Adsorption of Pb(II) from aqueous environment on artificially synthesized mordenite and its biochar composite

Biochar was used to enrobe artificially synthesized mordenite by co-pyrolysis, and the performance of the composite in removing lead from dilute solutions was compared with that of the mordenite alone. The optimal parameters for the hydrothermal synthesis of mordenite were as follows: crystallization temperature of 180℃, crystallization time of 4 days, pH value of 11, and aging time of 24 h. The optimal mass ratio of mordenite to biochar was 20:1, and the pyrolysis temperature was 500℃. Adsorption equilibrium was reached within 3 h, yielding a total average lead removal of 78.9 % for mordenite alone and 98.6 % for the composite. Pseudo-second-order kinetics and Langmuir isotherms adequately described the data. The predicted maximum adsorption capacities were 156.6 mg/g for the mordenite alone and 283.4 mg/g for the composite. When the initial concentration of Pb(Ⅱ) was 80 mg/L, the optimum doses were determined to be 6–7 g/L for the mordenite and 5–6 g/L for the composite. At pH less than 7, mordenite removed up to 80 % of the lead and the composite removed between 90 % and 100 %. Coexisting ions had little effect on mordenite, but K(I), Ca(II), and Mg(II) could significantly increase the adsorption efficiency by up to 5.7 % for the composite. Furthermore, tannic acid and fulvic acid showed a promoting effect. Adsorption mechanisms were proposed based on the characterization results of the materials before and after adsorption, batch experiments, and density functional theory calculations. Specifically, mordenite adsorption was found to be based primarily on electrostatic attraction, surface complexation and ion exchange. Adsorption by the composite also involved π-electronic interaction and coprecipitation. In general, the composite exhibited better adsorption performance than mordenite alone.