Mg(OH)₂-MgO@reduced graphene oxide nanocomposites: The roles of composition and nanostructure in arsenite sorption

Nanostructured MgO has been identified as a promising material for As(iii) removal, however its application is hindered by one engineering drawback (nanoparticles cannot be directly used in wastewater treatment) and its performance is limited by two unanswered scientific questions: (1) which composition is the optimized one for As(iii) sorption, MgO, Mg(OH)₂ or a mixed composition? (2) If the sorption of As(iii) relies on the formation of Mg(OH)₂ after hydration of MgO, why does nanostructured Mg(OH)₂ have generally low As(iii) sorption performance? To address the challenges, we report the synthesis of Mg(OH)₂-MgO@rGO nanocomposites with tunable compositions for arsenic sequestration for the first time. It is demonstrated that the nanocomposite with an optimized composition of Mg(OH)₂-MgO has an exceptionally higher As(iii) sorption capacity than pure Mg(OH)₂ or MgO composition, reaching a maximum arsenite sorption capacity of 681.3 mg g^-1 (equal to 866.0 mg g ^-1 As(iii)/MgO). It is revealed that only the adsorbent with a balanced composition and reduced crystallinity generates interconnected Mg(OH)₂ nanosheets in situ during the sorption process, providing abundant and accessible sites for As(iii) sorption with enhanced sorption capacity. The rGO in the nanocomposite has multiple functions, including (1) reducing the size and crystallinity of Mg(OH)₂-MgO, (2) favoring the formation of interconnected Mg(OH)₂ nanosheets and (3) facilitating the oxidation of As(iii) partially into As(v), all beneficial for enhanced arsenic sorption capacity. The Mg(OH)₂-MgO@rGO nanocomposite has been applied to treat As(iii) contaminated water in a household water treatment device with excellent performance. Our findings have provided new understanding in the rational design of highly efficient nanocomposite adsorbents for arsenite removal applications.