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

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.