Journal of Hazardous Materials 217–218 (2012) 391–397 Contents lists available at SciVerse ScienceDirect Journal of Hazardous Materials jou rn al h om epage: www.elsevier.com/loc ate/jhazmat Bioavailability assessment of hexachlorobenzene in soil as affected by wheat straw biochar Yang Song a,1 , Fang Wang a,1 , Yongrong Bian a , Fredrick Orori Kengara b , Mingyun Jia a , Zubin Xie a , Xin Jiang a,∗ a State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, PR China b Department of Chemistry, Maseno University, Private Bag Maseno 40105, Kenya a r t i c l e i n f o Article history: Received 9 November 2011 Received in revised form 12 March 2012 Accepted 17 March 2012 Available online 28 March 2012 Keywords: Biochar Bioavailability Chemical extraction Earthworm HCB a b s t r a c t Biochar incorporation with soil could increase sorption of organic contaminants, thereby reducing their bioavailability. In this study, the effects of wheat straw biochar on the sorption, dissipation and bioavail- ability of hexachlorobenzene (HCB), a typical persistent organic pollutant (POP), were investigated in laboratory experiments. We observed that HCB sorption by biochar was 42 times higher than that by soil and the sorption isotherm was linear for the concentration range studied. Biochar amendments reduced HCB dissipation, volatilization and earthworm (Eisenia foetida) uptake of HCB from soil. Hydroxypropyl- -cyclodextrin extraction correlated better with the earthworm bioassay than butanol extraction of HCB in biochar-amended soil. The results of both chemical extraction and earthworm bioassay indi- cate that biochar amendment of soil resulted in a rapid reduction in the bioavailability of HCB, even for the 0.1% biochar application rate. This suggested that wheat straw biochar could potentially be used in immobilizing POPs in contaminated sites. © 2012 Published by Elsevier B.V. 1. Introduction Application of biochar into soil has been shown to sequester C, reduce the emission of greenhouse gases, improve soil fertilization and thus plant growth [1–3]. Biochar has also been shown to have a very high affinity and capacity for sorbing organic contaminants since it has a large surface area and high microporosity, among other physico-chemical properties [4,5]. The strong sorption affin- ity of biochar influences the environmental fate and behavior of organic contaminants in soil [5], especially their bioavailability. Bioavailability of contaminants dominates their potential degradation and uptake in soil, since organisms mostly utilize contaminants dissolved in soil water [5]. The fact that decreased degradation of benzonitrile, atrazine and simazine has been observed in biochar-amended soil [6–10], and that reduced plant uptake of chlorpyrifos and carbofuran occurred with increasing biochar addition in soil [11], shows that sorption of contaminants by biochar reduces their bioavailability in soil [6,11]. The decreased bioavailability of the herbicides could also result in reduced herbi- cidal efficacy to weeds [6,12]. For example, barnyard grass survival ∗ Corresponding author. Tel.: +86 25 86881195; fax: +86 25 86881000. E-mail address: jiangxin@issas.ac.cn (X. Jiang). 1 These authors contributed equally to this work. rating increased with increasing biochar content at potentially damaging diuron or clomazone application rates [6,12]. However, studies have also reported that microbial activity could be stim- ulated by the elemental nutrients in biochar, thereby enhancing the biodegradation of pollutants such as PAHs and benzonitrile in biochar-amended soil [13,14]. Therefore, assessing the bioavail- ability of contaminants in biochar-amended soil is of importance. To assess the effect of biochar on the bioavailability of contam- inants in soil, bioassays such as microbial degradation and plant uptake have been performed [7,10,11]. However, these methods are time-consuming and laborious [15]. Chemical extraction meth- ods, such as mild-solvent extraction, have proved to be suitable in bioavailability assessment of contaminants in soils without biochar [15,16]. However, whether chemical extractions are suitable for biochar-amended soil needs to be evaluated. Most studies about the effects of biochar are based on polar organic contaminants [6–12]. However, reports on the effect of biochar on the bioavailability of non-polar persistent organic pollu- tants (POPs) are limited [17,18]. The objectives of the present study were therefore (1) to investigate the extent to which amendment of soil with different levels of wheat straw biochar affects the bioavail- ability of hexachlorobenzene (HCB) – a model non-polar POP and (2) to evaluate the suitability of chemical extraction methods to assess the bioavailability of contaminants in soil in the presence of biochar. 0304-3894/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.jhazmat.2012.03.055