digesting the COPR material using USEPA Method

3051A (USEPA 1996a) followed by USEPA Method 6010B

(USEPA 1996b) Hexavalent chromium concentration was

obtained using USEPA Method 3060 (USEPA 1996c) and

USEPA Method 7196A (USEPA 1992) Total carbon was

measured using ASTM D5291 (ASTM 2001) and it was as-

Wazne et al Leaching Mechanisms of Cr(VI) from Chromite Ore Processing Residue 2127

sumed to consist of inorganic carbonates Silicon concentration

was determined by fusing the sample with Na2CO

3 and precipi-

tating Si as SiO2 which was then measured using the gravimet-

ric method All sulfur was assumed present as sulfate

X-Ray Powder Diff raction AnalysesTh e XRPD was used for the qualitative characterization of the

particulate fraction of COPR Samples were air dried for 24 h

and then were pulverized using a McCrone micromill for 5 min

with cyclohexane Th e resulting slurry was air-dried and mixed

with 20 mass based corundum (αndashAl2O

3) as internal standard

and the resulting solids were subjected to XRPD analysis Step-

scanned x-ray diff raction data were collected by the Rigaku DXR

3000 computer-automated diff ractometer using Bragg-Brentano

geometry Th e diff ractometry was conducted at 40 kV and

40 mA using diff racted beam graphite-monochromator with Cu

radiation Th e data were collected in the range of 5deg to 65deg 2θ

with a step size of 002deg and a count time of 3 s per step Qualita-

tive analysis of XRPD patterns were conducted using the JADE

software version 71 and the International Centre for Diff raction

database reference patterns (ICDD 2002 ICSD 2005 MDI

2005) Th e JADE software was also used to quantify the identi-

fi ed crystalline mineral phases JADE uses ldquoWhole Pattern Fit-

tingrdquo which is based on the Rietveld method (Rietveld 1969)

Scanning Electron Microscopy AnalysesTh e samples were fi rst air-dried and attached to a substrate

using double-side carbon tape Scanning electron microscopy

(SEM) analyses were conducted using a LEO-810 Zeiss mi-

croscope equipped with an energy dispersive x-ray spectros-

copy (EDS) ISIS-LINK system

Leaching TestsTh e objective of these experiments was to study the quantities

of soluble chromium species ultimately released in COPR leachates

at various pH values for a mixing time of 1 wk Representative

air-dried samples COPR were pulverized to fi ner than 150 μm

(mesh-100) Th e pulverized COPR samples were mixed with DI

water using a liquid-to-solid ratio of 20 Incremental amounts of

concentrated HCl were added to cover a wide range of pH values

Th e mixtures were left on an end-over-end mixer for 1 wk before

the pH values of the mixtures were recorded Th e leachates were

analyzed for total Cr and Cr(VI) Th e residues were submitted for

XRPD analyses Upon sample acidifi cation part of the carbonate

species degassed as CO2 Th e samples were allowed to degas before

they were capped Duplicate samples were prepared with the same

acid dosage Th e duplicate samples were air-dried and the total

carbonate contents were determined using ASTM D5291 (ASTM

2001) Th e measured total carbonate concentrations at various pH

values were used in the model calculations

Another set of leaching tests was conducted to study the release

and exchange of soluble chromium species in COPR leachate for

shorter time scale (minutes) Th e time scale of these experiments

varied in the range of 0 to 80 h Representative COPR samples

were air-dried then pulverized to fi ner than 150 μm (mesh-100)

Th e pulverized COPR samples were added to a beaker with liquid-

to-solid ratio of 20 After a predetermined HCl acid dosage was

added to the samples the beakers were capped with a plastic covers

and the contents were mixed with magnetic stirrers Periodically

homogenous subsamples were withdrawn by a plastic syringe and

fi ltered using 045 μm syringe fi lter Th e fi ltrates were analyzed for

Cr(VI) and pH Two sets of experiments were performed at acid

dosages of 55 and 92 [H+] eqKg COPR

Synthetic Hydrotalcite Chromate Exchange CapacityAn experiment was conducted to assess the capacity of syn-

thetic hydrotalcite to exchange chromate from COPR leachates

Synthetic hydrotalcite [Mg3Al(OH)

8]

2CO

3nH

2O was prepared

by co-precipitation as described by Alvarez-Ayuso and Nugteren

(2005) Briefl y a solution containing 075 mol Mg(OH)26H

2O

and 025 mol Al(NO3)

39H

2O in 250 mL of deionized water

was added to a vigorously stirred solution (500 mL) containing

17 mol NaOH and 05 mol Na2CO

3 Th e resulting precipitate

washed thoroughly with deionized water was dried at 80degC over-

night Th e prepared precipitate was characterized as hydrotalcite

by x-ray diff raction 01 g of the prepared synthetic hydrotalcite

was added to 20 mL of COPR leachate in 50 mL vial After mix-

ing for 1 wk the pH values of the mixtures were recorded and the

leachate was analyzed for Cr(VI)

Geochemical ModelingTh e geochemical equilibrium computer program MINTEQ

(David and Allison 1999 USEPA 2006) was used to model

the concentration of Cr(VI) in the leaching tests and to deter-

mine the predominant solid phases in COPR Th e solid phase

thermodynamic database of MINTEQ was augmented with

the thermodynamic data of the relevant solid phases cited from

the literature based on the XRPD results (Table 1) Pure hydro-

talcite 4MgOAl2O

310H

2O was used in the model due to the

lack of thermodynamic data for other analogs Th e 2-pk diff use

double layer surface complexation model was used to describe

the adsorption of Cr(VI) In the model simulation iron and

aluminum were assumed to provide the adsorption sites Th e

adsorbents were assumed to consist of active and inactive sites

only the active sites were considered in the adsorption model

simulation Th e concentration of the active sites was determined

from the model by minimizing the sum of the square of the

diff erences between the experimental and the model predicted

values of the concentration of Cr(VI) (Jing et al 2006) Th e

acid-base surface reactions and the relevant adsorption reactions

were adopted from the MINTEQ thermodynamic database Th e

Davies equation (Snoeyink and Jenkins 1980) was used to calcu-

late the activity coeffi cients in the model Th e model calculated

ionic strengths were used for I lt 05 M For I gt 05 M the ionic

strength was fi xed at 05 M Th e calculated ionic strength was

equal to or less than approximately 05 M down to pH 9 and it

was lt085 M at pH values gt5 (the lowest pH values in the model

simulations) Th e values of Cr(VI) concentrations predicted by

the model simulation did not change appreciably (lt10) when

the high ionic strengths were fi xed at 05 M


Results and Discussion

Sample CharacterizationTh e COPR material originating from layer B1 is black to

gray fi ne to coarse sand sized with trace silt silty sand and san-

dy silt particles Th e B1 layer was encountered from the ground

surface to approximately 2 to 3 m (7ndash9 ft) below ground sur-

face (bgs) Th e measured water content ranged from approxi-

mately 4 to 23 with an average value of approximately 18

Conversely the COPR material in B2 layer is gray fi ne to

coarse sand sized with seams of silt and sandy silt particles trace

gravel size particles Th e B2 layer was encountered beginning

from approximately 2 to 3 m (7ndash9 ft) bgs to approximately 4 to

5 m (12ndash15 ft) bgs with thickness ranging from approximately

1 to 2 m (35ndash65 ft) Moisture contents ranged from approxi-


Th e elemental composition of the COPR sample is shown in

Table 2 Th e relatively high calcium concentration at approxi-

mately 24 is due to the addition of lime during the extraction

process Th e carbonate most likely was absorbed as CO2 by the

COPR material after the roasting process due to its alkaline

nature Th e sulfate may have entered the system from the oxida-

tion of sulfur in the kiln fuel oil (Bunker C fuel oil) during the

roasting process or from the sodium sulfate operations at the site

(Allied Signal 1982) Presence of sulfate is known to cause ex-

pansion in cement and cement-like material such as COPR

Leaching TestsTh e results of the short time scale leaching tests are shown in

Fig 1 For both acid dosages immediately after acidifi cation the

pH dropped to approximately 3 for the sample with the high

acid dosage and 5 for the sample with the lower acid dosage

However the pH values started to increase as the acid is neutral-

ized by the alkalinity of the COPR matrix It appears that the

pH values stabilized at approximately 8 and 9 for the higher and

lower acid dosages respectively after approximately 20 h

Th e measured Cr(VI) concentrations in the leachates

15 min after the addition of acid were approximately 125 mgL

and 200 mgL for the high and low acid dosages respectively

Cr(VI) concentrations for both samples reached maximum

values after approximately 2 h Th e Cr(VI) concentration of

the COPR sample with the smaller acid dosage (pH 9) peaked

at approximately 240 mgL and then it decreased to approxi-

mately 200 mgL after 76 h Conversely the Cr(VI) concentra-

tion in the COPR sample with the higher acid dosage (pH 8)

peaked at approximately 270 mgL and then it decreased to

approximately 250 mgL Th e moderate decrease in Cr(VI)

concentration of approximately 17 for the lower acid dosage

(pH 9) could be attributed to incorporation of chromate in

stable COPR minerals at that pH Adsorption is not expected

to play a signifi cant role at this pH range (Zachara et al 1987)

In addition had this decrease been due to adsorption higher

removal rates would have been expected at the high rather than

the low acid dosage since the rate of adsorption of anions is

known to be greater at lower pH values Th is information is

signifi cant for any remediation scheme incorporating leaching

techniques to solubilize Cr(VI) since no solid phase is reported

to control the solubility of Cr(VI) at pH lt105

Th e results of the long-term leaching tests (1 wk) are pre-

sented in Fig 2 where the concentration of total and hexavalent

chromium (left ordinate) and amount of acid in [H+] eqKg

COPR (right ordinate) are plotted vs pH Concentration levels

for total Cr and Cr(VI) in the leachate are virtually identical at

pH greater than approximately 5 Th is indicates that all leached

total Cr in this pH region is in the hexavalent state At pH val-

ues lt5 Cr concentration is greater than Cr(VI) concentration

Th e diff erence between total Cr and Cr(VI) concentrations at

pH values less than approximately 5 is attributed to Cr(III)

Figure 2 indicates that almost all of the chromium was released

when the pH was below 15 Approximately 32 equivalents of

acid [H+] per 1 kg of dry COPR were needed to attain pH 15

In addition the majority of Cr(VI) quantifi ed by alkaline di-

gestion was leached at pH 8 For example Cr(VI) concentration

in the leachate was approximately 260 mgL at pH of 8 with

a liquid-to-solid ratio of 20 meaning that 5200 mgKg Cr(VI)

was leached from the COPR sample constituting approximately

93 of the total leachable Cr(VI) However the entire Cr con-

centration was leached at pH lt 15 Th e measured Cr concentra-

tion was 1100 mgL which is equivalent to 22000 mgKg

Table 1 R eactions and parameters used in the model calculations (phases added to Minteqa2)dagger

Solid phase Chemical formula Log K

Afwillite Ca3Si

2O

4(OH)

6 6492a (1)Dagger

Brownmillerite C4AF Ca

4Al

2Fe

2O

1013913a (2)

C2AH

8Ca

2Al

2(OH)

103H

2O 5819b (3)

CAH10

CaAl2(OH)

86H

2O 3799b (4)

C4AH

13Ca

4Al

2(OH)

146H

2O 10725b (5)

C4AH

19Ca

4Al

2(OH)

1412H

2O 10368b (6)

Gehlenite Hydrate C2ASH

8Ca

2Al

2(OH)

6SiO

8H

8H

2O 4916c (7)

Hydrogarnet C3AH

6Ca

3Al

2(H

4O

4)

3 7827d (8)

Hydrotalcite M4AH

104MgOAl

2O

310H

2O 7378c (9)

Monosulfate Ca4Al

2(OH)

12SO

46H

2O 7197e (10)

Wairakite CaAl2SiO

10(OH)

41807b (11)

Surface reactions

= SOH + H+ rarr = S-OH2

+ 729f (12)

4SOH rarr = SOminus + H+ minus893f (13)

= SOH + CrO4 2minus rarr = SOHCrO

42minus 39f (14)

= SOH + CrO4 2minus + H+ rarr = SCrO

4minus +H

2O 1085f (15)

= SOH + CO3 2minus + H+ rarr = SCO3minus +H

2O 1278f (16)

= SOH + CO3 2minus + 2H+ rarr = SCO3Hminus +H

2O 2037f (17)

= SOH + H4SiO

4 rarr = SOSiO

2OH2minus + 2H+ + H

2O minus1169f (18)

= SOH + H4SiO

4 rarr = SOSi(OH)

2minus + H+ + H

2O minus322f (19)

= SOH + H4SiO

4 rarr = SOSi(OH)

3 + H

2O 428f (20)

dagger Parameters used in the model Best fi t active adsorbent sites = 1095 mmolL

gSurface site (SOH) density = 11 sitesnm2 gSpecifi c surface area = 600 m2g

Dagger Source(s) a Common Thermodynamic Database Project (2004) b EQ36

(Lawrence Livermore National Laboratory [1994]) c Bennet et al (1992) d

Reardon (1992) e Phreeqc Database Parkhurst and Appelo (1999) f David and

Allison (1999) g Dzombak and Morel (1990)

Table 2 Elemental composition of composite chromite ore processing residue sample B1B2

Element Al Ca CO3

2minus Cr(VI) Cr Fe K Mg Mn Na Si SO4

2minus

Percent mass basis 46 239 115 056 21 118 003 61 012 037 198 034


X-ray Powder Diff raction CharacterizationTh e XRPD mineralogical analyses indicate that brownmil-

lerite brucite calcite quartz hydrotalcite and katoite are the

major mineral phases in the COPR samples (Fig 3) Th e content

of brownmillerite ranged between approximately 38 and 205

of the solid residue for the ANC tests which indicated that it

is relatively stable down to pH 524 Th e amorphous content

as determined by the corundum internal standard was approxi-

mately 42 pH 1203 and it increased to approximately 645

at pH 524 Th e only known Cr(VI) bearing mineral identifi ed

was Calcium Aluminum Oxide Chromium Hydrates (CAC)

also known as Cr(VI)-hydrocalumite with molecular formula

(Ca4Al

2(OH)

12CrO

4nH

2O) Th e Cr(VI)-hydrocalumite is a

layered double hydroxide (LDH) mineral with chromate anions

held in the interlayers Th e Rietveld quantifi cation indicated

that CAC is present at 087 in the residue of the ANC test at

pH 1203 A Cr(VI)-hydrocalumite content of 087 indicate

a Cr(VI) concentration of approximately 667 mgKg which is

approximately 13 of total Cr(VI) Th is indicates that the ma-

jority of Cr(VI) is not encapsulated in the Cr(VI)-hydrocalumite

phase Th e Cr(VI) could be present in other mineral phases such

as hydrogarnet (katoite) as reported by Hillier et al (2007) or hy-

Fig 1 Plot of Cr(VI) concentration and pH as a function of time due to the additions of 55 and 92 eqKg [H+]

Fig 2 Concentration of Cr and Cr(VI) in the leachates vs pH after 1 wk of mixing with liquid-to-solid ratio of 20 for composite sample B1B2


drotalcites through anionic substitution Even though Cr(VI)-

ettringite is reported to control the solubility of chromate at

pH gt104 (David and Allison 1999 Jing et al 2006 USEPA

2006) it was not identifi ed in any of the COPR samples Con-

versely hydrotalcite phases such as quintinite and sjogrenite were

identifi ed in many COPR samples as evidenced by XRPD in Fig

3 and Table 3 and also by SEM as shown in Fig 4

Th e SEM images indicated the presence of hydrotalcite

phases with extensive chloride substitution in the interlayers

probably due to the use of HCl in the ANC tests However the

chromium peak at approximately 54 keV could be due to the

substitution of CrO42minus Cr3+ or both in the crystal structure

Hydrotalicte has platy like hexagonal structure as shown clearly

in Fig 4 Th e crystals were clearly identifi ed at pH 109 but as pH

decreased it seems that the crystal size decreased and there appears

to be some disorder in addition to some coating In general hydro-

talcite crystals were more diffi cult to identify at lower pH values

ModelingTh e diff use double layer surface complexation model (Dzom-

bak and Morel 1990 USEPA 2006) is used to describe Cr(VI)

adsorption data Carbonate adsorption is incorporated into the

model since carbonate is reported to adsorb onto iron hydrox-

ides (Zachara et al 1987 Van Geen et al 1994 Wijnja and

Schulthess 2001) Silicate has also been reported to adsorb onto

iron hydroxides (Meng et al 2000) and therefore it is included

in the model Th e total concentration of iron and aluminum in

the leaching solution are 9821 and 8703 mM respectively

Th e calculated best fi t for the active adsorbent concentra-

tion is 1095 mM which indicates that the molar ratio of the

active sites to the total iron and aluminum concentration is

lt6 It is worth noting that the adsorption sites are not as-

sumed to associate with any specifi c solid phase

Brownmillerite was the major mineral phase observed in the

residues of the leaching tests However brownmillerite is not

predicted by the model as it is not thermodynamically stable in

aqueous solution Tamas and Vertes (1972) reported complete

hydration of synthetic brownmillerite in less than 2 wk It ap-

pears that brownmillerite hydration is kinetically inhibited in

the COPR material and that may explain its persistence some

80 yr after its deposition Th e inclusion of magnesium in the

crystal structure of brownmillerite was reported to slow its hy-

dration as compared to pure brownmillerite (Jupe et al 2001)

this may explain the persistence of brownmillerite since mag-

nesium constitutes approximately 6 of the COPR material

Moreover the rate of brownmillerite hydration cannot be deter-

mined since the original composition of the COPR minerals af-

ter the quenching process is not known and there is no reliable

estimate of the initial brownmillerite content and its hydration

byproducts Also it is not clear whether brownmillerite under-

went further hydration after deposition Th e hydration byprod-

ucts of brownmillerite are katoite (a hydrogarnet) portlandite

and hematite (Tamas and Vertes 1972) Katoite was predicted

in the residue of the COPR leaching tests at pH gt 115 Th e

model also predicted the sequestration of magnesium in hydro-

talcite however the experimental data showed that magnesium

was sequestered in brucite and periclase in addition to hydrotal-

cites Brucite and periclase could be meta-stable states that will

transform into hydrotalcite if and when suffi cient aluminum is

released from brownmillerite hydration Hematite and diaspore

predicted by the model are the crystalline phases of the amor-

phous iron and aluminum hydroxides respectively

Th e model predicted the concentration of Cr(VI) to be con-

trolled by adsorption at pH lt 8 and by precipitation at pH gt

105 (Fig 5) It predicted Cr(VI) sequestration in Cr(VI)-

hydrocalumite and Cr(VI)-ettringite mineral phases at pH gt 11

and 105 lt pH lt 115 respectively (Fig 6) In the pH region

9 lt pH lt 105 chromate was predicted almost to be 100 in

the dissolved phase (Fig 5) However the experimental results

indicated that the model overpredicted Cr(VI) concentration

Fig 3 The x-ray diff raction patterns of the residues of the leaching tests at various pH values


Th e only stable mineral in that pH region that may contain

chromate through anionic exchange is hydrotalcite (Fig 5 and

6) Hydrotalcites are reported in the literature to be capable of

removing chromate from solution through anionic exchange

(Goswamee et al 1998 Lazaridis and Asouhidou 2003 Terry

2004 Alvarez-Ayuso and Nugteren 2005) Specifi cally the

data of Terry (2004) indicates Cr(VI) removal rates of approxi-

mately 96 and 95 from solutions with initial concentrations

of 5 and 20 mgL respectively using hydrotalcite concentra-

tions of 5 gL at pH values of 20 and 21 Lower Cr(VI) re-

Table 3 Rietveld quantifi cation of minerals and phases in the residues of the leaching tests

pH 1203 1104 1058 935 830 776 642 524

ndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashndashMineral phases

Calcium aluminum oxide chromium hydrate 087 069

Brownmillerite 2682 3384 2847 2966 3716 3794 3399 2057

Brucite 284 364 214 187 141 074 081

Calcite 853 604 638 684 718 684 501 117

Hydroandradite 319 398 231 162 159

Katoite 261 377 209 069 047

Periclase 221 240 209

Quartz 319 364 273 315 395 427 442 441

Quinitinite-2H 122 165 166 064

Sjoegrenite 145 185 123 472 147 114 086 060

Albite 517 700 450 472 559 604 878 711

Gibbsite 167

Percent crystalline phases 5811 6850 5362 4918 5883 5697 5386 3553

Noncrystalline phase 4194 3137 4638 5082 4111 4303 4614 6447

Percent total 10006 9986 10000 10000 9994 10000 10000 10000

Fig 4 Scanning electron microscopy (SEM) images of the leaching residue at pH 109 and pH 94


moval rates were obtained at higher pH values Approximately

30 Cr(VI) removal rates were obtained at pH value of ap-

proximately 9 for solutions with initial Cr(VI) concentration of

20 mgL It is worth noting that hydrotalcite is not expected to

be stable at pH 2 and the high Cr(VI) removal rates that Terry

attributed to hydrotalcite may in fact be due to the adsorption

of chromate anion onto aluminum hydroxide generated by the

dissolution of hydrotalcite Terry did not investigate the stabil-

ity of hydrotalcite at low pH values or present any data to sug-

gest that it is stable On the other hand the removal rates that

were achieved by Terry at higher pH values are similar to the

rates reported in this study that is 27 removal rate at pH of

approximately 95

On the other hand Alvarez-Ayuso and Nugteren (2005

Wat Res 39 2535ndash2542) reported that Cr(VI) adsorption at

high pH values decreased when much longer times than those

required to reach equilibrium were used Alvarez-Ayuso and

Nugteren deduced that chromium release occurred under such

conditions Accordingly these phenomena can be attributed

to the slow dissolution of ldquocalcinated hydrotalciterdquo andor to

the replacement of previously sorbed chromium by carbonate

However Alvarez-Ayuso and Nugteren did not report similar

desorption behavior for the ldquononcalcinated hydrotalciterdquo which

was used in this study

Th erefore to our knowledge chromate exchange from

COPR leachate by hydrotalcites or chromate substitution in

hydrotalcites in COPR has not been reported in the litera-

ture Hydrotalcite phases such as sjoegrenite Mg6Fe

2(CO

3)

(OH)14

5H2O and quintinite-2H MgAl

2(OH)

12(CO

3)4H

2O

were identifi ed in the COPR samples down to pH of 8 (Fig 3

and 4 Table 2) Th ese belong to a class of layered compounds

derived from the structure of mineral brucite Mg(OH)2

Brucite is comprised of layered sheets of neutral octahedrons

of magnesium hydroxide When a fraction x of Mg2+ ions

are isomorphously substituted by Al3+ ions the sheets acquire

the composition [Mg(1ndashx)Alx(OH)

2]x+ (02 lt x lt 033) and

become positively charged (Radha et al 2005) Anions (Clminus

NO3minus CO

3 2minus and others) are incorporated in the interlayer

region along with water molecules to balance the charge defi cit

and restore stability Many divalent ions such as Ni Co Cu

and Zn form LDHs with trivalent ions such as Al Cr Fe and

Ga produce a very large class of isostructural compounds (Trave

et al 2002 Bontchev et al 2003)

Th e decrease in Cr(VI) concentration at pH gt 9 (Fig 1) as

the leaching experiment progressed may provide an indication

of chromate substitution in hydrotalcites To further assess the

capacity of synthetic hydrotalcite to remove chromate from

COPR leachate an experiment was conducted using synthetic

hydrotalcite Approximately 27 of chromate was removed

from the COPR leachate when approximately 386 mgL

Cr(VI) COPR leachate was mixed with 01 gL synthetic hy-

drotalcite at pH 95 for 1 wk Th e calculated amount of Cr(VI)

exchanged is 208 mgg which is smaller than the value report-

ed by Lazaridis and Asouhidou (2003) of 33 mgg at pH 10

from 10 mgL synthetic chromate solution Th is indicates that

chromate in the COPR leachates can substitute for OHminus or

CO32minus in the interlayers of hydrotalcite Th is may indicate that

for treatment schemes pH has to be decreased to lt8 to destabi-

lize hydrotalcite phases

Th e underestimation of Cr(VI) concentration at pH 8

could be due to the competitive adsorption of silicates (stron-

ger than that predicted by the model) which strongly adsorbs

in that pH region Th is may have consumed most of the avail-

able adsorption sites and caused higher Cr(VI) leaching rates

Th e COPR mineralogy as it exits the roasting process is

comprised of a mixture of brownmillerite (Ca4Fe

2Al

2O

10) peri-

clase (MgO) and quicklime (CaO) which are considered to be

the ldquoparentrdquo COPR minerals in addition to minor impurities

(Allied Signal 1982) Based on experimental observations

COPR consists mainly of two groups of minerals that behave

diff erently fast reactants mainly hydrates and carbonates

and slow reactants such as brownmillerite and periclase Fast

reactants imply that the reactions will occur within minutes

and slow implies months or years to complete Cr(VI) is

Fig 5 Model prediction of Cr(VI) concentration as function of pH for the leaching tests


found mainly in calcium aluminium oxide chromium hydrates

(CACs) (monochromate or Cr(VI)-hydrocalumite) and is

partially present in hydrotalcites (this study) and hydrogarnets

(Hillier et al 2007) through anionic substitution Th e experi-

mental results indicated that Cr(VI) bearing minerals react

within minutes whereas brownmillerite hydration may take

years to complete However the two groups of minerals are

sometimes partially intertwined Modeling of the leaching data

should allow for the incorporation of chromates into various

mineral phases through anionic exchange with phases such as

hydrogarnets hydrotalcites and other phases (once identifi ed)

Moreover modeling should also account for the slow dissolu-

tion of brownmillerite and hence the availability of Cr(VI)

when chromate phases are encapsulated into the brownmillerite

nodules A major fi nding of this study is the strong evidence

about the presence of chromates in hydrotalcites However

more research is needed to determine the percentages of readily

available and nonreadily available (encapsulated in brownmil-

lerite nodules) chromates present in CAC(s) Cr(VI)-ettringite

hydrogarnts and hydrotalcites

Finally the complete hydration of brownmillerite and the

subsequent dissolution of its byproducts are expected to re-

lease additional alkalinity according to the following equation

Ca4Fe

2Al

2O

10 + 7H

2O rarr Ca(OH)

2 + Ca

3Al

2(OH)

12 + Fe

2O

3 [1]

Th is may cause pH to stabilize at values higher than those

measured in Fig 2 For example for a brownmillerite content

of 25 of the COPR matrix the complete dissolution of

brownmillerite requires additional 4[H+] eqKg Th e resulting

mineralogy at equilibrium at various pH values is shown in Fig

5 However the complete dissolution of brownmillerite is not

expected to signifi cantly aff ect the model prediction of aqueous

Cr(VI) concentration at various pH values because the dissolution

and precipitation of Cr(VI) minerals are relatively fast (minutes)

similarly the adsorption and desorption of chromate are also

relatively fast A model simulation with 25 additional adsorbent

sites shifted the adsorption edge lt1 pH unit to the right at pH

values lt9 (Fig 6) however the additional adsorbent had no eff ect

on the model simulation at pH gt 9 where anion exchange and

precipitation are expected to dominate Cr(VI) solubility

ConclusionsTh e leaching mechanism of Cr(VI) from COPR was inves-

tigated using the XRPD analyses and geochemical modeling

Th e model was able to simulate the adsorption edge at ap-

proximately pH 5 and the precipitation edge at approximately

pH 11 Th e experimental and modeling results suggested the

presence of a mineral phase controlling the solubility of Cr(VI)

in the pH region 8 lt pH lt 11 Experimental results indicated

that hydrotalcite may be controlling the solubility of chromate

though anionic exchange in that pH region Th e COPR at the

deposition sites is expected to continuously leach Cr(VI) at a

concentration equivalent to the solubility of Cr(VI) with re-

spect Cr(VI)-hydrocalumite If pH were to drop below pH 11

signifi cant increase in Cr(VI) leaching will ensue however this

seems unlikely due to the high buff ering capacity of COPR

Finally all chromium was released when 32[H+] eqKg of acid

was added to the COPR matrix Th is information is very useful

for stabilization or recovery of chromium in COPR matrices

AcknowledgmentsTh e authors wish to thank Honeywell International Inc

for the fi nancial support of this study

Fig 6 Model prediction of mineral phases present in the residues of the leaching tests for composite sample B1B2 at various equilibrium pHs


ReferencesAllied Signal 1982 Process description-Baltimore works Honeywell

Morristown NJ

Alvarez-Ayuso E and HW Nugteren 2005 Purifi cation of chromium(VI) fi nishing wastewaters using calcinated and uncalcinated Mg-Al-CO3-hydrotalcite Water Res 392535ndash2542

American Society for Testing Material 2001 Test methods for instrumental determination of carbon hydrogen and nitrogen in petroleum products and lubricants ASTM D 5291-96 Annual Book ASTM Stand 236ndash240 ASTM West Conshohocken PA

Bennet DG D Read M Atkins and FP Glasser 1992 A thermodynamic model for blended cements II Cement hydrate phases thermodynamic values and modeling studies J Nucl Mater 190315ndash325

Bontchev RP S Liu JL Krumhansl J Voigt and TM Nenoff 2003 Synthesis characterization and ion exchange properties of hydrotalcite Mg

6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H

2O (A Arsquo = Cl- Br- I- and NO

3- 2 ge x ge 0)

derivatives Chem Mater 153669ndash3675

Burke T J Fagliano M Goldoft RE Hazen R Iglewicz and T McKee 1991 Chromite ore processing residue in Hudson County New Jersey Environ Health Perspect 92131ndash137

Common Th ermodynamic Database Project 2004 CTDP homepage Available at httpwwwcigensmpfr~vanderleectdpindexhtml (verifi ed 6 Aug 2008)

Darrie RG 2001 Commercial extraction technology and process waste disposal in the manufacture of chromium chemicals from ore Environ Geochem Health 23187ndash193

David SB and JD Allison 1999 Minteqa2 an equilibrium metal speciation model Userrsquos manual 401 Environmental Res Lab USEPA Athens GA

Dermatas D R Bonaparte M Chrysochoou and DH Moon 2006b Chromite ore processing residue (COPR) Contaminated soil or hazardous waste J ASTM Int 3(7) doi 101520JAI13313

Dermatas D M Chrysochoou DH Moon DG Grubb M Wazne and C Christodoulatos 2006a Ettringite-induced heave in chromite ore processing residue (COPR) upon ferrous sulfate treatment Environ Sci Technol 40(18)5786ndash5792

Dzombak DA and FMM Morel 1990 Surface complexation modeling Hydrous ferric oxide John Wiley amp Sons New York

Farmer JG MC Graham RP Th omas C Licona-Manzur E Paterson and CD Campbell 1999 Assessment and modeling of the environmental chemistry and potential for remediative treatment of chromium-contaminated land Environ Geochem Health 21331ndash337

Farmer JG RP Th omas MC Graham JS Geelhoed DG Lumsdon and E Paterson 2002 Chromium speciation and fractionation in ground and surface waters in the vicinity of chromite ore processing residue disposal sites J Environ Monit 4235ndash243

Geelhoed JS JCL Meeussen S Hillier DG Lumsdon and RP Th omas 2002 Identifi cation and geochemical modeling of processes controlling leaching of Cr(VI) and other major elements from chromite ore processing residue Geochim Cosmochim Acta 66(22)3927ndash3942

Goswamee RL P Sengupta KG Bhattacharyya and DK Dutta 1998 Adsorption of Cr(VI) in layered double hydroxoides Appl Clay Sci 1321ndash34

Graham MC JG Farmer P Anderson E Paterson S Hillier DG Lumsdon and R Bewley 2006 Calcium polysulfi de remediation of hexavalent chromium contamination from chromite ore processing residue Sci Total Environ 36432ndash44

Higgins TE AR Halloran ME Dobbins and AJ Pittignano 1998 In-situ reduction of hexavalent chromium in alkaline soils enriched with chromite ore processing residue Air Waste Manage Assoc 48(11)1100ndash1106

Hillier S DG Lumsdon R Brydson and E Paterson 2007 Hydrogarnet A host phase for Cr(VI) in chromite ore processing residue (COPR) and other high pH wastes Environ Sci Technol 41(6)1921ndash1927

Inorganic Crystal Structure Database 2005 Fachinformationszentrum Karlsruhe Germany

International Centre for Diff raction Data 2002 Powder diff raction fi le PDF-2 database release International Centre for Diff raction Data Newtown Square PA

James BR 1996 Th e challenge of remediating chromium-contaminated soil Environ Sci Technol 30248Andash251A

Jing C S Liu GP Korfi atis and X Meng 2006 Leaching behavior of Cr(III) in stabilizedsolidifi ed soil Chemosphere 64379ndash385

Jupe AC JK Cockcroft P Barnes SL Colston G Sanker and C Hall 2001 Th e site occupancy of Mg in the brownmillerite structure and its eff ect on hydration properties An X-rayneutron diff raction and EXAFS study J Appl Crystallogr 3455ndash61

Kremser DD BM Sass GI Clark M Bhargava and C French 2005 Environmental forensics investigation of buried chromite ore processing residue Paper E-12 In BC Alleman and ME Kelley (conf chairs) In Situ and On-Site Bioremediationmdash2005 Proc Of the 8th Int In Situ and On-Site Bioremediation Symp Baltimore MD 6ndash9 June 2005 Battelle Press Columbus OH

Lawrence Livermore National Laboratory 1994 EQ36 Database Version 8 Release 6 Lawrence Livermore Natl Lab Livermore CA

Lazaridis NK and DD Asouhidou 2003 Kinetics of sorptive removal of chromium(VI) from aqueous solutions by calcinated Mg-Al-CO

3

hydrotalcite Water Res 372875ndash2882

Materials Data Inc 2005 Jade Version 71 Materials Data Inc Livermore CA

Meng X S Bang and GP Korfi atis 2000 Eff ects of silicate sulfate and carbonate on arsenic removal by ferric chloride Water Res 34(4)1255ndash1261

Parkhurst DL and CAJ Appelo 1999 Userrsquos guide to Phreeqc (Version 2)-A computer program speciation batch-reaction One-dimensional transport and inverse geochemical calculations US Geol Surv Denver CO

Radha AV PV Kamath and C Shivakumara 2005 Mechanisms of the anion exchange of the layered double hydroxides (LDHs) of Ca and Mg with Al Soil State Sci 7(10)1180ndash1187

Reardon EJ 1992 Problems and approaches to the prediction of chemical composition in cementwater systems Waste Manage 12221ndash239

Rietveld HM 1969 A profi le refi nement method for nuclear and magnetic structures J Appl Crystallogr 265ndash71

Snoeyink VL and D Jenkins 1980 Water chemistry John Wiley amp Sons New York

Tamas DF and A Vertes 1972 A mossbauer study of the hydration of brownmillerite Cement Concrete Res 3575ndash581

Terry P 2004 Characterization of Cr ion exchange with hydrotalcite Chemosphere 57541ndash546

Trave A A Selloni A Goursot D Tichit and J Weber 2002 First principles study of the structure and chemistry of Mg-based hydrotalcite-like anionic clays J Phys Chem B 10612291ndash12296

US Environmental Protection Agency 1992 Chromium hexavalent (colorimetric) Method 7196A USEPA Washington DC

US Environmental Protection Agency 1996a Microwave assisted acid digestion of sediments sludges soils and oils Method 3051A USEPA Washington DC

US Environmental Protection Agency 1996b Inductive coupled plasma-atomic emission spectrometry Method 6010B USEPA Washington DC

US Environmental Protection Agency 1996c Alkaline digestion for hexavalent chromium Method 3060A USEPA Washington DC

US Environmental Protection Agency 2006 Visual MINTEQ Version 25 USEPA Washington DC Available at httpwwwlwrkthseEnglishOurSoftwarevminteq (verifi ed 7 Aug 2008)

Van Geen A AP Robertson and JO Leckie 1994 Complexation of carbonate species at the goethite surface Implications for adsorption of metal ions in natural waters Geochim Cosmochim Acta 58(9)2073ndash2086

Weng CH CP Huang HE Allen AHD Cheng and PF Sanders 1994 Chromium leaching behavior in soil derived from chromite ore processing waste Sci Total Environ 15471ndash86

Wijnja H and CP Schulthess 2001 Carbonate adsorption mechanism on goethite studied with ATR-FTIR DRIFT and proton coadsorption measurements Soil Sci Soc Am J 65324ndash330

Yalčin S and K Uumlnluuml 2006 Modelling chromium dissolution and leaching from chromite ore processing residue Environ Eng Sci 23(1)187ndash201

Zachara JM DC Girvin RL Schmidt and CT Resch 1987 Chromate adsorption on amorphous iron oxyhydroxide in the presence of major groundwater ions Environ Sci Technol 21589ndash594


diff er in terms of leaching behavior and mineralogy due to varia-

tions in chromite ore composition extraction process and deposi-

tion practices Some ores could be rich in silica iron aluminum

or impurities which cause the resulting mineralogy to vary For

example the COPR at Glasgow has twice the amount of silicon as

the COPR at New Jersey Th is may have very strong implication

on the mineralogy of COPR as it may favor the formation of ce-

mentitious silicious phases Similarly ineffi cient extraction of sul-

fates during the sodium sulfate operation during processing or the

intrusion of sulfate due the oxidation of sulfur in certain kiln fuel

oil (Allied Signal) may change the resulting mineralogy of COPR

Th e introduction of sulfates to COPR may cause the formation of

ettringite a swell causing mineral in cement chemistry Reports of

catastrophic heave and swell are abound at the Jersey site however

no heave was reported at Glasgow (Geelhoed et al 2002 Der-

matas et al 2006a) Moreover often times COPR is mixed with

indigenous soils at some deposition sites whereas at other sites it

is deposited as pure COPR Even at pure COPR sites the man-

ner of deposition may have implications on COPR behavior For

example if COPR is deposited in homogenous layers its behavior

may diff er had it been deposited through conduits where the fi ner

material is expected to settle to the bottom of the heap and the

coarser material will collect on top similar to alluvial depositions at

deltas Th e mineralogical composition of the fi ne and coarse mate-

rials is expected to be diff erent (Dermatas et al 2006b) Moreover

due to its high alkalinity COPR absorbs signifi cant amounts of

atmospheric CO2 where it combines with COPR constituents to

form various carbonate phases Th e degree of atmospheric exposure

and manner of CO2 exposure may determine the amount of CO

2

absorbed For example CO32minus constitutes approximately 115 of

total mass of COPR at the Jersey site (total carbonate at Glasgow

is not reported) In addition hydrotalcites were clearly identifi ed

at the Jersey site but are not reported at Glasgow Th is has implica-

tions on COPR mineralogy and leaching behavior because hydro-

talcites which are magnesium aluminum carbonate hydrates are

reported to sequester chromates through anionic exchange (Alvar-

ez-Ayuso and Nugteren 2005) Consequently the leaching behav-

ior and the resultant mineralogy will diff er signifi cantly depending

on the parent ore extraction process and deposition practices

Moreover modeling techniques may diff er in terms of scope

and methodology For example Yalčin and Uumlnluuml (2006) reported

on a mechanistic model to simulate Cr(VI) leaching from COPR

obtained at an industrial plant in Turkey Th e model incorporates

a sequence of batch (dissolution) and fl ushing (leaching) opera-

tions It uses Runge Kutta method to solve the resultant coupled

diff erential equations and determine the parameters of the leaching

reactor Th e study does not address the geochemistry of COPR

leaching or mineralogy Conversely Geelhoed et al (2002) reports

on the leaching of Cr(VI) in COPR originating from a Glasgow

site Th e authors use leaching tests XRPD and equilibrium based

geochemical modeling to describe the leaching behavior of COPR

Th e concentration of leached Ca Al Si Mg and Cr(VI) are used

to calibrate for the solubility constants of some solid phases that

were used in the model calculations Th e mineral phases identi-

fi ed in the residues of the leaching tests and other phases that may

be present in COPR are used in their model Even though the

dissolution of COPR is kinetically controlled (dissolution time is

beyond experimental time frame) the authors used the ionic activ-

ity product instead of equilibrium solubility to account for the

discrepancy in the concentration of some elements between the

measured ones and the ones predicted by the model had they used

equilibrium constants as reported in the literature For example

they modifi ed the equilibrium constant of Cr(VI)-ettringite to

make the model fi t their experimental data down to pH 8 even

though they did not identify Cr(VI)-ettringite in any sample

through XRPD or scanning electron microscopy (SEM)ndashenergy

dispersive x-ray spectroscopy (EDS) Th e discrepancy between the

measured and predicted concentrations could be due to the slow

dissolution rate of some COPR minerals such as brownmillerite

which may encapsulate Cr(VI) minerals as reported by Kremser

et al (2005) Alternatively Cr(VI) could be encapsulated in other

phases that were not identifi ed in their study such as hydrotalcites

which are thermodynamically stable down to pH 8

In this study batch leaching tests complemented with

qualitative and quantitative XRPD analyses SEM-EDS and

geochemical modeling are used to investigate the leaching

behavior of Cr(VI) in COPR originating from a site in Hud-

son County New Jersey Th e experimental and modeling

results are then explained in terms of potential mineral phases

that could be controlling Cr(VI) leaching and the kinetics of

dissolution of thermodynamically unstable minerals present

on site Th e fi ndings of this study are relevant to assess the

potential of hexavalent chromium contamination emanating

from COPR sites in Hudson County New Jersey Th ey are

also relevant for the development of remediation strategies

and chemical treatment of Cr(VI) in COPR

Materials and Methods

MaterialsTh e COPR samples used in this investigation were ob-

tained from a study area in Jersey City NJ Th e samples were

collected from two stratigraphic layers (B1 and B2) during a

major bulk-sampling event from 14 borings across the site

Layer B1 represents the upper most unsaturated zone and layer

B2 lies directly below layer B1 Th e water table level fl uctuates

within layer B2 throughout the entire site A composite sample

B1B2 was prepared by mixing equal amounts of all samples

from layers B1 and B2 across the borings Th e samples were

thoroughly homogenized before any COPR material was used

All chemical reagents used were of ACS or higher-grade quality

and were obtained from Fisher Scientifi c (Suwanee GA) All

stock solutions were prepared using deionized (DI) water

Sample CharacterizationTh e elemental composition of the COPR samples was deter-

mined by digesting the COPR material using USEPA Method

3051A (USEPA 1996a) followed by USEPA Method 6010B

(USEPA 1996b) Hexavalent chromium concentration was

obtained using USEPA Method 3060 (USEPA 1996c) and

USEPA Method 7196A (USEPA 1992) Total carbon was

measured using ASTM D5291 (ASTM 2001) and it was as-




3 and precipi-




























































8]

2CO

3nH

2O was prepared



2O

and 025 mol Al(NO3)

39H


















talcite 4MgOAl2O

310H





































































































Afwillite Ca3Si

2O

4(OH)

6 6492a (1)Dagger


4Al

2Fe

2O

1013913a (2)

C2AH

8Ca

2Al

2(OH)

103H

2O 5819b (3)

CAH10

CaAl2(OH)

86H

2O 3799b (4)

C4AH

13Ca

4Al

2(OH)

146H

2O 10725b (5)

C4AH

19Ca

4Al

2(OH)

1412H

2O 10368b (6)


8Ca

2Al

2(OH)

6SiO

8H

8H

2O 4916c (7)

Hydrogarnet C3AH

6Ca

3Al

2(H

4O

4)

3 7827d (8)

Hydrotalcite M4AH

104MgOAl

2O

310H

2O 7378c (9)

Monosulfate Ca4Al

2(OH)

12SO

46H

2O 7197e (10)

Wairakite CaAl2SiO

10(OH)

41807b (11)

Surface reactions


+ 729f (12)



42minus 39f (14)


4minus +H

2O 1085f (15)


2O 1278f (16)


2O 2037f (17)

= SOH + H4SiO

4 rarr = SOSiO

2OH2minus + 2H+ + H

2O minus1169f (18)

= SOH + H4SiO

4 rarr = SOSi(OH)

2minus + H+ + H

2O minus322f (19)

= SOH + H4SiO

4 rarr = SOSi(OH)

3 + H

2O 428f (20)








Element Al Ca CO3


2minus














(Ca4Al

2(OH)

12CrO

4nH































































































pH 1203 1104 1058 935 830 776 642 524



Brownmillerite 2682 3384 2847 2966 3716 3794 3399 2057

Brucite 284 364 214 187 141 074 081

Calcite 853 604 638 684 718 684 501 117


Katoite 261 377 209 069 047


Quartz 319 364 273 315 395 427 442 441


Sjoegrenite 145 185 123 472 147 114 086 060

Albite 517 700 450 472 559 604 878 711

Gibbsite 167



Percent total 10006 9986 10000 10000 9994 10000 10000 10000















approximately 95
















2(CO

3)

(OH)14


2(OH)

12(CO

3)4H

2O








2]x+ (02 lt x lt 033) and


NO3minus CO






























2Al

2O

10) peri-


































Ca4Fe

2Al

2O

10 + 7H

2O rarr Ca(OH)

2 + Ca

3Al

2(OH)

12 + Fe

2O

3 [1]






































Morristown NJ





6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H


3- 2 ge x ge 0)
























3

























3 and precipi-




























































8]

2CO

3nH

2O was prepared



2O

and 025 mol Al(NO3)

39H


















talcite 4MgOAl2O

310H





































































































Afwillite Ca3Si

2O

4(OH)

6 6492a (1)Dagger


4Al

2Fe

2O

1013913a (2)

C2AH

8Ca

2Al

2(OH)

103H

2O 5819b (3)

CAH10

CaAl2(OH)

86H

2O 3799b (4)

C4AH

13Ca

4Al

2(OH)

146H

2O 10725b (5)

C4AH

19Ca

4Al

2(OH)

1412H

2O 10368b (6)


8Ca

2Al

2(OH)

6SiO

8H

8H

2O 4916c (7)

Hydrogarnet C3AH

6Ca

3Al

2(H

4O

4)

3 7827d (8)

Hydrotalcite M4AH

104MgOAl

2O

310H

2O 7378c (9)

Monosulfate Ca4Al

2(OH)

12SO

46H

2O 7197e (10)

Wairakite CaAl2SiO

10(OH)

41807b (11)

Surface reactions


+ 729f (12)



42minus 39f (14)


4minus +H

2O 1085f (15)


2O 1278f (16)


2O 2037f (17)

= SOH + H4SiO

4 rarr = SOSiO

2OH2minus + 2H+ + H

2O minus1169f (18)

= SOH + H4SiO

4 rarr = SOSi(OH)

2minus + H+ + H

2O minus322f (19)

= SOH + H4SiO

4 rarr = SOSi(OH)

3 + H

2O 428f (20)








Element Al Ca CO3


2minus














(Ca4Al

2(OH)

12CrO

4nH































































































pH 1203 1104 1058 935 830 776 642 524



Brownmillerite 2682 3384 2847 2966 3716 3794 3399 2057

Brucite 284 364 214 187 141 074 081

Calcite 853 604 638 684 718 684 501 117


Katoite 261 377 209 069 047


Quartz 319 364 273 315 395 427 442 441


Sjoegrenite 145 185 123 472 147 114 086 060

Albite 517 700 450 472 559 604 878 711

Gibbsite 167



Percent total 10006 9986 10000 10000 9994 10000 10000 10000















approximately 95
















2(CO

3)

(OH)14


2(OH)

12(CO

3)4H

2O








2]x+ (02 lt x lt 033) and


NO3minus CO






























2Al

2O

10) peri-


































Ca4Fe

2Al

2O

10 + 7H

2O rarr Ca(OH)

2 + Ca

3Al

2(OH)

12 + Fe

2O

3 [1]






































Morristown NJ





6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H


3- 2 ge x ge 0)
























3




































































































Afwillite Ca3Si

2O

4(OH)

6 6492a (1)Dagger


4Al

2Fe

2O

1013913a (2)

C2AH

8Ca

2Al

2(OH)

103H

2O 5819b (3)

CAH10

CaAl2(OH)

86H

2O 3799b (4)

C4AH

13Ca

4Al

2(OH)

146H

2O 10725b (5)

C4AH

19Ca

4Al

2(OH)

1412H

2O 10368b (6)


8Ca

2Al

2(OH)

6SiO

8H

8H

2O 4916c (7)

Hydrogarnet C3AH

6Ca

3Al

2(H

4O

4)

3 7827d (8)

Hydrotalcite M4AH

104MgOAl

2O

310H

2O 7378c (9)

Monosulfate Ca4Al

2(OH)

12SO

46H

2O 7197e (10)

Wairakite CaAl2SiO

10(OH)

41807b (11)

Surface reactions


+ 729f (12)



42minus 39f (14)


4minus +H

2O 1085f (15)


2O 1278f (16)


2O 2037f (17)

= SOH + H4SiO

4 rarr = SOSiO

2OH2minus + 2H+ + H

2O minus1169f (18)

= SOH + H4SiO

4 rarr = SOSi(OH)

2minus + H+ + H

2O minus322f (19)

= SOH + H4SiO

4 rarr = SOSi(OH)

3 + H

2O 428f (20)








Element Al Ca CO3


2minus














(Ca4Al

2(OH)

12CrO

4nH































































































pH 1203 1104 1058 935 830 776 642 524



Brownmillerite 2682 3384 2847 2966 3716 3794 3399 2057

Brucite 284 364 214 187 141 074 081

Calcite 853 604 638 684 718 684 501 117


Katoite 261 377 209 069 047


Quartz 319 364 273 315 395 427 442 441


Sjoegrenite 145 185 123 472 147 114 086 060

Albite 517 700 450 472 559 604 878 711

Gibbsite 167



Percent total 10006 9986 10000 10000 9994 10000 10000 10000















approximately 95
















2(CO

3)

(OH)14


2(OH)

12(CO

3)4H

2O








2]x+ (02 lt x lt 033) and


NO3minus CO






























2Al

2O

10) peri-


































Ca4Fe

2Al

2O

10 + 7H

2O rarr Ca(OH)

2 + Ca

3Al

2(OH)

12 + Fe

2O

3 [1]






































Morristown NJ





6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H


3- 2 ge x ge 0)
























3


































(Ca4Al

2(OH)

12CrO

4nH































































































pH 1203 1104 1058 935 830 776 642 524



Brownmillerite 2682 3384 2847 2966 3716 3794 3399 2057

Brucite 284 364 214 187 141 074 081

Calcite 853 604 638 684 718 684 501 117


Katoite 261 377 209 069 047


Quartz 319 364 273 315 395 427 442 441


Sjoegrenite 145 185 123 472 147 114 086 060

Albite 517 700 450 472 559 604 878 711

Gibbsite 167



Percent total 10006 9986 10000 10000 9994 10000 10000 10000















approximately 95
















2(CO

3)

(OH)14


2(OH)

12(CO

3)4H

2O








2]x+ (02 lt x lt 033) and


NO3minus CO






























2Al

2O

10) peri-


































Ca4Fe

2Al

2O

10 + 7H

2O rarr Ca(OH)

2 + Ca

3Al

2(OH)

12 + Fe

2O

3 [1]






































Morristown NJ





6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H


3- 2 ge x ge 0)
























3








































































































pH 1203 1104 1058 935 830 776 642 524



Brownmillerite 2682 3384 2847 2966 3716 3794 3399 2057

Brucite 284 364 214 187 141 074 081

Calcite 853 604 638 684 718 684 501 117


Katoite 261 377 209 069 047


Quartz 319 364 273 315 395 427 442 441


Sjoegrenite 145 185 123 472 147 114 086 060

Albite 517 700 450 472 559 604 878 711

Gibbsite 167



Percent total 10006 9986 10000 10000 9994 10000 10000 10000















approximately 95
















2(CO

3)

(OH)14


2(OH)

12(CO

3)4H

2O








2]x+ (02 lt x lt 033) and


NO3minus CO






























2Al

2O

10) peri-


































Ca4Fe

2Al

2O

10 + 7H

2O rarr Ca(OH)

2 + Ca

3Al

2(OH)

12 + Fe

2O

3 [1]






































Morristown NJ





6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H


3- 2 ge x ge 0)
























3



































approximately 95
















2(CO

3)

(OH)14


2(OH)

12(CO

3)4H

2O








2]x+ (02 lt x lt 033) and


NO3minus CO






























2Al

2O

10) peri-


































Ca4Fe

2Al

2O

10 + 7H

2O rarr Ca(OH)

2 + Ca

3Al

2(OH)

12 + Fe

2O

3 [1]






































Morristown NJ





6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H


3- 2 ge x ge 0)
























3














































Ca4Fe

2Al

2O

10 + 7H

2O rarr Ca(OH)

2 + Ca

3Al

2(OH)

12 + Fe

2O

3 [1]






































Morristown NJ





6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H


3- 2 ge x ge 0)
























3
























Morristown NJ





6Al

2(OH)

16(A)

x(Arsquo)

2-x 4H


3- 2 ge x ge 0)
























3






















2125

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