Humic acid residue (HAS), as a waste product from the extraction of humic acid from low-rank coal such as weathered coal and lignite, is characterized by its abundant yield and low price. However, the low efficiency and high pH value of HAS in passivating heavy metals limit their application in the passivation of heavy metals in soils. To improve the passivation efficiency of humic acid residues against heavy metals, in this study, modified humic acid residues (N-HAS) were prepared by pretreatment with HNO 3 and modification with NH 4 H 2 PO 4 . The results showed that the passivation efficiency of N-HAS was up to 87.75% and 57.07% for Hg and Pb in the effective state, and the adsorption kinetics were under the pseudo-second-order model. The addition of N-HAS to the soil resulted in a change in the morphology of the heavy metals, with the acid extractable state of Hg and Pb reduced by 66.92% and 30.44% respectively, and the results of the Toxicity Leaching Test (TCLP) showed that the addition of N-HAS significantly reduced the leaching toxicity and potential ecological risk index (RI). The soil physicochemical properties changed after the addition of N-HAS and there was a correlation between the soil physicochemical properties and the effective state of Hg and Pb content. In conclusion, N-HAS has a positive effect on the remediation of heavy metal-contaminated soil and has a broad application prospect.
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.
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