Effect mechanism of litter extract from Alternanthera philoxeroides on the selective absorption of heavy metal ions by amphoteric purple soil

Abstract

Plant litter causes a serious waste of resources. Thus, plant litter extract (LE) should be used in the soil remediation of heavy metals. In this study, different proportions of LE from the Alternanthera philoxeroides were used to modify dodecyl dimethyl betaine (BS)-modified purple soil (P). The basic physicochemical properties of LE + BS-modified Ps (LE + BS-Ps) were determined, and the microscopic morphology of LE + BS-Ps was studied by using scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, and specific surface area detection. The isothermal adsorption characteristics of heavy metal ions (Pb²⁺, Cu²⁺, and Cr⁶⁺) on different LE + BS-Ps were investigated by the batch method, and the effect mechanisms of temperature, pH, ionic strength, and LE + BS-P's property were compared. Results showed that the cation exchange capacity and specific surface area of LE + BS-Ps increased, pH of LE + BS-Ps decreased, and TOC of LE + BS-Ps increased first and then decreased with increasing proportion of LE. FTIR, SEM, and EDS results proved that LE was modified on the surface of BS-P. Langmuir and Freundlich models could be used to describe the adsorption isotherms of heavy metal ions on different LE + BS-Ps, and the fitting correlation of the Langmuir model was higher than that of the Freundlich model. The maximum adsorption capacity (q_m) of Pb²⁺, Cu²⁺, and Cr⁶⁺ were 107.60-295.66, 133.00-342.11, and 33.59-75.41 mmol/kg, respectively. The q_m of Pb²⁺, Cu²⁺, and Cr⁶⁺ on LE + BS-Ps all increased first and then decreased with increasing proportion of LE, and the peak value was observed in 20%LE + BS-Ps. High pH improved Pb²⁺ and Cu²⁺ adsorption but inhibited Cr⁶⁺ adsorption. The adsorption amounts of Pb²⁺, Cu²⁺, and Cr⁶⁺ all increased first and then decreased with incresing ionic strength and were maintained at the maximum value of 0.1-0.2 mol/L. The Pb²⁺, Cu²⁺, and Cr⁶⁺ adsorption mechanisms on different LE + BS-Ps showed a positive temperature effect and presented spontaneous, exothermic and entropy-adding processes.

Keywords: Adsorption amount; Dodecyl dimethyl betaine; Heavy metal; Litter extract; Purple soil.