Application of Fe-Al layered double hydroxides on silica for phosphate and arsenate removal from water

In present study FeAl-LDH@SiO2 adsorbent was used for phosphate and arsenate removal from aqueous solutions. Silica, obtained from rice husks, was used as a substrate for the deposition of FeAl based layered double hydroxides (FeAl-LDH) particles. Synthesis of FeAl-LDH, obtained at Fe/Al molar ratio of 3:1, was performed via co-precipitation and concomitant deposition on SiO2 carrier at 1:1, 2:1, and 3:1 LDH/silica mass ratios. The prepared materials were characterized by scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), N2 adsorption/desorption isotherms (BET method) and Fourier transform infrared spectroscopy (FTIR). SEM analysis revealed uniform precipitate of LDH on SiO2 substrate, while XRD analysis confirmed crystallographic structure of LDH deposit. Optimization of adsorption properties of synthesized adsorbents confirmed best performances of FeAl-LDH@SiO2, obtained at 3:1 LDH/silica mass ratio. Thus, adsorption experiments were carried out with respect to adsorbent dosage, phosphate and arsenate concentration at initial pH 4. Phosphate concentrations were determined using spectrophotometer and arsenate concentrations were determined using atomic absorption spectrometer. The results showed that the maximum sorption capacities for phosphates was 62.2 mg g-1, and for arsenate was 59.0 mg g-1. Kinetic study confirmed intra-particle diffusion as rate-limiting step, while thermodynamic study indicates feasible and spontaneous process.