The Potential of Biochar Amendments to Remediate Contaminated Soils

Abstract

There is a legacy of polluted soils worldwide, contaminated with a variety of different chemicals from a wide range of industrial (e.g., electricity generation, oil refi ning, mining), agricultural (e.g., pesticide application) and urban (e.g., waste disposal, motor vehicle discharges) sources. Soils are considered polluted when they have an excess of an element or compound which, through direct or indirect exposure, causes a toxic response to biota resulting in unacceptable risks to the environment or human health (Adriano 2001, Abrahams 2002, Vangronsveld et al. 2009). This is a cause for concern in many countries such as the U.S where over 100,000 contaminated sites have been identifi ed (Connell 2005), or the E.U. member states thathave reported 250,000 polluted sites that need urgent remediation (Mench et al. 2010). The remediation of contaminated soils is therefore receiving increasing attention from governments, legislators, industries and the general population. Due to the wide range and dispersive nature of many contaminant sources, soil contamination is often too widespread for ex situ remediation options (e.g., excavating and burying polluted soils in landfi ll sites) to be practically, environmentally or fi nancially viable. This has resulted in the development of more sustainable, in situ remediation treatments that, in many cases, involve the application of amendments directly to contaminated soils. For soils contaminated with inorganic pollutants like heavy metals, these amendments include clay minerals, zeolites, lime or composts (Simon 2000, Mench et al. 2003, Vangronsveld et al. 2009), whereas for organic contaminants like polycyclic aromatic hydrocarbons (PAHs), or polychlorinated biphenyls (PCBs), activated carbons have been favored (Brändli et al. 2008, Ghosh et al. 2011). The application of this kind of in situ amendments does not remove the contaminants from the soil, so their success is based on a reduction in the bioavailability and/or mobility of the contaminants in question. This reduction is achieved by altering the physico-chemical and biological characteristics of the soil, ultimately reducing the risk of contaminant uptake by fauna and fl ora, or leaching into waters. Biochars have received interest recently as an amendment for soil remediation purposes due to their potential to reduce the bioavailability of organic contaminants (Gomez-Eyles et al. 2011, Hale et al. 2011), inorganic contaminants (Beesley and Marmiroli 2011), and both organic and inorganic contaminants simultaneously (Cao et al. 2009, Beesley et al. 2010b). This is demonstrated by a rapid increase in the number of studies published featuring, within their title, the words ‘biochar’, ‘soil’, and ‘contaminated’ or ‘polluted’ from just 1 in 2007 to almost 30 in 2011 (Fig. 1).