Multi-cycle aqueous arsenic removal by novel magnetic N/S-doped hydrochars activated via one-pot and two-stage schemes

Abstract Arsenic attracts worldwide concern. Novel hydrochars derived from dairy cattle manure and co-activated by thiourea and Fe(NO3)3 via one-pot and two-stage schemes were innovatively fabricated to adsorb aqueous As(V) in multi-cycles. Thiourea introduced oxygen/sulfur/nitrogen functional groups, and Fe(NO3)3 served as iron composition precursors. From one-pot scheme, ample functional groups were obtained. Magnetic γ-Fe2O3 were uniformly aggregated as iron-centered clusters onto internal carbon matrix due to self-generated pressure and complicated interactions, providing ample Fe-O for As(V) adsorption. As(V) reacted with Fe and generated stable Fe-As compositions. From two-stage scheme, precursory compact textures were rebuilt into hierarchical structures with sponge-like constructions, spherical nanoparticles, enlarged surface areas, and micro/mesopores, transferring As(V) into internal matrix. In-situ Fe3+ was partially reduced to Fe0 by carbon matrix and reductive gases, and deposited on external surfaces at atmospheric pressure. As(V) was not reduced by Fe0 due to multi-layers and oxidative functional groups. Two-stage-synthesized hydrochars were better for one-round adsorption, but worse for multi-adsorptions due to lower stability. Maximum one-round adsorption capacity and partition coefficient were 98.74 mg/g and 85.67 mg/(g μM), obviously superior to previous adsorbents. Physisorption and chemisorption were proposed to respectively dominate initial and subsequent adsorption. Ten extra consecutive adsorption-desorption indicated NaOH was better for As(V) desorption than HNO3. One-pot-synthesized hydrochars with high stability and desorption rates were superior for multi-adsorptions especially in early cycles. Maximum accumulative adsorption amount was up to 235.86 mg/g. This study innovatively explored As(V) multi-adsorption by activated hydrochars and provided deep insights into two activation schemes with scientific potentials.