This study details the preparation of Fe-Mn binary oxide/mulberry stem biochar composite adsorbent (FM-MBC) from mulberry stems via the multiple activation by potassium permanganate, ferrous chloride, triethylenetetramine, and epichlorohydrin. The characteristics of FM-MBC had been characterized by SEM-EDS, BET, FT-IR, XRD, and XPS, and static adsorption batch experiments such as pH, adsorption time, were carried out to study the mechanism of Cr(VI) adsorption on FM-MBC and the impact factors. The results indicated that in contrast with the mulberry stem biochar (MBC), the FM-MBC has more porous on surface with a BET surface area of 74.73 m²/g, and the surface loaded with α-Fe₂O₃ and amorphization of MnO₂ particles. Besides, carboxylic acid, hydroxyl, and carbonyls functional groups were also formed on the FM-MBC surface. At the optimal pH 2.0, the maximum adsorption capacity for Cr(VI) was calculated from the Langmuir model of 28.31, 31.02, and 37.14 mg/g at 25, 35, and 45 °C, respectively. The aromatic groups, carboxyls, and the hydroxyl groups were the mainly functional groups in the adsorption of Cr(VI). The mechanism of the adsorption process of FM-MBC for Cr(VI) mainly involves electrostatic interaction, surface adsorption of Cr(VI) on FM-MBC, and ion exchange.

Phytoextraction using Celosia argentea Linn. by Mn pretreatment can potentially decontaminate Cd-contaminated soils. However, the mechanism that accelerates the Cd bioaccumulation is still unknown. In order to study the effect and mechanism of Mn pretreatment on Cd bioaccumulation in C. argentea, the hydroponic experiments were set to determine the chlorophyll content, antioxidant enzyme activity, malondialdehyde content, and root exudation of C. argentea. The results indicated that after seven days of Mn pretreatment, both the biomass and Cd concentrations in plants increased compared to the control group. One of the mechanisms for this was the improvement in the physiological resistance of C. argentea following pretreatment with Mn. Compared with Cd stress alone, Mn pretreatment increased photosynthesis and reduced membrane lipid peroxidation. Meanwhile, the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) were significantly reduced in leaves of C. argentea after Mn pretreatment through the reduction in the production of reactive oxygen species. In addition, Mn promoted the exudation of organic acids in the roots of C. argentea. The contents of citric and malic acids increased by 55.3% and 26.4%, respectively, which may be another important reason for Mn pretreatment increasing Cd bioaccumulation in C. argentea. Therefore, the present work shows that the pretreatment of seedlings with Mn can provide a meaningful strategy to improve the remediation efficiency of Cd-contaminated soils by C. argentea.

In order to identify an efficient and environmentally friendly washing agent for heavy metal-contaminated soil, this study selected seven natural amino acids—arginine (Arg), alanine (Ala), glycine (Gly), cysteine (Cys), lysine (Lys), threonine (Thr), and glutamic acid (Glu)—based on their water solubility, effectiveness, and functional group characteristics. According to the removal efficiencies for zinc (Zn), lead (Pb), and cadmium (Cd), Cys, which contains a specific sulfhydryl group (-SH), was chosen as the target leaching agent for the remediation of composite-contaminated soil. The optimal process conditions were determined as follows: 0.02 mol/L of cysteine concentration, liquid-to-soil ratio of 20:1 (mL/g), 10 min of ultrasonic time, and pH = 8.0. The order of removal efficiency was Pb (40.8%) > Zn (21.6%) > Cd (19.9%). The leaching process selective effects on the speciation fractions of Zn, Pb, and Cd in the soil, and these differences can be explained by the hard–soft acid–base theory and the strength of coordination between the metals and cysteine. Mechanism analysis revealed that soil washing essentially achieves selective extraction of the target metal through strong chemical interactions between functional groups of cysteine and active metal sites of secondary minerals in the soil. Cysteine is a green remediation agent with high selectivity and environmental compatibility for contaminated soil. Its application requires precise design and risk assessment based on the chemical properties of the target metals, while ensuring the sustainability of the soil to maintain the ecological functions and long-term health of the remediated soil.