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CHAPTER-7

Adsorption characteristics of phosphate-treated Ashok bark

(Saraca indica): Removal of Ni(II) from Electroplating wastewater

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7.1 Introduction

Because of heavy metal toxicity and non-biodegradable nature, the

introduction of heavy metals in water is becoming a serious environmental and public

health concern. Heavy metals in human bodies tend to bioaccumulate, which may

result in damaged or reduced mental, central nervous function and blood composition,

lungs, kidneys and liver. The heavy metal such as nickel is a naturally occurring

element. Small amount of this element is common in our environment and actually

necessary for our health. But large amount of it may cause acute or chronic toxicity

[1-3].The regulatory level of Ni(II) in drinking water by WHO is 0.07 mg L-1[4].

A number of technologies have been developed to remove toxic heavy metals

from wastewaters. The most important technologies for heavy metals ions removal

from wastewaters include precipitation, ion exchange, adsorption, coagulation,

evaporation and reverse osmosis. Adsorption on solid matrices has been shown to be

an economically feasible alternative method [5-8]. Adsorption play an important role

in the elimination of metals ions from aqueous solution in water pollution control [9,

10]. The main advantages of this technique are the low operating cost, improved

selectivity for metals of interest , removal of heavy metals from effluent irrespective

of toxicity, short operation time and no product of secondary compounds which might

be toxic[11]. Recently various biomasses have been used for removal of Ni(II) ions

from aqueous solution [12-16].

Ashok tree (Saraca indica) is a plant belongs to Caesalpiniaceae subfamily of

Legume family. It is important in the cultural traditions of the Indian subcontinent and

adjacent areas. The Ashok tree is prized for its beautiful foliage and fragrant flowers.

It is very handsome erect evergreen tree, with deep green leaves growing in dense

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clusters. Its flowering seasons is around February to April. The Ashok flowers comes

in heavy, lush bunches. They are bright yellow in color, turning red before wilting.

The bark of the Ashok tree is used to make a drug, which is reported to posses a

stimulating effect on the endometrium and ovarian tissue. The present study deals

with the adsorption efficiency of phosphate-treated Ashok bark and its possible role in

the removal and recovery of Ni(II) from electroplating wastewater.

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7.2. Experimental procedure

7.2.1. Preparation and activation of adsorbent

Bark of Ashok tree (Saraca indica) biomass was collected from A.M.U

campus. The biomass was washed several times with double distilled water (DDW) to

remove dirt and dust. The washed biomass was dried in an oven at 60 0C. The dried

biomass was then crushed and sieved to 100-300 µm particle size. It was then treated

with an aqueous solution of 0.1N Na3PO4.12H2O for 24 hrs and then washed several

times with double distilled water (DDW) to remove excess phosphate ions. The

washed biomass was dried in an oven at 60 0C and stored in an airtight container in

order to avoid moisture and used as such for the adsorption studies.

7.2.2. Preparation of adsorbate solution

The stock solution of Ni(II) was prepared (1000 mg L-1) by dissolving the

desired amount of nickel nitrate (AR grade) salts. Solutions of other salts were

prepared by dissolving their nitrates.

7.2.3. Characterization of adsorbent

Scanning electron microscopy (SEM) analysis technique was employed to

observe the surface morphology of the adsorbent with 3500x magnification. The type

of binding groups present on the adsorbent were identified by Fourier transform

infrared spectroscopy (FTIR) analysis using Perkin Elmer 1600 infrared spectrometer

with pellets of powdered KBr and biomass.

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7.2.4. Determination of active sites

Acidic sites on Ashok bark were determined by acid-base titration method

[17]. 0.5g adsorbent was treated separately with 50 mL each 0.1N NaOH, 0.1N

Na2CO3 and 0.1N NaHCO3 in 250 mL conical flasks. The flasks were agitated at

constant temperature (200C) and left there for 5 days. Afterwards a sample of 10 mL

was titrated with 0.1N HCl solution using pH meter.

7.2.5. Determination of Point of zero charge (pHpzc)

The zero surface charge characteristics of the Ashok bark were determined by

using the solid addition method [18] as described in earlier chapters.

7.2.6. Adsorption studies

Batch process was employed for adsorption studies. 0.5 g adsorbent was

placed in a conical flask having 50 mL Ni(II) solution and the mixture was shaken in

a shaker incubator at100 rpm. The mixture was then filtered at predetermined time

interval and the final concentration of metal ions was determined in the filtrate by

Atomic Absorption Spectrophotometer (GBC 902). Amount of Ni(II) adsorbed was

then calculated by subtracting final concentration from initial concentration.

Adsorption studies were carried out by varying the adsorbate concentration (10–100

mg L−1), the agitation time (1–120 min), adsorbent amount (0.1–1.0 g) and

temperature (30, 40 and 50 ◦C). A series of experiments with pH of the initial Ni(II)

solution varying between 2 and 9 (by adding 0.1N HCl and 0.1N NaOH solutions)

were also carried out using 0.5 g adsorbent at room temperature.

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7.2.7. Breakthrough studies

Breakthrough studies were carried out as follows. 0.5g of adsorbent was taken

in glass column (0.6 cm internal diameter) with glass wool support. 1000 mL Ni(II)

solution was passed through the column at 1mL min-1 flow rate. The initial Ni(II) ions

concentration (C0) was 50 mgL-1. The effluent was collected in 50 mL fractions. The

concentration of metal ions (C) in each fraction was determined by AAS. The

breakthrough curve was obtained by plotting C/C0 versus volume of the effluent.

7.2.8. Desorption of Ni(II) by batch process

Desorption studies were carried out as follows. 50 mg L-1 Ni(II) solution was

treated with 0.5 g adsorbent in a conical flask for 24 hrs. The adsorbent was washed

several times with DDW (pH 6.5) in order to remove traces of metal ions remained

unadsorbed. The adsorbent was then treated with 50 mL 0.1N HCl for 24 hrs. The

desorbed Ni(II) ions were then determined in the solution by AAS.

7.2.9 Analysis of electroplating wastewater

Electroplating wastewater was collected from one of the lock factory in

Aligarh city. The pH of the waste was measured immediately. It was filtered and

stored in a polythene bottle. Analysis of heavy metals ions was carried out by AAS.

Total dissolved salts (TDS) were determined by evaporating 100 mL filtered

wastewater in a china dish.

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7.2.10 Treatment of electroplating wastewater by batch process

50 mL wastewater was taken in a conical flask and its pH was adjusted to 5.6

and then 0.5 g adsorbent was added. The mixture was shaken and then kept for 24 hrs.

It was filtered and filtrate was analyzed for heavy metals by AAS.

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7.3. Results and discussion

7.3.1. Characterization of adsorbent

Scanning Electron Microscope (SEM) Analysis:

A scanning electron microscopy (SEM) was used to examine the surface of the

adsorbent. The surface of untreated adsorbent appears to be irregular and porous (Fig.

7.1a).The pores are prominent on the surface of the adsorbent before adsorption. After

adsorption of Ni(II) onto untreated bark the pores are filled showing adherence of

adsorbate ions on the surface (Fig. 7.1b). Similarly SEM images of phosphate-treated

Ashok bark before and after adsorption of Ni(II) are shown in Figs. 7.2a and 7.2b.

However the effect of Phosphate treatment is not visible in SEM image.

FTIR Analysis:

FTIR spectrum of untreated Ashok bark is shown in Fig. 7.3a. The prominent

peaks due to the presence of various groups are shown in Table 7.1 [19]. FTIR

spectrum of phosphate-treated Ashok bark is shown in Fig. 7.4a. All the dominant

peaks remained unaltered except that two new peaks at 1157 and 3786 cm-1 appeared.

The appearance of peaks at 1157 may be due to the presence of phosphate [20], while

peak at 3786cm-1 may be due to the –OH group of PO4-3 [21]. Fig.7.4b represents the

FTIR of phosphate treated Ashok bark after Ni(II) adsorption. The peak at 3786 cm-1

disappeared indicating that Ni(II) interacts with –OH group of phosphate [21].

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7.3.2. Effect of phosphate treatment on the adsorption of Ni(II)

The % adsorption of Ni(II) increased from 80 to 95% when adsorbent was

treated with Na3PO4. Therefore further studies were carried out on phosphate-treated

Ashok bark.

7.3.3. Determination of active sites

The total numbers of acidic sites matching carboxylic, phenolic, and lactonic

sites were neutralized using alkaline solutions (0.1N NaOH, 0.1N NaHCO3, and 0.1N

Na2CO3). The carboxylic and lactonic sites were titrated with 0.1N Na2CO3 solution,

the carboxylic sites were determined with 0.1N NaHCO3 solution and the phenolic

sites were estimated by the difference [17]. The results shown in Table 7.2 indicated

that total numbers of acidic sites are 3.09 meq g-1. These sites suggest the high

adsorption capacity of the adsorbent.

7.3.4. Effect of Contact time and initial concentration

Adsorption of Ni(II) onto phosphate-treated Ashok bark at various initial

concentrations was studied at different time intervals (1 – 180 min.). The equilibrium

uptake capacity (qe) for Ni(II) was found to be 0.99, 1.97, 4.42, 6.85 and 7.90 mg g-1

at 10, 20, 50, 70 and 100 mg L-1 initial Ni(II) concentrations respectively (Fig. 7.5).

When the initial concentration of Ni(II) is increased, the rate of adsorption decreased,

but the amount of Ni(II) adsorbed increased. In the first stage, the rate of adsorption is

rapid initially and then increases slowly with time until attains an equilibrium. The

equilibrium is achieved easily when the initial concentration is low, because at the

first stage the ratio of available surface of adsorbent is large for the adsorption of

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Ni(II) and as the contact time increases the available sites gradually decreases until it

attains equilibrium [22].

7.3.5. Effect of pH and electrolyte concentration

The effect of solution pH on the % adsorption of Ni(II) was studied by

varying the pH in the range 2-9. It can be observed from Fig. 7.6 that adsorption of

Ni(II) was strongly affected by pH. The % adsorption was 75.4 % at pH 2 and then

increased by increasing the pH; reached a maximum value (90%) at pH 4. However,

beyond pH 4 the adsorption attained the same maximum value (90%). The variation

in the adsorption of Ni(II) with change in the pH can be explained by considering the

surface charge of the adsorbent and speciation of Ni(II).At pH 2 the H+ ions are

adsorbed along with Ni(II) ions resulting an increase in final or equilibrium pH (pHf =

6.2).When initial pH (pHi) is adjusted to 3, the final pH (pHf) increases rapidly to 7.6

and at same time appreciable amount of Ni(II) is adsorbed (88.4%). However,

increasing the initial pH above 3 does not affect final pH value (pHf = 7.6) and

adsorption of Ni(II) remain maximum (90 %). Fig. 7.7 represents the point of zero

charge in presence of varying concentration of electrolyte. The point of zero charge

(pHpzc) was found to be 8.7. Hence surface of the adsorbent is positive at pH < 8.7;

neutral at pH = 8.7 and negative at pH>8.7. Further, Ni(II) is present as Ni2+ ions at

pH ≤ 6 and Ni(OH)2 at pH ≥ 7.2 [23]. Therefore adsorption of Ni2+ in the pH range 2-

4 (lower than pHpzc) cannot be explained on the basis of electrostatic attraction

between adsorbent and Ni2+ ions. The initial solution pH is not the only factor to

explain the adsorption behavior. The increase or decrease of final pH after contact

may also affect metal adsorption and may bring about changes in the chemical

speciation of the metal [24]. The final pH recorded at which maximum adsorption of

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Ni(II) occurred was 7.6. Therefore it can be concluded that adsorption of Ni(II) above

pH 2 might have occurred in the form of micro precipitation on the surface of the

adsorbent.

7.3.6. Adsorption isotherms

In order to optimize the design of adsorption system for the removal of Ni(II)

from aqueous solution, experimental data were fitted in the Langmuir, Temkin,

Freundlich and Dubinin-Radushkeuich models at various temperatures. The linear

form of Langmuir isotherm is represented as

1/qe = (1/qm) × (1/b) × (1/Ce) + 1/qm ----------------- (1)

Where qe is the amount of metal adsorbed per unit weight of adsorbent, qm is the

maximum adsorption capacity (mg g -1) determined by the number of reactive surface

sites in an ideal monolayer system, Ce is the concentration of metal ions at

equilibrium (mg L-1) and b is a constant, related to bonding energy associated with pH

dependent equilibrium constant. Plots of 1/qe versus 1/Ce at 300, 400 and 500C gave

straight lines (Fig. 7.8) and values of b and qm were calculated from the slope and

intercept of the plots (Table 7.3). The essential characteristic of Langmuir isotherm

can be expressed in terms of dimensionless constant separation factor or equilibrium

parameter RL, given by the following relation

RL = 1/(1+ b×C0) ------------------- (2)

Where b is the Langmuir constant and C0 is the initial concentration of Ni(II) (mg L-

1). RL value predicts the shape of the isotherm. If RL >1 unfavorable; RL = 1 linear; 0<

RL <1 favorable and RL = 0 for irreversible adsorption [25]. The RL values at 300, 400

and 500C are also shown in Table 7.3.The values of RL in the range 0-1 at all these

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temperatures show favorable adsorption of Ni(II). Table 7.4 presents the comparison

of adsorption capacity (qm) of phosphate-treated Ashok bark with various adsorbents

reported earlier [14, 26-32].The Ni(II) adsorption capacity of phosphate-treated

Ashok bark is higher than these adsorbents.

The Linear form of Freundlich isotherm can be represented as

log qe = log Kf + (1/n) log Ce ---------------- (3)

Where Kf is Freundlich constant and n is another constant that informs about the

heterogeneity degree of the surface sites. Plots of log qe verses log Ce gave straight

lines at 300, 400 and 500C (Fig. 7.9) and values of n and Kf were calculated from the

slope and intercept of these plots (Table 7.3).

Temkin isotherm assumes that the decrease in the heat of adsorption is linear

rather logarithmic, as implied in the Freundlich isotherm. The linear form of Temkin

equation can be represented as [33].

qe = Bt × ln At + Bt × ln Ce ----------------- (4)

Where Bt = (RT/bt), and R is universal gas constant, T is absolute temperature and bt

is another constant. At (g L-1) and Bt are Temkin constants related to adsorption

potential and heat of adsorption. The values of At and Bt were calculated from the

slope and intercept of the plot of qe versus ln Ce (Table 7.3).

Dubinin-Redushkeuich (D-R) isotherm does not assume a homogenous

surface or a constant adsorption potential [34].The linear form of this equation is

represented as

ln qe = ln qm – βε2 ---------------------- (5)

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Where ε is the Polyanyi potential, qm is the monolayer capacity (mol g-1), Ce is the

equilibrium concentration (mol L-1), and β is a constant related to adsorption energy

[mol)2(kJ)-2]. The parameters qm and β can be obtained from the intercept and slope of

the plot of ln qe versus ε2. The Polyanyi potential (ε) and mean free energy of

adsorption (E, k J mol-1) can be calculated from the equations

ε = RT ln (1+1/Ce) ------------ (6)

E = 1/ √-2 β--------------------- (7)

The fitting procedure was performed using R software, version 2.10.1 (2009-12-14).

To evaluate the fitness of the data, correlation coefficients (R2), error analysis

(residual standard error (RSE), sum of square error (SSE)) and P-values were

calculated. The values of constants obtained from different models were fitted and

corresponding qe values was calculated (designated as qe(cal)) from each model. The

values of qe found experimentally (designated as qe(exp)) were compared with qe(cal)

using chi-square test (χ2). Chi-square test values were calculated from the following

relation

χ² = ∑ [qe(exp) – qe(cal)] ²/ qe(cal) --------- (8)

The SSE values were calculated using the following relation

SSE= Nqq calee /)( 2exp ----------------(9)

Where, N is the number of observations. Lower the value of χ2 and SSE better is the

fit. The parameters calculated from different models at 300, 400 and 500C are reported

in Table 7.3. All the models were obeyed by the system at these temperatures as

indicated by their regression coefficient values (R2 vary from 0.98 to 0.99). The p-

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values in all cases are less than 0.05. However, R2 is not the only parameter that

indicates the fitness of the model because in linear model, experimental values are

regressed. The chi-square test (χ2) is more significant because experimental qe values

are compared with qe calculated from the model on the same abscissa and ordinate

[35]. The Langmuir model for instance shows high correlation coefficient values (R2

=0.995) at 300C and χ2 value is least (χ2 = 0.039) when compared at 400C and 500C

therefore it can be concluded that Langmuir model is best obeyed at 300C but

Freundlich model shows least χ2 value (0.003) at 400C. The RSE and SSE are also

least (Table 7.3) at this temperature hence Freundlich model is also best fitted at 400C

though R2 at 400C is 0.992. Similarly D-R models are also obeyed at all these

temperatures but Temkin model shows higher χ2 values hence is not a better fit.

7.3.7. Thermodynamic studies

The temperature range used in this study was 30-500C.The equilibrium

constants (Kc) at 300, 400 and 500C were calculated from the following relation [36].

Kc = CAC/Ce --------------------------------- (10)

Where CAC and Ce are the equilibrium concentrations (mg L-1) of Ni(II) on the

adsorbent and in solution, respectively. Free energy change (∆G0) can be calculated as

∆G0 = - RT ln Kc --------------------------------- (11)

The value of enthalpy change (∆H0) and entropy change (∆S0) were calculated

from the following relation.

ln Kc = (∆S0/R) – (∆H0/R) × (1/T ) -------------------- (12)

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∆H0 and ∆S0 were calculated from the slope and intercept of linear plot of ln Kc

versus 1/T (Fig. 7.10). The values of Kc, ∆H0, ∆S0and ∆G0 are reported in Table 7.5.

The positive value of ∆H0 indicates endothermic process. The negative values of ∆G0

indicate that the process is spontaneous and spontaneity increases with increase in

temperature. The positive value of ∆S0 suggests increased randomness at the solid-

liquid interface during adsorption. The value of mean free energy (energy required to

transfer one mole of adsorbate from infinity to the adsorbent surface) gives an idea

about the nature of adsorption. If the value of mean energy is greater than 8.0 kJ mol-

1, the adsorption is chemical in nature [37], i.e. adsorption occurs due to the chemical

bonding between Ni(II) ion and adsorbent. The value of mean free energy was found

to be 9.25 k J mol-1 at 300C and increases only slightly with increase in temperature

(Table 7.3), indicating that adsorption is chemical in nature.

7.3.8. Adsorption Kinetics

In order to analyze the adsorption kinetics, the pseudo-first-order and pseudo-

second-order kinetic models were tested using experimental data and rate constants

were calculated at different concentrations. Pseudo-first-order kinetics equation as

expressed by Lagergren [38] can be written as

log (qe – qt) = log qe – (K1 / 2.303) × t --------------- (13)

Where qe and qt are the amounts of metal adsorbed (mg g-1) at equilibrium and at time

t respectively and K1 is the pseudo-first-order adsorption rate constant (min-1). A plot

of log (qe – qt) versus t gives straight line and rate constant K1 can be calculated from

the slope (Fig. 7.11).

Pseudo-second-order kinetics equation may be expressed as [38]

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t / qt = 1 / (K2 × qe2) + (1/ qe) × t ------------------ (14)

Where, K2 is the pseudo-second-order adsorption rate constant (g mg-1 min-1). A plot

of t/qt versus t gives straight line (at different concentrations) (Fig. 7.12). The values

of K2 can be calculated from the intercept of the plot. The data show that the

correlation coefficient values (R2) for pseudo-first-order are very low in comparison

to pseudo-second-order model (Table 7.6). The values of adsorption capacity

calculated from the model (qe(cal)) are very near to experimental values (qe(exp)) for

second-order-kinetics but for first-order-kinetics equation these values are deviated.

Higher correlation coefficient (R2) values and similar qe(cal) and qe(exp) values indicate

the better applicability of pseudo-second-order kinetics model.

For batch process the temporal approach to equilibrium can be illustrated by a

plot of the fractional uptake F against time t (figure not shown), where F = qt/qe. The

time needed to reach equilibrium increases with increasing the initial Ni(II)

concentration. It also shows that the fractional uptake F decreases with increasing the

initial Ni(II) concentration, although this tendency is not so obvious within the high

concentration range at short adsorption times. Boyd [39] and Webber [40] models are

widely used for predicting the nature of adsorption. The detail of the model has been

explained in Chapter 2.This model is expressed as.

∞ F = 1- 6/π²Σ 1/n2 exp (-n2Bt) ________________ (15) n = 1 Where, Bt is diffusivity constant. From Eq. (15), it is not possible to calculate the

values of Bt for each fraction adsorbed. By applying the Fourier transform and then

integration, Reichenberg [41] obtained the following approximation

For F > 0.85, Bt = −0.4977 − ln (1 − F) ------------------ (16)

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and for F < 0.85, Bt = (√π - √ (π – (π2 F/3))2 --------------- (17)

The values of F obtained were F>0.85. This shows that Eq. (16) is applicable

in this case. If the plot of Bt versus time is linear and passes through the origin then

pore-diffusion controls the rate of mass transfer. If the plot is nonlinear or linear but

does not pass through the origin, then it is concluded that film-diffusion or chemical

reaction controls the adsorption rate. The plots of Bt versus time for the adsorption of

Ni(II) on Phosphate-Ashok bark at different concentrations show that the lines do not

pass through the origin. These observations suggest that film diffusion or chemical

reaction controls the rate of adsorption during this period.

7.3.9. Intra-particle diffusion

The Weber and Morris [40] intra-particle diffusion model can be expressed as

qt = Kid × t1/2 + I ----------------------- (18)

Where qt (mg g-1) is the amount of Ni(II) adsorbed at time t, I (mg g-1) is the intercept

and Kid (mg g-1 min-1/2) is the intra-particle diffusion rate constant. The Kid values

obtained from the slope of the curves of different initial Ni(II) concentrations

(Fig.7.13) are shown in Table 7.7. The R2 values (between 0.8502 and 0.9577)

suggest that adsorption of Ni(II) follows intra-particle diffusion model to some extent.

However, plots did not pass through the origin (the intercept values are between

0.9434 and 5.6894 mg g-1) indicating that intra-particle diffusion is not the only rate–

limiting step. The increase in intercept values with increase in concentration is

indicative of increased boundary layer effect [42].

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7.3.10. Breakthrough studies

Fig.7.14 shows breakthrough curve of Ni(II) from the column at an initial

concentration of 50 mg L-1.The breakthrough occurred at 50 mL effluent volume. The

breakthrough and exhaustive capacities were found to be 5.0 and 65.0 mg g-1,

respectively [43].

7.3.11. Desorption of Ni(II) by batch process

In order to make the process more economical, attempt were made to desorb

Ni(II). The desorption of Ni(II) by Batch process is shown in Table 7.8. It has been

found that 2.22 mg Ni(II) was adsorbed and 2.20 mg was recovered with 0.1N HCl

solution.

7.3.12. Removal and recovery of Ni(II) from electroplating wastewater

The analysis of electroplating wastewater is reported in Table 7.9. The results

after treatment of wastewater by batch processes are shown in Table 7.10. It can be

inferred that after treatment the concentration of Ni(II) present in the electroplating

wastewater is reduced to much extent. The adsorbed Ni(II) could be recovered to the

extent of 69.2 % with 0.1N HCl solution. Further, it is also possible to desorb

completely Pb(II), Cu(II) and Zn(II) ions with 0.1N HCl as desorbing agent.

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7.4. Conclusions

Ashok bark showed high affinity towards Ni(II) ions. The adsorption capacity

increased remarkably when bark was treated with Na3PO4 solution.

Thermodynamic parameters indicated endothermic and spontaneous nature of

the adsorbent .The mean free energy value indicated that the adsorption was

chemical in nature. Langmuir and Freundlich isotherms were better obeyed at

300C and 400C respectively as compared to Temkin and D-R isotherms as

indicated by chi-square test. Adsorption of Ni(II) was appreciable at pH 2 and

increased pH favored adsorption due to deprotonation of functional groups.

Pseudo-second order kinetics model is better obeyed than pseudo-first order

model in all the experimental concentrations. The phosphate-treated Ashok bark

could be utilized for the removal and recovery of Ni(II) from electroplating

wastewater.

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Table7.1: FTIR peak values for different functional groups present in native and phosphate- treated Ashok bark before and after adsorption.

Functional Ashok bark Phosphate treated Phosphate treated groups Ashok bark Ashok bark Ni(II) loaded -OH( PO4

3- ) - 3786.1 - - OH (water) 3427.3 3433.1 3435.7 aliphatic 2927.5 2926.1 2926.9 C – H NH2 2361.9 2361.5 2360.8 C = C 1619.6 1620.8 1621.8 COO- 1450.5 1445.0 (s) 1423.6 phenolic OH 1381.9 1381.0 (s) 1381.0 1320.7 1322.6 1321.6 H2PO4 - - 1157.3 1157.0 (weak)

C-O 1022.6 1019.2 1023.2

Table 7.2: Concentration of active sites on phosphate-treated Ashok bark

Actives sites Concentration(meq g-1) Carboxylic 0.12 Phenolic 0.57 Carboxylic + Lactonic 2.40 Total sites 3.09

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Table 7.3: Adsorption isotherm parameters for the adsorption of Ni(II) on phosphate-treated Ashok bark.

Isotherms Parameters 300C 400C 500C Langmuir qm (mg g-1) 22.900 12.900 11.410 b (L mg-1) 0.059 0.186 0.302 RL 0.253 0.097 0.062 R2 0.995 0.991 0.990 χ2 0.039 0.082 0.283 RSE 0.008 0.009 0.014 SSE 0.121 0.204 0.342 P-value <0.05 <0.05 <0.05 Freundlich K 1.742 2.330 2.680 n 1.520 1.730 1.810 R2 0.980 0.992 0.994 χ2 0.016 0.003 0.052 RSE 0.038 0.006 0.223 SSE 0.077 0.005 0.018 P-value <0.05 <0.05 <0.05

Temkin At 0.698 1.386 2.280 Bt 4.440 3.310 2.930 R2 0.994 0.984 0.989 χ2 0.114 0.112 0.261 RSE 0.321 0.421 0.741 SSE 4.08 × 10-6 4.47 × 10-7 5.96 × 10-6 P-value <0.05 <0.05 <0.05 D-R qm(mol g-1) 0.002 0.001 0.001 β ( mol2 (kJ)-2) 5.83 × 10-9 4.28 × 10-9 3.88 × 10-9

E (k J mol-1) 9.250 10.80 11.350 χ2 2.40 × 10-6 4.86 × 10-8 1.21 × 10-7

RSE 0.068 0.042 0.056 SSE 1.05× 10-6 1.77× 10-8 1.213× 10-6

R2 0.998 0.996 0.993 P-value <0.05 <0.05 <0.05

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Table 7.4: Adsorption capacities of various adsorbents reported earlier. Adsorbent Adsorption capacity Reference (mg g-1) Peat 8.52 [14] Modified pine bark 9.50 [23] Wheat Straw 7.90 [24] Bagasse 0.001 [25] Fly ash 0.03 [25] Bituminous coal 6.47 [26] Coir pith 9.50 [27] Sheep manure wastes 7.20 [28] Nanoiron 11.53 [29] Phosphate-treated Ashok bark 22.90 Present study Table 7.5: Thermodynamics parameters at different temperatures for the adsorption of Ni(II) on phosphate-treated Ashok bark.

Temperature Kc ∆G˚ ∆H˚ ∆S˚ R² (˚C) (kJ molˉ¹) (kJ molˉ¹) (kJ molˉ¹ Kˉ¹) 30 19.0 -7.42 40 30.3 -8.87 55.59 12.476 0.9733 50 40.7 -9.95 Table 7.6: Pseudo-first-order and pseudo-second-order rate parameters for the adsorption of Ni(II) at different concentrations on phosphate-treated Ashok bark.

Concentration Pseudo-first-order Pseudo-second-order (Co) (mgLˉ¹) qe(exp) qe(cal) K1 R² qe(exp) K2 h R² (mg gˉ¹) (mg gˉ¹) 10 0.99 0.057 0.746 0.8687 0.99 28.360 27.8 1.0000 40 1.97 0.140 0.108 0.9555 1.98 10.200 40.0 0.9997 50 4.42 2.144 0.052 0.9089 4.43 0.606 11.83 0.9925 80 6.85 1.056 0.036 0.9058 6.54 0.3290 15.46 0.9999 100 7.90 1.960 0.103 0.9573 8.00 0.1470 9.20 0.9995

221

Table 7.7: Intra-particle diffusion parameters for the adsorption of Ni(II) on phosphate- treated Ashok bark.

Table 7.8: Desorption of Ni(II) from aqueous solution by batch process (eluent = 0.1N HCl) on phosphate- treated Ashok bark. Amount % Adsorption Amount Amount %Recovery loaded adsorbed desorbed (mg) (mg) (mg) 2.5 88.8 2.22 2.20 99.1 Table 7.9: Analysis of electroplating wastewater.

Parameters Concentration (mgL-1) pH 5.60 TDS 710.00 Na+ 165.00 K+ 3.10 Ca2+ 30.00 Cu(II) 1.60 Zn(II) 0.42 Ni(II) 1.70 Cr(VI) 27.00

Concentration Kid I R² (mg L-1) (mg g-1 min-1/2) (mg g-1) 10 0.0230 0.9434 0.9542 40 0.0395 1.8007 0.9577 50 0.2019 3.6334 0.9246 80 0.1829 5.5448 0.8904 100 0.4274 5.6894 0.8502

222

Table 7.10: Removal and recovery of Ni(II) and some other heavy metals from electroplating wastewater by batch process. Heavy metals Amount %Adsorption Amount Amount %Recovery loaded adsorbed desorbed (mg) (mg) (mg) Pb(II) 0.035 100.0 0.035 0.035 100.0 Cu(II) 0.080 81.3 0.065 0.065 100.0 Ni(II) 0.085 76.5 0.065 0.045 69.2 Cr(VI) 1.000 15.0 0.150 0.070 46.7 Zn(II) 0.021 85.7 0.018 0.018 100.0

223

Fig.7.1a SEM image of Ashok bark (native).

Fig.7.1b SEM image of Ashok bark after Ni(II) adsorption.

224

Fig.7.2a SEM image of Phosphate-treated Ashok bark before Ni(II) adsorption.

Fig.7.2b SEM image of Phosphate-treated Ashok bark after Ni(II) adsorption.

225

Fig.7.3a FTIR spectrum of native Ashok bark.

515.0 668.5

781.9

1022.6

1320.7

1381.9 1450.5

1513.4

1619.6

2361.9

2927.5

3427.3

6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 21 22 23

%T

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

226

Fig.7.4a FTIR spectrum of phosphate-treated Ashok bark before Ni(II) Adsorption.

Fig.7.4b FTIR spectrum of Phosphate-treated Ashok bark after Ni(II) Adsorption.

433.9

659.5

779.6

1023.2 1321.6

1423.6

1621.8

2360.8

2926.9

3435.7

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

%T

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

408.4

649

778.7

1019.2

1157.3

1322.6

1620.8

2361.5 2926.1

3433.1

3786.1

49 50

51

52

53

54

55

56

57

58

59

60

61

%T

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

227

05

1015

2025

30

20

40

6080

1000

1

2

3

4

5

6

7

8

Fig.7.5 Effect of conc and contact time

conc(mg/L)

qt(m

g/g)

Time(min)

228

Fig.7.6 Effect of pH and electrolyte on the adsorption of Ni(II)

0

10

20

30

40

50

60

70

80

90

100

2 3 4 6 8 9

pHi

% A

dsor

ptio

n

0

2

4

6

8

10

pHf

DDW0.1N KNO3pH change

Fig.7.7 Point of zero charge

-6

-5

-4

-3

-2

-1

0

1

2

0 2 4 6 8 10

pHi

pHi-p

Hf

0.1N KNO3

0.01N KNO3

DDW

229

Fig. 7.8 Langmuir isotherms at different temperatures

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2

1/Ce

1/qe

30C

40C

50C

Fig. 7.9 Freundlich isotherms at different temperatures

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5

log Ce

log

qe

30C

40C

50C

230

Fig. 7.10 Van't Hoff Plot

0

0.5

1

1.5

2

2.5

3

3.5

4

0.003 0.0031 0.0032 0.0033 0.0034

ln Kc

1/T

Fig. 7.11 Pseudo first order kinetics for the adsorption of Ni(II) at different concentrations

-12

-10

-8

-6

-4

-2

0

2

0 10 20 30 40

time(min)

log

(qe-

qt)

10mg/L

20mg/L

30mg/L

80mg/L

100mg/L

231

Fig. 7.12 pseudo-second order kinetics for the adsorption of Ni(II) at different concentrations

0

1

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30 35

time(min)

t/qt

10mg/L

20mg/L

30mg/L

80mg/L

100mg/L

Fig. 7.13 Intraparticle diffusion

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10t1/2

qt

10mg/L

20mg/L

30mg/L

80mg/L

100mg/L

232

Fig. 7.14 Breakthrough capacity curve for the adsorption of Ni(II) on phosphste-treated Ahsok bark

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000

volume (mL)

C/Co