Characteristics and mechanisms of nickel adsorption on biochars produced from wheat straw pellets and rice husk

The adsorption characteristics and mechanisms of Ni2+ on four-standard biochars produced from wheat straw pellets (WSP550, WSP700) and rice husk (RH550, RH700) at 550 and 700 °C, respectively, were investigated. The kinetic results show that the adsorption of Ni2+ on the biochars reached an equilibrium within 5 min. The increase of the solid to liquid ratio resulted in an increase of Ni2+ removal percentage but a decrease of the adsorbed amount of Ni2+ per weight unit of biochar. The Ni2+ removal percentage increased with the increasing of initial solution pH values at the range of 2-4, was relatively constant at the pH range of 4-8, and significantly increased to ≥98% at pH 9 and stayed constantly at the pH range of 9-10. The calculated maximum adsorption capacities of Ni2+ for the biochars follow the order of WSP700 > WSP550 > RH700 > RH550. Both cation exchange capacity and pH of biochar can be a good indicator of the maximum adsorption capacity for Ni2+ showing a positively linear and exponential relationship, respectively. This study also suggests that a carefully controlled standardised production procedure can make it reliable to compare the adsorption capacities between different biochars and investigate the mechanisms involved.