Synthesis of Fe3O4@rice husk biochar for As(V) remediation by adsorption: Is sodium silicate formation involved?

Rice husks (RH) were pretreated by premixing with aqueous FeCl 3 and 4 M NaOH at pH 10, then pyrolyzed at 600 °C for 1 h to generate Fe 3 O 4 -deposited on biochar, Fe 3 O 4 @RHBC, used to remediate aqueous As(V). The iron oxide phase was mostly Fe 3 O 4 after pyrolysis with small amounts of Fe 2 O 3 and α-FeOOH. Exposing RH to aqueous NaOH raises the possibility that RH's SiO 2 content could generate sodium silicate, affecting its adsorption behavior. However, we demonstrate the absence of evidence for detectable sodium silicate formation on Fe 3 O 4 @RHBC as probed by FT-IR, XPS, and PZC studies. Since many silica-rich biochars have been pretreated with alkali, we were prompted to examine silica-to-silicate transformations by premixing 4 M NaOH with RH, without iron oxide modification, and heating to a series of individual temperatures, from 100 to 700 °C. Each sample was analyzed by FT-IR to determine if sodium silicate had formed from silica. As(V) uptake was determined as a function of pH, adsorbate-adsorbent contact time, initial As(V) concentration, adsorbent dose, and temperature. Optimized As(V) adsorption occurred at pH 5. As(V) adsorption kinetics on Fe 3 O 4 @RHBC best fit the pseudo-second-order model, and adsorption isotherms (5, 25, and 40 °C) were endothermic and fit the Langmuir isotherm model, with a maximum adsorption capacity of 5.56 mg/g at 25 °C. Competitive PO 4 3− , SeO 4 2− , MoO 4 2− , Cr 2 O 7 2− , CO 3 2− , and F − adsorption during As(V) removal was investigated. The As recovery from the exhausted Fe 3 O 4 @RHBC by aqueous NaOH stripping gave ∼9.0% and ∼13.8% reductions in adsorption capacity after three batch As(V) regenerations.