3.1.2. Contaminated soils
Limited studies are available on biochar applications to remediate the soils contaminated with organic pollutants as compared to water remediation (Table 3). Jones et al. (2011) evaluated the long-term biochar effect on soil contaminated with simazine. Strong simazine sorption into the micropores of biochar suppresses biodegradation and leaching of simazine into groundwater (Jones et al., 2011). A high application rate (25 t ha−1) and small particle size (<2 mm) of biochar were most effective for simazine adsorption. Yang et al. (2010) and Yu et al. (2009) reported the similar findings in which the biochars produced from woodchips and cotton straw pyrolyzed at 850 °C resulted in a remarkable decrease in the dissipation of chlorpyrifos, carbofuran, and fipronil from soil due to their high sorption, which consequently reduced their bioavailability. Those authors also reported a pronounced decrease in the uptake of these pesticides by the plants grown in contaminated soils. Comparatively less efficiency was reported by the biochar produced at <450 °C. Low pesticide adsorption in soils may be attributed to the potential association of biochar with dissolved organic matter from soil, which could coat biochar particles, reducing the accessibility of pesticides to the sorption sites (Zhang et al., 2010). Sorption of atrazine onto the organic C content of biochar produced from dairy manure at 450 °C shows that the higher dissolved organic C content in soil may reduce atrazine sorption by blocking the biochar pores (Cao et al., 2011).
Overall, the biochars produced at higher temperatures exhibit higher sorption efficiency for organic contaminant remediation in soil and water. This is probably due to the high surface area and microporosity of biochars. Additional sorption mechanisms include electrostatic attractions between charged surfaces of biochars and ionic organic compounds. However, the partitioning and subsequent diffusion into the non-carbonized and carbonized fractions of biochar could be an effective sorption mechanism for non-ionic compounds. Therefore, the biochars should be produced under well-defined pyrolysis conditions. The biochar properties should also be carefully examined before the applications for the remediation of specific organic contaminants in soil or water.
3.2. Remediation of inorganic contaminants in soil and water
Inorganic contaminants, particularly metals in the environment, originate mostly from a range of anthropogenic sources, such as mining, smelting, metal finishing, fertilizers, animal manure, pesticides, leaded gasoline, battery manufacture, power plants, waste water, and sewage sludge (Adriano, 2001, Ok et al., 2011, Usman et al., 2012 and Lim et al., 2013). Unlike organic contaminants, metals are non-biodegradable and their bioavailability makes them highly toxic to living organisms (Adriano, 2001 and Zhang et al., 2013). Carbonaceous materials have been deliberately used for in situ remediation of metal contaminated soil and water ( Park et al., 2011a). Biochar has recently been applied as a novel carbonaceous material to adsorb metals in soil and water. However, contradicting explanations on the mobility of metals within biochar have been reported ( Beesley et al., 2010). Therefore, specific investigations into the mechanisms related to metal binding, transformation, and release are required.
3.2.1. Metal contaminated waters
Heavy metal removal studies using biochar together with advanced spectroscopic techniques have recently been conducted to elucidate the adsorption capacities and binding mechanisms in aqueous media (Ippolito et al., 2012). Table 4 summarizes the recent studies on biochar applications for remediating soil and water contaminated with metals. Lima et al. (2010) compared eight different biochars derived from broiler litter, alfalfa stems, switch grass, corn cobs, corn stover, guayule bagasse, guayule shrubs, and soybean straw with their activated counterparts for their ability to adsorb Cu2+, Cd2+, Ni2+, and Zn2+ from water. Activated biochars were better metal adsorbents due to their high surface area and easy access to their functional groups. Copper showed greater affinity to biochars compared to other divalent metals, which may be attributed to the formation of surface complexes between Cu2+ and active functional groups (–COOH and –OH) on the biochars (Tong et al., 2011). Furthermore, an X-ray absorption fine structure (XAFS) spectroscopic investigation predicted that Cu2+ sorption onto biochar is pH dependent (Ippolito et al., 2012). The authors concluded that Cu2+ is sorbed to the organic functional groups of biochar at pH 6 and 7, whereas of azurite (Cu3(CO3)2(OH)2) and tenorite (CuO) precipitate within the biochar at pH 8 and 9 (Ippolito et al., 2012). Atomic/ionic size of the metal is another parameter of concern while considering adsorption onto biochar. Generally, the smaller ionic radius of metals, the greater adsorption capacity due to enhanced penetration into biochar pores (Ko et al., 2004 and Ngah and Hanafiah, 2008).
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