Effects of phosphorus and manganese modified apple pomace biochar on soil characteristics and its immobilization effect on Cd in polluted soil

Environmentally friendly and efficient biochar precursor materials and their modification methods are of great significance in solving the problem of soil cadmium pollution.

This study prepared apple pomace biochar (ABC) from discharged apple pomace by pyrolysis at 500 °C. ABC was modified by phosphoric acid impregnation (P-ABC), MnO₂ loading (Mn-ABC), and MnO₂ loading after phosphoric acid impregnation (Mn-P-ABC). The biochar and modified biochar were described by infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The impacts of original and modified biochar on soil characteristics and the immobilization of original and modified biochar with different additions (4, 7, 10 and 13%) on Cd in contaminated soils were studied.

The outcomes show that ABC, P-ABC, Mn-ABC, and Mn-P-ABC are porous and amorphous materials rich in functional groups. All four types of biochar can enhance soil organic matter content, cation exchange capacity, and pH value, effectively improving soil properties. After application of biochar materials, the extractable Cd in soil was obviously reduced compared to the control, and 13% Mn-P-ABC reduced the extractable Cd in the soil by 43.84%. The results of Cd speciation analysis indicate that after the application of biochar materials, the acid soluble Cd in the soil reduced and the residual Cd increased.

The treatment with 13% Mn-P-ABC resulted in a 35.57% decrease in acid soluble Cd in soil and a 30.18% increase in residual Cd, indicating the best immobilization effect on Cd.