Chemosphere 78 (2010) 1167–1171

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Chemosphere

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Technical Note

Agronomic properties of wastewater sludge biochar and bioavailability of metalsin production of cherry tomato (Lycopersicon esculentum)

Mustafa K. Hossain a, Vladimir Strezov a,*, K. Yin Chan b, Peter F. Nelson a

a Graduate School of the Environment, Faculty of Science, Macquarie University, NSW 2109, Australiab NSW Department of Primary Industries, Locked Bag 4, Richmond, NSW 2753, Australia

a r t i c l e i n f o

Article history:Received 31 October 2009Received in revised form 7 January 2010Accepted 8 January 2010

Keywords:BiocharChromosol soilPyrolysisWastewater sludge

0045-6535/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.chemosphere.2010.01.009

* Corresponding author. Tel.: +61 2 9850 6959; faxE-mail address: vstrezov@gse.mq.edu.au (V. Strezo

a b s t r a c t

This work presents agronomic values of a biochar produced from wastewater sludge through pyrolysis ata temperature of 550 �C. In order to investigate and quantify effects of wastewater sludge biochar on soilquality, growth, yield and bioavailability of metals in cherry tomatoes, pot experiments were carried outin a temperature controlled environment and under four different treatments consisting of control soil,soil with biochar; soil with biochar and fertiliser, and soil with fertiliser only. The soil used was chromo-sol and the applied wastewater sludge biochar was 10 t ha�1. The results showed that the application ofbiochar improves the production of cherry tomatoes by 64% above the control soil conditions. The abilityof biochar to increase the yield was attributed to the combined effect of increased nutrient availability (Pand N) and improved soil chemical conditions upon amendment. The yield of cherry tomato productionwas found to be at its maximum when biochar was applied in combination with the fertiliser. Applicationof biochar was also found to significantly increase the soil electrical conductivity as well as phosphorusand nitrogen contents. Bioavailability of metals present in the biochar was found to be below the Austra-lian maximum permitted concentrations for food.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Sludge wastes produced during wastewater treatment are someof the most difficult waste materials to manage due to the increas-ing quantities produced and the pathogenic organisms and metalcontents present in the sludge. Approximately 190 000 t of bioso-lids are produced each year at wastewater treatment plants inthe Sydney basin alone (Bamforth et al., 2004). The safe and bene-ficial use of wastewater sludge is a subject of considerable interestfor the society. Application of unprocessed wastewater sludge andwastewater sludge compost have been previously trialled asorganic fertilisers with some success (Singh and Agrawal, 2007;Roca-Perez et al., 2009). Thermal processing of wastewater sludgeprovides an additional option to manage this waste and for itsupgrading to bio-gas, bio-oil and biochar (Werther and Ogada,1999; Inguanzo et al., 2002; Hossain et al., 2009). The biochar isparticularly attracting international attention for the followingtwo reasons. Firstly, biochars can be used as soil amendments forimproving soil properties and crop yield and secondly, storingbiochars in soils is regarded as means for permanently sequester-ing carbon (Glaser et al., 2002a,b; Lehmann et al., 2003, 2006).The conversion of wastes by pyrolysis to produce biochar has

ll rights reserved.

: +61 2 9850 7972.v).

potential to increase conventional agricultural productivity(McHenry, 2009). Understanding of agricultural effects of biocharis very limited and based on few biomass feedstock materials.

Previous work investigated the beneficial effects of biochar pro-duced from green-waste and poultry litter on the yield of agricul-tural crop and properties of soil (Glaser et al., 2002b; Chan et al.,2007, 2008). It was found that biochar from poultry litter signifi-cantly improves the yield of radish crops (Chan et al., 2008) butthe risk of using poultry as a soil amendment is still unknown. Bio-char addition to soils also improved nitrogen fertiliser use effi-ciency through improvement of the chemical properties ofchromosol soil. Application of biochar to the soil was found to in-crease soil cation exchange capacity (CEC) by up to 40% and soil pHby up to one pH unit (Mikan and Abrams, 1995). The application ofwastewater sludge biochar as a soil amendment has a potential toprovide a viable option for nutrient recovery when applied to soils,improve the wastewater sludge management practice and seques-ter carbon in the soils.

However, there is currently no published data available onquantification of the effect of wastewater sludge biochar to the soil,plant nutrients and the bioavailability of metals in plant. This studyis focused on the agronomic potential of wastewater sludge bio-char and its impact on soil quality, plant growth, yield and bio-availability of metals in fruit using cherry tomato (Lycopersiconesculentum) as an agricultural crop in a glasshouse pot experiment.

1168 M.K. Hossain et al. / Chemosphere 78 (2010) 1167–1171

2. Materials and methods

2.1. Soils

The soil used for this study was collected from the flat paddockat the Centre for Recycled Organic Agriculture site Menangle nearCamden, south-western region of Sydney, Australia. The soil se-lected was with fair agricultural properties in order to better deter-mine the effect of treatment conditions using biochar on plantgrowth and production. According to the Australian Soil Classifica-tion, the soil collected for this research is classified as chromosol(Isbell, 1996). A composite sample was collected down to 0.1 mof the top soil layer, followed by sieving through a 6 mm sieve.The chemical properties of the collected surface soil are shown inTable 1. The soil was found to be low in total nitrogen (0.13%),ammonium nitrogen (3.6 mg kg�1), phosphorus (15 mg kg�1) andwas found to be acidic in nature.

2.2. Biochar preparation

The feedstock used for biochar production for this study waswastewater sludge collected from a wastewater treatment plantin Sydney region, Australia. The wastewater sludge was first air-dried and then pyrolysed under controlled conditions to ensureuniform heating and treatment conditions. Biochar productionwas carried out using a fixed bed reactor set at a heating rate of10 �C min�1 up to 550 �C. Nitrogen gas was flown through the sam-ple at a rate of 100 mL min�1 to ensure inert heating conditions.Approximately 300 g of wastewater sludge biochar were pyrolysedwith each experiment. The pyrolysis trials were repeated toachieve a total of 1 kg of biochar which was then used for chemicalanalysis and pot experiments. The chemical properties of thewastewater sludge biochar used in this experiment are shown inTable 1. The biochar was found to be alkaline in nature (pH 8.2)and low in total nitrogen (2.3%).

2.3. Pot trial

Glasshouse pot trial using L. esculentum as a crop species werecarried out to determine the agronomic properties and the effectof the wastewater sludge biochar on growth, yield and risk of met-als. The pot trials were carried out in a temperature controlled (20–26 �C) glasshouse environment. Cylindrical plastic pots 19 cm inheight, 15 cm in diameter at the bottom and 20 cm in diameterat the top were used for the pot trials. The experimental designwas factorial randomised block design with four treatments andsix replications. The four treatments were: (i) control soil (CP);(ii) soil with biochar (SB); (iii) soil with biochar and fertiliser(SBF) and (iv) soil with fertiliser (SF).

Table 1Chemical properties of soil and biochar used in pot experiment.

Properties Unit Soil Biochar

EC dS m�1 0.09 1.9pH (CaCl2) pH unit 4.6 8.2Total N % 0.13 2.3P (Colwell) mg kg�1 15 1100Ammonium N (KCl extract) mg kg�1 3.6 11Nitrate N (KCl extract) mg kg�1 4.9 0.49

Exchangeable cationsAl cmol kg�1 0.37 <0.03Ca cmol kg�1 5 33Mg cmol kg�1 1.5 1.8K cmol kg�1 0.17 0.24Na cmol kg�1 0.2 0.5CEC cmol kg�1 7.2 35

In each pot 6 kg of air-dried soil was packed and the appliedbiochar was 10 t ha�1. In absence of standards or recommendedapplication dose of biochars to soils, the amount of biochar appliedin this research is based on previous investigations conducted byChan et al. (2008) and Van Zwieten et al. (2009) who demonstratedbenefits to soil and plants by application of 10 t ha�1 of biocharproduced from papermill sludge and poultry litter, respectively.The fertiliser application used in the current experiment wasequivalent to 120 kg of nitrogen; 70 kg of phosphorus and 80 kgof potassium ha�1 (Murrison and Huett, 1987).

The pots were wetted up to the field capacity using de-ionisedwater. Five seeds of tomato were sown in each pot and germinatedfor 5–6 d. After 15 d the germinated seedlings were thinned andthe healthiest plant from each pot was retained. A shallow traywas placed under each pot and the plants were watered approxi-mately up to the field capacity throughout the duration of the trial.The pot trials were carried out for a total of 16 wk.

2.4. Soil and fruit analysis

The soil from each pot was collected and air-dried at a temper-ature of 36 �C until constant weight. The soils from each pot weremixed and passed through a 4 mm sieve to separate the plant deb-ris. Sub-samples were further ground to pass through a 2 mm sieveand analysed for electrical conductivity (EC), pH, total nitrogen,extractable phosphorus (Colwell), exchangeable cations and CEC.Total N were measured by Dumas combustion method using anElementar vario MAX CN analyser with combustion chamber setat 900 �C and oxygen flow rate of 125 mL min�1. The pH was mea-sured in 0.01 M CaCl2 (1:5) according to method 4B2 of Raymentand Higginson (1992). Available phosphorus (Colwell), mineralnitrogen (KCl extraction and extractable) and micronutrients weremeasured according to methods 9B1, 7C2 and 12A1 of Raymentand Higginson (1992), respectively. Exchangeable cations weredetermined using the Gillman and Sumpter (1986) method. Atthe end of the pot experiment the fruits and plants were harvestedfrom each pot and oven dried at 70 �C to constant weight beforeweighing to determine the dry matter.

Accumulation of metals and trace elements in fruits were ana-lysed by ICP according to the US EPA method 6010. The sampleswere homogenised and a sub-sample (0.2–0.5 g) was digested withre-distilled nitric acid on a DigiPrep block for 1 h until vigorousreaction was complete. Samples were then transferred to a Mile-stone microwave for further digestion. The digested sample wasanalysed for metals and trace elements using ICP-AES.

2.5. Statistical analysis

All data were statistically studied by analysis of variance usingGENSTAT 9.1 software (Lawes Agricultural Trust, 2006). The treat-ment means were compared using least significant differences forthe main effect of biochar on plant growth properties. Unlessotherwise stated, the differences were significant at p 6 0.05 level.

3. Results

3.1. Effect of biochar application on soil parameters

Table 2 shows the changes of chemical properties of the soils ofdifferent treatments due to application of biochar. Application ofwastewater sludge biochar was found to significantly change mostof the chemical properties of the soil. The biochar increased EC, pH,total nitrogen, extractable phosphorus and CEC of the soil. EC wasfound to have the highest value (0.53 dS m�1) in SBF treatmentwhile pH was the highest (pH 4.7) in the SF treatment. SBF treat-

Table 2Changes in soil chemical properties and standard deviation by biochar application.

Properties Unit CP SB SBF SF LSD

EC dS m�1 0.05 (±0.00) 0.29 (±0.03) 0.53 (±0.04) 0.33 (±0.04) 0.14pH pH unit 4.3 (±0.01) 4.5 (±0.03) 4.7 (±0.06) 4.7 (±0.06) 0.13Total N % 0.14 (±0.01) 0.18 (±0.01) 0.22 (±0.01) 0.19 (±0.01) 0.02P (Cowell) mg kg�1 26 (±0.87) 56 (±2.4) 290 (±24.2) 303 (±16.0) 36Ammonium N (KCl extract) mg kg�1 3.6 (±0.27) 5.1 (±0.57) 102 (±20.1) 88 (±19) 41Nitrate N (KCl extract) mg kg�1 9.3 (±1.26) 54 (±21.6) 193 (±19.6) 125 (±23.7) 64

Exchangeable cationsAl cmol kg�1 0.38 (±0.01) 0.23 (±0.01) 0.08 (±0.01) 0.09 (±0.01) 0.04Ca cmol kg�1 4.7 (±0.07) 5.9 (±0.1) 6.5 (±0.11) 6.0 (±0.1) 0.3Mg cmol kg�1 1.6 (±0.03) 1.6 (±0.02) 1.4 (±0.04) 1.6 (±0.02) 0.1K cmol kg�1 0.28 (±0.02) 0.25 (±0.09) 1.3 (±0.16) 1.3 (±0.09) 0.35Na cmol kg�1 0.39 (±0.01) 0.39 (±0.03) 0.29 (±0.03) 0.38 (±0.01) 0.07CEC cmol kg�1 7.3 (±0.09) 8.5 (±0.14) 9.5 (±0.18) 9.4 (±0.17) 0.45

LSD = least significant difference.

0

2040

60

80100

120

Dry

mat

ter (

gm/p

lant

)

0

50

100

150

200

250

Frui

ts n

umbe

r/pla

nt

050010001500200025003000

CP SB SBF SFTreatments

Yiel

d (g

m/p

lant

)

a

b

c

Fig. 2. Cherry tomato production using different soil treatments: (a) dry matterproduction of cherry tomato per plant; (b) cherry tomato fruit number; (c) cherry

M.K. Hossain et al. / Chemosphere 78 (2010) 1167–1171 1169

ment contained the highest concentration of total nitrogen (0.22%).The available phosphorus (Cowell) exhibited the highest value(303 mg kg�1) in case of the SF treatment but it was not signifi-cantly different to the SBF treatment at 290 mg kg�1. A significantincrease of mineral nitrogen was detected in the SBF treatment(Table 2). For the exchangeable cations, the biochar application in-creased only the calcium concentration, while the aluminium de-creased in all treatments.

3.2. Effect of biochar on plant height

The height of each plant in all treatments was measured start-ing from wk 5 to 15. The results, as shown in Fig. 1, revealed signif-icant effect of biochar on the height of the plant. However, SBFtreatment conditions showed the highest average performance(69.3 cm), followed by SF (64.0 cm), SB (57.7 cm) and CP(44.5 cm). The maximum plant height at the end of 15 wk trialwas estimated at 96.8 cm for CP, 107.5 cm for SB, 113.5 cm forSBF and 109 cm for SF conditions. The maximum plant growth ratewas observed during the 8th wk for SBF (13.0 cm), the 9th wk forSF (13.5 cm), and 12th wk for SB (15.8 cm).

3.3. Effect of biochar on plant dry matter weight

The dry matter weight of cherry tomato plant shoot varied sig-nificantly among the different treatments, as shown in Fig. 2a. Theaverage dry weight of shoot production ranged from 61.9 g plant�1

for CP to 92 g plant�1 recorded for SBF treatment. There was nosignificant difference between the SB (73.8 g plant�1) and SF(79.7 g plant�1) treatments. The SBF treatment showed the highestperformance due to the addition of fertiliser in combination withbiochar, which improved the growth of the cherry tomato plant.

0

20

40

60

80

100

120

5 6 7 8 9 10 11 12 13 14 15weeks

heig

ht (c

m)

CPSBSBFSF

Fig. 1. Weekly plant height of cherry tomato.

tomato yield.

3.4. Effect of biochar on the number of produced fruits

The cherry tomato fruits produced from the plants for all of theconsidered treatments were harvested during a period of 12–16 wk. All plant treatments using biochar, fertiliser or combinationof both, exhibited larger number of fruits per plant, comparing toCP condition (Fig. 2b). Plants grown with SBF had the maximumaverage fruit number which was 167% above the CP condition,63% above SB and 122% above SF treatments.

3.5. Effect of biochar on crop yield

Biochar in combination with fertiliser also showed the most sig-nificant effect on the yield of cherry tomato production followed by

1170 M.K. Hossain et al. / Chemosphere 78 (2010) 1167–1171

SF and SB treatments, as shown in Fig. 2c. The maximum averageyield per plant was harvested from SBF conditions, (2514 g) whichwas 20% greater than the case of SF conditions and 97% above theyield produced in SB conditions. CP produced the lowest (776 g)yields of cherry tomatoes. The difference in crop yield betweenCP and SB conditions was also significant (Fig. 2c). SB treatmentproduced 64% greater yield compared to control treatment.

3.6. Heavy metal concentrations in fruits

The accumulation of heavy metals, especially arsenic, cadmium,chromium, copper, lead, nickel, selenium and zinc, are of great con-cern in agricultural product due to potential threat for human andanimal health. The amount of heavy metals present in the appliedbiochar and their bioavailability in fruits are shown in Table 3.There were 16 metals and trace elements measured in the waste-water sludge biochar. The results of fruit analysis show that all ele-ments are uptaken by the fruits but their amounts are notsignificant. The presence of selenium (<0.05), lead (<0.01) and tin(<0.05 mg kg�1) are below the detection limit in all treatments.In the SBF treatment the presence of arsenic (<0.01) and chromium(<0.05 mg kg�1) in the fruit are also below the detection limit. Theaccumulation of cadmium in fruit was estimated at 0.85% for bothSB and SBF treatments, and 1% in case of the SF treatment. Silver(<0.01 mg kg�1) is less than detection limit in SB and CP treat-ments. Copper and zinc show the lowest bioavailability in theSBF treatment followed by SB and SF treatments. Availability ofthe other trace elements in the fruit was estimated as very low (Ta-ble 3). Only the concentration of cadmium present in the fruit forSF treatment conditions was found to be equivalent to the Austra-lian maximum permitted concentration of cadmium in food. How-ever, the remaining metals, including antimony, arsenic, cadmium,copper, lead, mercury, selenium, tin and zinc, were measured be-low the Australian maximum permitted concentrations for foodproducts (Table 3).

4. Discussion

Results obtained from the pot experiments in this work indicatethe potential of the application of wastewater sludge biochar to a

Table 3Concentration (mg kg�1) of heavy metals and trace elements in wastewater sludgebiochar and their accumulation in fruits of four different treatments comparing toAustralian food standard limitations for heavy metals in food (mg kg�1).

Elements Biochar Treatments Present Australian MPCa

CP SB SBF SF

Arsenic 8.8 0.02 0.02 BDL 0.01 1.0Cadmium 4.7 0.03 0.04 0.04 0.05 0.05–2.0Chromium 230 BDL BDL BDL 0.06 –Copper 2100 5.9 6.2 4.6 6.2 10–70Lead 160 BDL BDL BDL BDL 1.5–2.5Nickel 740 8.2 1.2 0.61 0.55 –Selenium 7 BDL BDL BDL BDL 1.0Zinc 3300 18 22 18 22 150Antimony 8 BDL 0.01 BDL BDL 1.5Boron 20 15 15 9.6 15 –Silver 29 BDL BDL 0.01 0.01 –Barium 750 3.6 0.91 0.35 2.9 –Beryllium 1 BDL BDL BDL BDL –Cobalt 21 0.06 0.03 0.3 0.27 –Tin 310 BDL BDL BDL BDL 50Strontium 390 5 3.1 2.6 4.6 –

MPC = maximum permitted concentration.BDL below detection limit of <0.05 mg kg�1 for As, Cr, Se and Sb, and <0.01 mg kg�1

for Pb, Sb and Be.a Source: Anon (1987).

chromosol soil for improvement of the yield of cherry tomato pro-duction by 64%. This value is of a similar magnitude to the yield ob-served for radish using poultry litter biochar when applied at thesame dose on a similar soil (Chan et al., 2008). The lower agricul-tural properties of the soil used in the pot experiment was dueto the low nutrient availability of the chromosol soil selected forthe study (Table 1). It was observed that the average fruit yieldproduced under the control treatment conditions was lower andplants were thinner than the fruits and plants from the other treat-ments. The weight of the dry shoot (Fig. 2a) and the number offruits (Fig. 2b) produced from CP treatment were also lower thanunder the remaining treatment conditions.

It has been already identified that biochar applied to soils im-proves the availability of phosphorus, total nitrogen and major cat-ions (Glaser et al., 2002b; Lehmann et al., 2003). Additionally,biochar has positive liming effect when applied to low pH soils(Van Zwieten et al., 2007), thereby the application of biochar toacidic soils increases the soil pH and thereof improves nutrientuse efficiency. Our study shows significant improvement in thenumber of fruits per plant when biochar was applied at 10 t ha�1

to the chromosol with low pH soil (Fig. 2b), suggesting release ofadditional nutrients from the biochar. The wastewater sludge bio-char used in the current work was high in extractable phosphorus(1100 mg kg�1). Presence of phosphorus in soils is particularly fa-voured for growth of a tomato root system increasing fruit produc-tivity (Filgueira, 2000). According to Poulton et al. (2002) soil withhigh phosphorus content improves vegetative and reproductivetraits of tomato plants, therefore additional phosphorus has animportant role in incremental fruit formation when biochar isadded to the soil. The wastewater sludge biochar produced for thiswork had a higher level of total nitrogen (2.3%) while the mineralnitrogen was low at 0.49%. This suggests that the wastewatersludge biochar might have the ability to increase mineralisationof soil organic nitrogen upon its incorporation into soil as a resultof priming effect (Hamer et al., 2004). Another possibility forimprovement of the nitrogen release is through the mineralisationof wastewater sludge biochar and therefore release of the availablenitrogen after addition to soil during the pot experiments. The car-bon and nitrogen ratio of the biochar used in the pot trials was only8.7 (Table 1), therefore mineralisation is expected upon its applica-tion to soil (Sullivan and Miller, 2001). According to Hamer et al.(2004), biochars from maize and rye residues in soils can promotemineralisation of carbon compounds as well as biochar by enhanc-ing the growth of micro-organisms.

The highest yield of cherry tomato in the current work was har-vested from the combined biochar with fertiliser treatment(2514 g plant�1) which was 20% above the yield produced underthe SF treatment. Since the nutrient addition through a fertiliserwas optimal for cherry tomato growth and production, the im-proved yield observed in the combined biochar and fertiliser treat-ment, when compared to fertiliser only, suggests additionalbeneficial effects of biochar inclusion, which are beyond the solenutrient effect. These additional benefits include improved soilproperties as well as the liming effects. The fertiliser effect of bio-char is additionally supported by increased water retention andCEC of the soil by the large surface area of the biochar (Steinbeisset al., 2009).

Biochar produced from wastewater sludge pyrolysis has poten-tial to reduce the quantity of fertiliser requirement for cultivationof agricultural crops, however application of wastewater biochar at10 t ha�1 can not fully substitute for the requirement of fertilisers.The composition of sewage sludge is variable and contains toxicmetals (Smith, 1992) which limit the land application due to foodchain contamination (Chaney, 1990). Singh and Agrawal (2007)found increased concentrations of Pb, Cr, Cd, Cu, Zn and Ni in Betavulgaris (leafy vegetables) when grown in a greenhouse environ-

M.K. Hossain et al. / Chemosphere 78 (2010) 1167–1171 1171

ment with sewage sludge amended soil, comparing to unamendedsoil. As previously reported, biochar from wastewater sludge con-tains high concentration of heavy metals, such as arsenic, sele-nium, cadmium, chromium, copper and zinc (Hossain et al.,2009). Metal behaviour in soils and plant uptake are dependenton the nature of the metal, sludge, soil physico-chemical propertiesand plant species (Kidd et al., 2007). The study presented here de-tected insignificant bioaccumulation of the trace metals present inthe produced fruits using wastewater sludge biochar. The level ofall of the heavy metals measured in the fruits were below the Aus-tralian maximum permitted concentrations for food safety.

Our results also highlight the potential benefits of application ofwastewater sludge biochar for improving the chemical propertiesof the chromosol soils, which are widespread in Australia (Mullinset al., 1990). The biochar is known to have positive effect on soilquality as it enhances soil aeration, increasing water holdingcapacity and making good environmental situation for the growthand development of the plant root system (Glaser et al., 2002b;Lehmann et al., 2003; Chan et al., 2007).

Acknowledgements

The authors gratefully acknowledge the financial support forthis research provided by the Higher Degree Research Unit, Mac-quarie University, Sydney, Australia. The authors also wish tothank the Sydney Water for supply of wastewater sludge samplesfor the study.

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