Adsorption of lead ions from aqueous solution using NH4H2PO4 modified humic acid residue

Humic acid (HA) has led to promising developments in the improvement of soil remediation. The low utilization efficiency of the residue produced in the proceeding of HA production causes environmental pollution and the waste of resources. The humic acid residue (HAS) was used as the feedstock for the preparation of an NH4H2PO4 modified material (NH4H2PO4-HAS) adsorbent for the adsorption of Pb2+ in aqueous solutions. Response surface methodology (RSM) using the Box-Behnken design(BBD) to investigate the effects of concentration, temperature, and volume on the adsorption effectiveness of Pb2+ by the modified material. The optimal parameters of the modified materials are as follows: concentration of NH4H2PO4 3%volume 27.7 mL, temperature 71.8 °C. The adsorption equilibrium shows a well fit with the Langmuir isotherm model. The maximum adsorption amounts of Pb2+ by HAS and NH4H2PO4-HAS composites were 272.185 mg/g and 343.166 mg/g, respectively; the kinetic adsorption data of HAS and NH4H2PO4-HAS exhibit a well fit with the pseudo-second-order model. The desorption of NH4H2PO4-HAS was less than that of HAS at different temperatures and pH. The thermodynamic parameters, ΔHº, ΔGº, and ΔSº, obtained from the isotherm data indicate that the process of Pb2+ adsorption by HAS and NH4H2PO4-HAS is spontaneous and the uptake of Pb2+ by HAS and NH4H2PO4-HAS composite is more favored at high temperatures. The adsorption mechanism of NH4H2PO4-HAS on Pb2+ mainly includes physical adsorption, surface complexation adsorption, and precipitation.