Effect of pH on the adsorptive and cycling performance of amphoteric clay-loaded biochar

Abstract

To investigate the effect of solution pH on the adsorption and reuse of carbon-based compos-ites, xerophilic and hydrophytic Alternanthera philoxeroides biochars (XAp-C and HAp-C) were loaded with 0%, 50%, and 100% dodecyl dimethyl betaine (BS-12)-modified bentonite (B, 50BS-B, and 100BS-B) to form Ap-C/B, Ap-C/50BS-B, and Ap-C/100BS-B, respectively. Then, the isother-mal adsorption and morphological changes of Cu2+ and cycling performance by the tested materials under different pH gradients were studied with XAp-C and HAp-C as the control. Results revealed the following. (1) The adsorption isotherms of Cu2+ for the tested materials were all consistent with the Langmuir model, and the maximum adsorption amount remained between 100.82–286.31 mmol/kg. Under the same conditions, the adsorption amount of XAp-C/BS-B was better than that of HAp-C/BS-B, and that of XAp-C/100BS-B was the best. (2) Thermodynamic parameter analysis showed that Cu2+ adsorption of the tested materials was a spontaneous, endo-thermic, and entropy-increasing reaction process. Kinetic fitting further showed that Cu2+ adsorp-tion conformed to the pseudo-second-order kinetic model. (3) Within pH 2–6, the amount of Cu2+ adsorbed onto the tested materials all initially increased and then decreased, and the value at pH 5 was the best. (4) The Cu adsorbed onto the tested materials primarily existed in exchangeable and organic matter forms, followed by the carbonate form and then the Fe–Mn oxide form. With increased pH, Cu in carbonate form gradually transformed into the organic matter form, whereas Cu in exchangeable and Fe–Mn oxide forms remained basically unchanged. In sum-mary, using XAp-C/100BS-B and adjusting the pH to 5 was the most favorable for Cu2+ adsorption, and the adsorption amount was 62.6% of the raw material after three times of regeneration.