Publication date: 1 September 2018
Source:Science of The Total Environment, Volume 635
Author(s): Ting Yang, Mark E. Hodson
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.
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