Wastewater is “used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff or stormwater, and any sewer inflow or sewer infiltration”. Arsenic is naturally occurring hazardous metal in the earth crust and is distributed in the environment. It mostly exists in the environment in two inorganic forms i.e. arsenite (AsIII) and arsenate (AsV) but its organic forms also exist. It is more toxic in inorganic form as compare to organic form (Shi et al. 2004). Arsenic acute toxicity is associated with nausea, vomiting, abdominal pain and severe diarrhoea. The chronic toxicity through drinking As contaminated water involves diseases such as skin lesions, hyperpigmentation and respiratory symptoms. Ingestion of inorganic As also causes lungs and bladder cancer.
The permissible level of As in drinking water is 10 µg L-1 and 50 µg L-1 according to WHO and NDWQS respectively. Its ranking involves 12th in human body, 14th in ocean, 20th in crustal abundance (Agency for Toxic Substances and Disease Registry. Arsenic in trace amount is found in soil, water and organisms. Arsenic is also present is in groundwater as a result of pollution through human activities. Low level of As in organic and inorganic form with the concentration of 0.25 mg/kg occurs in food. Highest concentration of arsenic is present in seafood. Inorganic form of As is present in poultry, meat and dairy products and as well as organic in sea food, fruit and vegetables. Apart from As, chromium (Cr) is also widely used in various industries and is released to the environment through wastewater of industrial leather tanning, electroplating, photography, pigmentation and metal cleaning. The most important Cr oxidation states in the environment are þ3 and þ6. Cr(III) is an essential trace element for human health that plays an important role in metabolic disorders, reducing blood glucose and cholesterol levels while controlling diabetes. On the other hand, Cr(VI) is hazardous and toxic, having adverse effects on humans. Based on drinking water guidelines recommended by the EPA, total chromium concentration should not exceed 100 mg/l. The corresponding limit set by the European Commission is 50 mg/l.
The conventional methods used for heavy metal removal from water include adsorption on activated carbons, precipitation, use of ion exchange resins and membrane filtration. In the case of a heavy metal contaminated soil, soil removal and landfilling, physicochemical extraction, stabilization/solidification, soil washing, phytoremediation and bioremediation are usually applied. However, lately there is an intense interest regarding heavy metal immobilization using biochars in waters and soils.
Biochar is a carbon rich, solid by-product resulting from the pyrolysis of biomass under oxygen-free and low temperature conditions. Biochar’s proven ability to remain stable against chemical and biological degradation, when applied to soils, makes it a pioneer means of mitigating climate change. In addition, biochar can improve soil productivity, not only because it may be a valuable nitrogen and phosphorous source, but also it affects soil cation exchange capacity, pH and retention of water and nutrients. Finally, biochar has the potential to restore and remediate contaminated soils as it can adsorb both organic and inorganic pollutants. Biochar’s ability to adsorb heavy metals is possibly attributed to electrostatic interactions between carbon negative surface charge and metal cations, as well as to exchange of ions between biochar surface protons and metallic cations. In addition, the presence of mineral impurities (e.g. ash and metal oxides), acidic oxygen groups (e.g. carboxylic and lactonic groups) and basic nitrogen groups could further enhance the adsorption capacity of carbonaceous materials.
The use of biochar, as a cost effective sorbent for heavy metal removal from contaminated water and soils, has already been re-ported by many researchers. The majority of studies are focused on the immobilization of metal cations, such as Pb, Cu, Ni and Cd, while limited research has been conducted on As(V) and Cr(VI) removal by biochar. For instance, Mohan et al. (2007) studied As(III) removal from water by woody biomass-derived biochars, showing that oak bark char has a significant potential for As(III) adsorption, whereas Beesley et al. (2011) and Gomez-Eyles et al. (2011) dealt with As immobilization in multi-element contaminated soils. In addition, Shen et al. (2012), Dong et al. (2011) and Mohan et al. (2011) studied the use of biochars for Cr(VI) removal from water and found that biochar can efficiently adsorb chromate, while its maximum sorp-tion capacity was, in some cases, 123 mg/g.