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 NH 4 H 2 PO 4 modified material (NH 4 H 2 PO 4 -HAS) adsorbent for the adsorption of Pb 2 + 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 Pb 2 + by the modified material. The optimal parameters of the modified materials are as follows: concentration of NH 4 H 2 PO 4 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 Pb 2 + by HAS and NH 4 H 2 PO 4 -HAS composites were 272.185 mg/g and 343.166 mg/g, respectively; the kinetic adsorption data of HAS and NH 4 H 2 PO 4 -HAS exhibit a well fit with the pseudo-second-order model. The desorption of NH 4 H 2 PO 4 -HAS was less than that of HAS at different temperatures and pH. The thermodynamic parameters, Δ Hº, Δ Gº, and Δ Sº, obtained from the isotherm data indicate that the process of Pb 2 + adsorption by HAS and NH 4 H 2 PO 4 -HAS is spontaneous and the uptake of Pb 2 + by HAS and NH 4 H 2 PO 4 -HAS composite is more favored at high temperatures. The adsorption mechanism of NH 4 H 2 PO 4 -HAS on Pb 2 + mainly includes physical adsorption, surface complexation adsorption, and precipitation.