Simultaneous removal of cadmium(II) and arsenic(III) from aqueous solutions using pyrite-modified biochar: Effects and mechanisms

Abstract

Simultaneous removal of cadmium [Cd(II)] and arsenic [As(III)] from aqueous systems is still a great challenge. Through mechanical ball milling and thermal activation, pyrite-modified biochar (PMB) was successfully prepared and used to evaluate its synchronous removal effect and mechanism of Cd(II) and As(III) from aqueous solutions. PMB exhibited a maximum adsorption capacity of 51.0 mg·g−1 for Cd(II) (pH=6) and 65.9 mg·g−1 for As(III) (pH=4) at 25 °C, surpassing that of biochar by 1.5 and 5.7 times, respectively. Interestingly, in the mixed Cd-As system, the presence of Cd(II) hindered the adsorption of As(III) onto PMB, while the presence of As(III) enhanced the adsorption of Cd(II). Additionally, the removal of Cd(II) and As(III) by PMB remained in the range of 70%− 80% under the competition of cations (K+, Mg2+) and anions (NO3-, SO42-), except for the severe inhibition of Cd(II) and As(III) removal by Ca2+ and PO43- due to their specific physicochemical properties. The adsorption of Cd(II) by PMB followed the Langmuir and pseudo-second-order models, involving ion exchange (formation of CdS) and complexation as the main removal mechanisms. And the adsorption of As(III) followed the Freundlich and pseudo-second-order models, involving oxidation of As(III) to arsenic(V) and further complexation as the primary removal mechanisms. After four adsorption-desorption cycles, the adsorption performance of PMB for Cd(II) and As(III) could still reach 78% and 68% of the initial performance. Overall, the results demonstrated that PMB was the preferred eco-friendly material for efficiently removing Cd(II) and As(III) from co-pollution aqueous solutions.