The potential of ferrihydrite-synthetic humic-like acid composite to remove metal ions from contaminated water: Performance and mechanism

Abstract

Ferrihydrite-natural humic acid composite (FH-NHA) has been recently applied as an adsorbent for toxic metal ions removal. However, the structural characteristics of natural humic acids are various and uncontrollable due to the different sources, which may lead to the fluctuant adsorption capacities of FH-NHA and restrict its practical application. In this work, an optimized synthetic humic-like acid (SHLA) with controllable structural characteristics was synthesized by the abiotic humification process, and a novel synthetic method for ferrihydrite-synthetic humic-like acid composite (FH-SHLA) has been proposed. FH-SHLA will be expected to show a better adsorption ability and more controllable structural characteristics compared to the FH-NHA. The equilibrium adsorption data showed the best fit to the pseudo-second-order kinetics (0.9998 ≤R2 ≤1.0000; P ≤ 0.0001) and Langmuir models (0.9897 ≤R2≤0.9997; P ≤ 0.0001), suggesting a monolayered chemisorption process of both Pb2+ and Cd2+ onto FH-SHLA. At 25 °C, the maximum adsorption capacities (Qm) of Pb2+ and Cd2+ were 87.41 and 47.10 mg/g, respectively, which were 41.6% and 109.6% higher than those of FH-NHA, indicating that adsorption capacities of FH-SHLA were enhanced tremendously. The thermodynamic parameters (ΔH°>0, ΔG°<0 and ΔS°>0), RL values (0.003–0.250) and n (3.34–6.72) indicated that the adsorption processes of both Pb2+ and Cd2+ onto FH-SHLA were endothermic, spontaneous and favourable. The adsorption mechanisms of Pb2+ and Cd2+ onto FH-SHLA involved surface complexation, cation-π interaction and electrostatic attraction. Overall, FH-SHLA is a promising adsorbent to remove heavy metal ions from contaminated water.