Latest trends in heavy metal removal from wastewater by biochar based sorbents

Biochar, a solid carbon-rich residue is derived from wide variety of abundantly available raw materials like forest and agricultural waste, industrial by products and waste, and municipal solid waste through pyrolysis. Besides enhancing soil fertility thereby agricultural productivity and reducing greenhouse gas emissions, biochar based sorbents are widely being employed in waste water treatment owing to their attributes viz., high porosity, large surface area, affinity towards metals, surface modifications, stability, recyclability and even safe disposal. In this comprehensive review, latest trends w.r.t. synthesis protocols, modifications, mechanisms, metal removal comparison, thermo-kinetics, mechanisms, regeneration and safe disposal methods have been presented and discussed. Synthesis through slow pyrolysis (300-700 degrees C, 0.01-2 degrees C/s), nanoscale modifications with metal oxides especially based on iron to magnetise the biochar, regeneration through acids and complexation agents are found to be highly promising. While ion exchange and electrostatic interactions are found to be common mechanisms of metal removal which in turn depends on nature of biomass, process conditions and metal, it is found that modifications and mechanisms are strongly interrelated. From our studies, biochar based sorbents gave heavy metal adsorption capacities in mg/g of 1217 (Ag), 560 (Pb), 288 (Cu), 216 (Cd), 204 (As), 130 (Cr), 58 (Ni), 48 (Hg) etc under optimal conditions. Though the nature of kinetic models and isotherms do depend on the material and process conditions, pseudo kinetic models (first and second order) along with Langmuir and Fruendlich isotherms were most widely reported. Disposal of spent biochar could be using it in construction (cement and bricks) and electronic (super-capacitors) industries besides phytoremediation.