Bioresource Technology 112 (2012) 61–66

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Bioresource Technology

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The sorption of pentachlorophenol by aged sediment supplementedwith black carbon produced from rice straw and fly ash

Liping Lou a, Ling Luo b, Guanghuan Cheng a, Yanfei Wei c, Rongwu Mei c, Bei Xun a,Xinhua Xu a, Baolan Hu a,⇑, Yingxu Chen a

a The Department of Environmental Engineering, Zhejiang University, Hangzhou 310029, Chinab School of Biological Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, SAR, Chinac The Environment Science Research & Design Institute of Zhejiang Province, China

a r t i c l e i n f o

Article history:Received 12 October 2011Received in revised form 6 February 2012Accepted 13 February 2012Available online 21 February 2012

Keywords:Black carbon (BC)BiocharPentachlorophenol (PCP)SorptionAging

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.biortech.2012.02.058

⇑ Corresponding author.E-mail address: blhu@zju.edu.cn (B. Hu).

a b s t r a c t

Black carbon (BC) is a potential material for controlling hydrophobic organic contaminants in sedimentbecause it has a high sorption capacity. In the present study, the sorption of pentachlorophenol (PCP)onto sediments supplemented with rice straw biochar (RC) and fly ash (FC) aged for different timesand at temperatures were investigated. The sorption of PCP increased with increasing amounts of BCand decreased with aging time and storage temperature of the BC-supplemented sediments. The sorptionof PCP onto RC-supplemented sediments was higher than those supplemented with FC regardless ofwhether or not BCs were aged in sediments. For aged sediments containing 2% BCs, the sorption capacitywas 9.15- and 2.87-fold higher than that of FC when supplemented with RC aged at 25 and 45 �C, respec-tively. Therefore, biochar is better than fly ash for controlling organic pollutants even when the RC waspresent in sediment for a long time.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Black carbon (BC) is a chemically heterogeneous, biologicallyrefractory class of carbon compounds that are produced duringburning of biomass and combustion of fossil fuel (Cornelissenet al., 2005). BC is proposed to play an important role in sorptionand desorption of contaminants in sediment due to its intrinsicallyhigh surface activity (Accardi-dey and Gschwend, 2002; Bucheliand Gustaffson, 2000; Chun et al., 2004; Cornelissen et al.,2004a,b, 2005). For example, Bucheli and Gustaffson (2000) re-ported that BC accounted for the high sorption affinities for hydro-phobic organic contaminants (HOCs) by sediment. In support ofthese findings, Accardi-dey and Gschwend (2002) demonstratedthat sorption by combustion-derived BC was important for bindingof PAHs to sediments. Moreover, it has been shown that the addi-tion of BC reduces the ecological risks of HOCs in sediment (Cuiet al., 2009; Yu et al., 2009). Previous studies showed that BC pro-duced from rice straw could effectively adsorb pentachlorophenol(PCP) and reduce the toxicity of PCP (Lou et al., 2011).

Aging processes, such as surface precipitation, surface oxida-tion, diffusion into micropores, or incorporation into crystal lat-tices, can cause a decline in mobility and bioavailability/toxicityof materials added to soil/sediment over time (Alexander, 2000;

ll rights reserved.

Ma et al., 2006; Wendling et al., 2009; Yang and Sheng, 2003).Studies have demonstrated an increase in the resistant fractionwith time (Chai et al., 2008; Ma et al., 2006), but few studies havebeen conducted on the influence of aging BCs in sediment onadsorption capacity. Kwon and Pignatello (2005) reported thataging of wood char placed in a soil–water suspension at 45 �C ledto a reduction in surface area and sorption capacity of the char. Pig-natello et al. (2006) assumed that this reduction was mainly due toblockage of pores by the interaction of functional groups of BC withhumic and fulvic acids in the soil. Moreover, it was found that theBC properties and the strength of contaminant sorption were af-fected over time with following the amendment of a BC materialto a contaminated soil or sediment (Hale et al., 2011). Thus, agingof BC can be considered to alter BC properties and affect BC’s sorp-tion abilities.

Currently, it is problematic to accurately and effectively reflectthe sorption capacity of pollutants by sedimentary BC using wetchemical oxidation or the CTO-375 method (Song et al., 2002; Xiaoet al., 2004). However, sedimentary BC, whose sorption behaviourmight differ from the extracted or fresh BC, can be simulated by add-ing BC to the sediment and then aging the BC-inclusive sediment fordifferent times and temperatures (Kwon and Pignatello, 2005).

Biochar (from vegetation burning) and fly ash (from fossil fuelcombustion) are the two main sources of BC due to their ubiquityand abundance in the environment (Accardi-dey and Gschwend,2002; Bucheli and Gustaffson, 2000; Luo et al., 2011; Lou et al.,

62 L. Lou et al. / Bioresource Technology 112 (2012) 61–66

2011). In this study, these two BCs were used to investigate thefeasibility of applying BC to reduce the ecological risk of HOCs insediment. The sorption properties of sediment supplemented withrice straw biochar and fly ash, and the effects of aging for differentlengths of time and at different temperatures on the sorption prop-erties of the BC-supplemented sediments were evaluated. Further-more, the contribution rate of BC in the aged BC-sediment systemto sorption was estimated.

2. Methods

2.1. Chemicals and materials

PCP with a purity >98% was purchased from Sigma–Aldrich(China). Sediment was collected from the Qiantang River in Hangz-hou, Zhejiang province, China. The properties of the sediment weredetermined previously (Lou et al., 2011). Several pollutants werepresent in the sediment, such as PCB (0.0459 mg/kg), Cr (0.0429mg/kg), Zn (0.1683 mg/kg), Cd (0.0021 mg/kg) and Cu(0.0764 mg/kg).

The rice straw biochar (RC) and the fly ash (FC) were collectedand prepared as reported by Lou et al. (2011) and Luo et al.(2011). RC and FC had, respectively, a carbon content of 18.49%and 29.68%, a surface area of 72.1 and 21.0 m2/g, a porosity of0.133 and 0.0387 mg/L, acidic groups at 3.741 and 0.984 mmol/g,and no basic groups due to the acid treatments (Luo et al., 2011;Lou et al., 2011).

2.2. Sorption of PCP to sediment supplemented with BC

To achieve the supplement amounts of 0% (control), 0.5%, 1%,2%, 5% and 10% (dry weight basis) BC in sediment, the sedimentwas mixed with specific quantities of each BC, and the supple-mented sediments were thoroughly mixed. A solution containing1 mM CaCl2, 1 mM MgCl2, 0.5 mM Na2B4O7�10H2O and 200 mg/LNaN3, was prepared for making PCP standard solutions rangingfrom 0.1 to 20 mg/L (Chen et al., 2004). The pH of PCP standardsolution was adjusted to pH 7.0 (Chen et al., 2004). The BC-supple-mented sediments were weighed into 50-mL glass vials, and 30 mLPCP standard solution was added to each vial. The sorbent-to-water ratio (w/v) was adjusted to achieve 30–70% of the totalsorbed solute. The vials were shaken at 200 rpm on a horizontalshaker at 25 ± 1 �C for 24 h in the dark. After the sorption equilib-rium was established, the sorbent and aqueous phase were sepa-rated by centrifugation at 1006g for 20 min, and the supernatantwas analysed using high performance liquid chromatography(HPLC) according to Chen et al. (2004). Each treatment was repli-cated three times.

Two different models were used to fit the adsorption data. TheFreundlich model (FM), which is commonly used for quantifyingHOC sorption equilibria for soils and sediments, has the followingform:

qe ¼ K f Cne ð1Þ

where qe is the solid-phase concentration (mg/kg); Ce is the liquid-phase equilibrium concentration (mg/L); Kf [(mg/kg)/(mg/L)n] is theFreundlich model capacity factor and n is the isotherm linearityparameter, an indicator of site energy heterogeneity (He et al.,2006).

The dual mode model (DMM), which describes the solid-phasedissolution, or partitioning, by a linear term and describes hole-fill-ing by a Langmuir term, is given by the following equation:

qe ¼ KDCe þQ max;LCe

KL;D þ Ceð2Þ

where KD (kg/L) is the dissolution domain partition coefficient;Qmax,L (mg/kg) is the capacity coefficient of the hole-filling domainand KL,D (mg/L) is the affinity coefficient of the hole-filling domain(Huang et al., 2003). The Qmax,L/(KL,D � KD) value represents theintrinsic affinity of the solute for the hole-filling domain relativeto the dissolution domain given infinite dilution (He et al., 2006).

2.3. Sorption of PCP to aged sediment supplemented with BC

The percentages of BC in the supplemented sediments were 0%,0.5% and 2.0% (w/w). The sediments supplemented with BC werethoroughly mixed and the sediment without BC was labelled CK;the sediments containing 0.5% and 2% RC were labelled R0.5 andR2, and those containing 0.5% and 2% FC were labelled F0.5 andF2, respectively. The mixed sediments (1 g) were added to 50-mLglass vials and the original water content was re-established byadding appropriate amounts of distilled water. The samples weresealed with caps to prevent water loss. The mixtures were agedat either 25 or 45 �C in incubators (PGX-350B, Haishusaifu Com-pany, Ningbo, China) for designated times (0–90 days) (Kwon andPignatello, 2005; Pignatello et al., 2006).

At the end of the designated aging time, the vials were removedfrom the incubators, and 30 mL of PCP electrolyte solution (10 mg/L) was added to each vial. The sorption experiments were con-ducted following the protocol described in Section 2.2, except thataged sediments supplemented with BC were used instead of freshBC-supplemented sediments. Each treatment was repeated threetimes.

2.4. PCP analysis

The PCP concentrations in initial and equilibrated supernatantswere analysed using an Agilent 1100 series HPLC (USA) with adiode array UV-detector and a C18 reversed-phase column (ODS,5 lm, 2.1 mm � 250 mm). A solution of methanol and 1% aceticacid in water (90% v/v) was used as mobile phases at a flow rateof 1.0 mL/min, and the injection volume was 20 lL. The wave-length for the detection of PCP was 220 nm with a 20-nm band-width, and 300 nm with a 50-nm bandwidth was used as areference wavelength. The compound concentrations were quanti-fied using an external standard (Chen et al., 2004).

3. Results and discussion

3.1. Sorption of PCP onto BC-supplemented sediment

The data for the sorption of PCP onto the sediments supple-mented with different amounts of BC are plotted in Fig. 1. Thesorption capacity of the BC-supplemented sediments increasedwith an increase in BC content in the sediment. The parameters fit-ted by the Freundlich and dual mode models are listed in Table 1,which indicates that both models described the sorption data well.The Freundlich sorption coefficient Kf increased, the n value de-creased, and the dissolution domain partition coefficient KD andthe sorptive capacity Qmax,L increased with increasing BC content.

The Kf values of RC-supplemented sediments were higher thanthose of sediments supplemented with FC, but the n values of theformer were lower than that of the latter at the same BC content.When the amount of the BC in the sediment was 10%, the Kf valuesof RC- and FC-supplemented sediments were 951.69 and 56.63,and the n values were 0.37 and 0.42, respectively. These resultsare consistent with the findings of Luo et al. (2011) and Lou et al.(2011), which indicated that the sorption capacity of PCP onto pureRC was higher than onto pure FC, and RC enhanced the sorptioncapacity and nonlinearity of the sediment.

Fig. 1. Sorption isotherms of sediments supplemented with BC, (a) RC, (b) FC.

L. Lou et al. / Bioresource Technology 112 (2012) 61–66 63

The Qmax,L values showed that the maximum sorption capacityof PCP onto RC-sediment was higher than onto FC-sediment. Whenthe amount of BC in the sediment was 10%, the maximum sorptioncapacity of PCP by the RC-sediment was approximately 28 timesthat of the FC-sediment.

There is increasing recognition that the mechanisms of hole-fill-ing, surface coverage, multilayer adsorption, condensation in cap-illary pores and absorption into the polymeric matrix, which areinfluenced by the surface area and porosity of BC, contribute tothe sorption of contaminants by BC (Accardi-dey and Gschwend,

Table 1The sorption parameters fitted by Freundlich and dual mode models.

BC content (%)s Freundlich model Dual mod

Kf n R2 KD

RC 0 7.02 ± 0.36 0.60 ± 0.02 0.995 1.49 ± 00.5 30.82 ± 2.40 0.52 ± 0.04 0.986 3.55 ± 01.0 77.74 ± 2.01 0.44 ± 0.02 0.991 5.54 ± 02.0 203.60 ± 3.85 0.33 ± 0.02 0.985 6.31 ± 05.0 421.72 ± 16.44 0.40 ± 0.03 0.985 22.73 ± 2

10.0 951.69 ± 74.79 0.37 ± 0.03 0.981 81.95 ± 2

FC 0 4.06 ± 0.34 0.59 ± 0.05 0.964 1.18 ± 00.5 5.64 ± 0.50 0.58 ± 0.05 0.958 1.86 ± 01.0 6.98 ± 0.73 0.65 ± 0.06 0.969 2.27 ± 02.0 14.85 ± 0.73 0.49 ± 0.03 0.983 2.74 ± 05.0 30.87 ± 1.05 0.48 ± 0.02 0.990 6.64 ± 1

10.0 56.63 ± 5.37 0.42 ± 0.05 0.923 10.86 ± 2

2002; Bucheli and Gustaffson, 2000; Chun et al., 2004; Cornelissenet al., 2004a,b; Cui et al., 2009; Lou et al., 2011; Luo et al., 2011).Moreover, it is reported that the functional groups, especially car-boxyls and lactones, affect the sorption of pollutants through p–pinteractions and intermolecular hydrogen bonding (Mathialaganand Viraraghavan, 2009; Qiu et al., 2009). Additionally, it was pro-posed that soil with a high Qmax,L/(KL,D � KD) value might have adominating, hole-filling sorption mechanism of adsorption (Heet al., 2006).

In the current study, the Qmax,L/(KL,D � KD) values increased from5.79 to 36.39 for RC and from 7.01 to 29.57 for FC as the RC and FCcontents were increased from 0% to 10% in the sediment. Thus,hole-filling was considered to be the main sorption mechanismdue to the increased Qmax,L/(KL,D � KD) value upon the addition ofBC into the sediment. In particular, RC had a larger surface area,greater porosity and more functional groups than FC. That is whythe sorptive ability of the RC-supplemented sediment was strongerthan the sediment supplemented with FC.

3.2. Sorption of PCP onto aged BC-supplemented sediment

3.2.1. Effect of aging timeThe data for the sorption of PCP by aged BC-sediments are plot-

ted in Fig. 2. The sorption capacity of each sample decreased withincreasing aging time at 25 ± 1 �C. The sorption capacity of PCP toBC-supplemented sediments decreased by approximately half after60 days of aging compared to day 0, with the exception of R2 sed-iment which only showed a decline in sorption capacity of 6.6%.These results are consistent with the results reported by Kwonand Pignatello (2005). For sediments containing 2% BCs, the PCPsorption capacity of the aged RC-sediment was 9.15-fold higherthan that of the aged FC-sediment. Moreover, the results fromthe variance analysis indicated that sorptive properties of the FC-sediment significantly decreased with aging time, whereas thesorptive properties of the sediment supplemented with 2% RC wereless affected.

It was proposed that organic matter and/or native sorbates inthe environmental sediment most likely occupied or blocked BCsorption sites, causing them to become less available for the addedphenanthrene (PHE-d10) (Cornelissen and Gustafsson, 2004; Jonkerand Koelmans, 2002). It was also assumed that substances such ashumic acid, minerals, metal oxides and native pollutants in thesediment might alter the physical or chemical properties of theBC surface and affect PCP sorption. These properties were changedthrough the way of interactions such as surface coverage, poreblockage, and surface oxidation (Cornelissen et al., 2004a,b; Huanget al., 2003).

In the present study, RC and FC were rich in porosity and func-tional groups. Once the BCs were introduced into the sediment,

e model

Qmax,L KL,D R2 Qmax,L/(KL,D ⁄ KD)

.26 15.26 ± 4.16 1.77 ± 0.85 0.992 5.79

.81 32.85 ± 9.62 0.80 ± 0.57 0.976 11.57

.92 67.51 ± 10.33 0.61 ± 0.26 0.986 19.98

.64 212.40 ± 6.48 0.43 ± 0.04 0.998 78.28

.43 437.35 ± 29.12 0.67 ± 0.10 0.997 28.720.53 1520.78 ± 199.33 0.51 ± 0.17 0.979 36.39

.24 4.88 ± 1.89 0.59 ± 0.05 0.970 7.01

.26 4.95 ± 1.56 0.23 ± 0.04 0.967 11.57

.74 10.29 ± 2.44 1.11 ± 0.36 0.970 4.08

.58 21.22 ± 4.93 0.66 ± 0.35 0.981 11.73

.24 34.89 ± 8.43 0.35 ± 0.07 0.970 15.01

.63 54.60 ± 7.92 0.17 ± 0.02 0.927 29.57

Fig. 2. Effects of aging time on sorption of PCP onto sediment supplemented withBC (25 ± 1 �C). qe is the amount of PCP sorbed per kg of sediment and the error barsrepresent standard deviations (n = 3).

Fig. 3. Contribution of BC to sorption by aged BC-supplemented sediments(25 ± 1 �C).

64 L. Lou et al. / Bioresource Technology 112 (2012) 61–66

pores to an internal population of adsorption sites might have be-come blocked by organic matter and pollutants in the sediment.The functional groups of RC and FC, such as carboxyls, lactonesand quinonyls, would have been able to react with substances insediment. Consequently, the sorption capacity of PCP onto theBC-supplemented sediments would have decreased with agingtime. Moreover, due to a higher surface area and larger numberof functional groups on RC, the percentage of reduced qe in theRC-supplemented sediment was lower than that of the FC-supple-mented sediment. Therefore, according to the results reported byLuo et al. (2011) and our results, it can be concluded that the addi-tion of RC to sediment has better adsorption and stability of PCPcompared to FC, whether or not the BC amended sediments wereaged.

It is notable that the sorption capacity of PCP to CK also de-creased with increasing aging time. This reduction was also ob-served by Yang et al. (2009), who reported that aging of soilresulted in a reduction in diuron sorption. The most likely reasonmight be chemical reactions involving organic matter and contam-inants in sediment during aging (Yang and Sheng, 2003).

3.2.2. Contribution of BC to sorptionThe contribution percentage of each BC to the sorption was

calculated as: the contribution rate = (Tqe–Cqe)/Tqe � 100, in whichTqe means total sorption capacity of PCP to whole BC-sedimentsystem, while Cqe represents the sorption capacity of sedimentwithout BC (Lou et al. 2011). The contribution of each BC to sorp-tion varied with the amount and source of the BC at room tem-perature (25 ± 1 �C) (Fig. 3). The contribution of RC to sorptionwas higher than that of FC in the aged BC-sediment system. Addi-tionally, the results demonstrate that the RC-sediment systemshowed an increase in contribution of BC to sorption althoughthe RC-sediments had been through the aging process, whereasthe FC-supplemented sediment displayed a decreasing trend.After the BC-supplemented sediments aged for 90 days, the con-tribution percentages of the R0.5, R2, F0.5 and F2 sediments tothe whole BC-sediment system were 49.15%, 82.74%, �57.89%and �57.89%, respectively. Compared with CK, RC significantly(P < 0.01) enhanced the sorption capacity of PCP with increasingaging times, but no significant contribution (P > 0.05) was foundin the FC-sediment systems.

It was reported that the surface area and sorptive ability of sed-iment supplemented with char was higher than that of sedimentalone, even though the aging effect could reduce the surface areaand porosity of the sediment supplemented with char (Kwon andPignatello, 2005). Moreover, the functional groups on the surfaceof BC have been proposed to affect sorption of pollutants(Mathialagan and Viraraghavan, 2009; Qiu et al., 2009). Conse-quently, the reason why RC showed an increasing trend in contri-bution percentage to sorption may be that RC still had some freesorption sites, even if natural substances in the sediment, such asnative organic matter, PCB and metal oxides, competed for theadsorption sites or blocked the pores.

However, it is reasonable to assume that 0.5% and 2% FC did nothave enough sorption sites to coat the pollutants and organic mat-ter in the sediment at room temperature because the FC had a rel-atively lower surface area, porosity and number of functionalgroups. Furthermore, the sedimentary organic matter, which canadsorb PCP, might interact with the functional groups of the FC.Therefore, the sorption capacity of the PCP by the FC-sediment sys-tem was less than that of sediment alone.

3.2.3. Effect of aging temperatureThe sorption capacity of PCP to the aged R2- and F2-sediment

systems at different temperature are shown in Fig. 4, indicatingthat the sorption capacity of PCP to each BC-supplemented sedi-ment decreased as the aging temperature increased. This is consis-tent with data that showed a lower sorption capacity ofhydroquinone than fresh BC, especially at higher aging tempera-ture (Cheng and Lehmann, 2009). By comparison, the sorptioncapacity of sediment with RC aged at 45 �C was approximately2.87-fold higher than that of FC.

Cheng et al. (2008) and Cheng and Lehmann (2009) proposedthat temperature was one of the major factors influencing the BCaging process as the elemental composition, surface chemistryand sorption characteristic of BC changed with age in any terres-trial regime. It was reported that the surface area and porosity ofBC decreased (Hale et al., 2011) and that acidic functional groupsof BC, especially carboxylic groups, increased with aging tempera-ture (Cheng et al., 2006). Natural oxidation between sediment/soiland BC was assumed to be an important reason for these changesas this oxidation is initiated on the surface of BC particles and leads

Fig. 4. Effects of aging temperature on sorption of PCP onto sediment containing 2%BC. qe is the amount of PCP sorbed per kg of sediment and the error bars representstandard deviations (n = 3).

L. Lou et al. / Bioresource Technology 112 (2012) 61–66 65

to a subsequent change of BC at ambient temperature, while theinterior of BC particles is also oxidised and the changes of BC aremore obvious at higher temperatures (Cheng et al., 2006, 2008;Cheng and Lehmann, 2009). Therefore, it is reasonable to concludethat temperature could accelerate the aging effects of the sedimentsupplemented with BC.

Moreover, several researchers proposed that natural organicmatter (NOM) could change from a glassy (rigid-chain) state to arubbery (flexible-chain) state as the temperature increased (LeB-oeuf and Weber, 1997; Lu and Pignatello, 2004). The sorptioncapacity of PCP to sediment might decrease under these condi-tions. Additionally, there were several pollutants in the sediment,including PCB, Cr, and Zn. It was assumed that the higher temper-ature could accelerate the rate of diffusion between the moleculesof organic matter and pollutants through the solution toward andinto the adsorbent (Mollah and Robinson, 1996). Consequently,this action might partly block BC sorption sites, thereby reducingthe sorption capacity of PCP onto BC-sediments aged at a highertemperature.

In summary, the effect of temperature on aging process of BC-sediment as shown in this study were a product of several compli-cated reactions, not of a single chemical reaction. Therefore, furtherstudies are necessary to determine the mechanism of how the tem-perature affects the aging process of BC-sediment.

4. Conclusions

The sorption capacity of PCP onto BC-sediment increased withincreasing amounts of BC added. The PCP adsorbed onto the BC-supplemented sediments decreased with increasing aging timeand temperature. By comparison, the sorption capacity of PCP tobiochar-sediment and the contribution of biochar to the sorptionwere higher than those of fly ash-supplemented sediment, regard-less of whether they were aged in sediment. Therefore, biocharcould be considered in contaminated sediment remediationregardless of its age, while aged fly ash is unsuitable for remedyingcontaminated sediment although fresh fly ash could adsorb pollu-tants effectively.

Acknowledgements

The work was financially supported by Grants from the Na-tional Natural Science Foundation of China (No. 40801198), the

Zhejiang Provincial Major Science and Technology Special Projects(No. 2007C13060), the Zhejiang Provincial Natural Science Foun-

dation of China (No. R5090033) and the Zhejiang Provincial KeyLaboratory Foundation of Environmental Pollution Control (No.2010-11).

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