Improved lead removal from aqueous solution using novel porous bentonite - and calcite-biochar composite

Biochar-mineral (bentonite/calcite) composite (BC-CM) prepared at different temperatures were tested under varied conditions for effective removal of lead (Pb) from aqueous solution. With increasing pyrolysis temperature, increased surface area, pore volume, bentonite decomposition and less or no decomposition of calcite occurred. Bentonite-biochar (BCS) and calcite-biochar (CCS) prepared at 700 °C were found most suitable for efficient removal of Pb (99.9%). Bentonite and calcite acted as catalyst and contributed to changes in yield, pH, texture, functional groups, minerals and carbonization that facilitated efficient Pb removal by BCS 700 and CCS 700. Pb concentration, pH, dose of BCS and CCS, and contact time were further optimized using response surface methodology (RSM) for maximizing removal percentage (R%) of Pb and adsorption capacity (qt). Both BCS 700 and CCS 700 showed similar effects (positive/negative) of factors on R% and qt. Under optimized conditions, 0.21 g of BCS 700 effectively removed 99.2% of 431 mg/L in 3.6 h at solution pH of 4.2, while 0.07 g CCS 700 removed 97.06% of 232 mg/L in 3.5 h at 5.5 pH. Removal of Pb onto both BCS and CCS was by monolayer adsorption with maximum adsorption capacity of 500 mg/g. Rapid Pb removal was observed within 2 h of contact time (CCS 700 > BCS 700) and equilibrium was achieved within 10 h. BCS 700 followed first order and CCS 700 followed second order kinetic model. Electrostatic attraction between Pb ions and mineral groups present in BCS 700 and CCS 700 also played important role in Pb removal. This study clearly demonstrated that composite of biochar with bentonite or calcite under optimized conditions significantly improved Pb removal and adsorption capacity that can be further utilized for larger scale applications.