Investigating the potential of synthetic humic-like acid to remove metal ions from contaminated water.

Humic acid can effectively bind metals and is a promising adsorbent for remediation technologies. Our studies initially focussed on Cu2+ as a common aqueous contaminant. Previous studies indicate that carboxylic groups dominate Cu2+ binding to humic acid. We prepared a synthetic humic-like acid (SHLA) with a high COOH content using catechol (0.25 M) and glycine (0.25 M) with a MnO2 catalyst (2.5% w/v) at pH = 8 and 25 °C and investigated the adsorption behaviour of Cu2+ onto it. The SHLA exhibited a range of adsorption efficiencies (27%-99%) for Cu2+ depending on reaction conditions. A pseudo-second-order kinetic model provided the best fit to the experimental data (R2  = 0.9995-0.9999, p ≤ 0.0001), indicating that chemisorption was most likely the rate-limiting step for adsorption. The equilibrium adsorption data showed good fits to both the Langmuir (R2  = 0.9928-0.9982, p ≤ 0.0001) and Freundlich (R2  = 0.9497-0.9667, p ≤ 0.0001) models. The maximum adsorption capacity (qm ) of SHLA increased from 46.44 mg/g to 58.78 mg/g with increasing temperature from 25 °C to 45 °C. Thermodynamic parameters (ΔG0  = 2.50-3.69 kJ/mol; ΔS0  = 0.06 kJ/(mol·K); ΔH0  = 15.23 kJ/mol) and values of RL (0.0142-0.3711) and n (3.264-3.527) show that the adsorption of Cu2+ onto SHLA was favourable, spontaneous and endothermic in nature. Over six adsorption/desorption cycles using 0.5 M HCl for the desorption phase, there was a 10% decrease of the adsorption capacity. A final experiment using a multi-metal solution indicated adsorption efficiencies of up to 84.3-98.3% for Cu, 86.6-98.8% for Pb, 30.4-82.9% for Cr, 13.8-77.4% for Ni, 9.2-62.3% for Cd, 8.6-51.9% for Zn and 4.6-42.1% for Co. Overall, SHLA shows great potential as an adsorbent to remove metals from water and wastewater.