Chitosan is a biopolymer derived from chitin, which is the second most abundant and renewable polymer in nature after cellulose. Low-molecular-weight chitosan (LMWC) is the degradation product of chitosan through depolymerization. Compared with conventional chitosan, LMWC is considered as one of the most promising functional materials due to its characteristics of lower polymerization degree, lower viscosity, good water solubility, reactivity and degradability. This review focused on the preparation and characterization methods as well as the application in environmental remediation of LMWC. The three main methods of LMWC preparation including chemical, physical and enzymatic methods were summarized and compared in this paper. The mechanism, advantages and disadvantages of various preparation methods were also discussed. In addition, the applications of LMWC in environmental fields such as water treatment, soil remediation and air purification were briefly reviewed. With the continuous progress of science and technology and the improvement of environmental awareness, it is believed that more efficient, economical and environmentally friendly chitosan degradation methods will be developed, providing strong support for the wide application of LMWC in the field of environmental protection.

Low C/N wastewater results from a wide range of factors that significantly harm the environment. They include insufficient carbon sources, low denitrification efficiency, and ${\mathrm{NH}}_4^{+}$-N concentrations in low C/N wastewater that are too high to be treated. In this research, the membrane biofilm reactor and hydrogen-based membrane biofilm reactor (MBR-MBfR) were optimized and regulated under different operating parameters: the simulated domestic sewage with low C/N was domesticated and the domestic sewage was then denitrified. The results of the MBR-MBfR experiments indicated that a C/N ratio of two was suitable for ${\mathrm{NH}}_4^{+}$-N, ${\mathrm{NO}}_2^{-}$-N, ${\mathrm{NO}}_3^{-}$-N, and chemical oxygen demand (COD) removal in partial nitrification-denitrification (PN-D) and hydrogen autotrophic denitrification for further treatment. The steady state for domestic wastewater was reached when the MBR-MBfR in the experimental conditions of HRT = 15 h, SRT = 20 d, 0.04 Mpa for H₂ pressure in MBfR, 0.4–0.8 mg/L DO in MBR, MLSS = 2500 mg/L(MBR) and 2800 mg/L(MBfR), and effluent concentrations of ${\mathrm{NH}}_4^{+}$-N, ${\mathrm{NO}}_3^{-}$-N, and ${\mathrm{NO}}_2^{-}$-N were 4.3 ± 0.5, 1.95 ± 0.04, and 2.05 ± 0.15 mg/L, respectively. High-throughput sequencing results revealed the following: (1) The genus Nitrosomonas as the ammonia oxidizing bacteria (AOB) and Denitratisoma as potential denitrifiers were simultaneously enriched in the MBR; (2) at the genus level, Meiothermus,Lentimicrobium, Thauera,Hydrogenophaga, and Desulfotomaculum played a dominant role in leading to ${\mathrm{NO}}_3^{-}$-N and ${\mathrm{NO}}_2^{-}$-N removal in the MBfR.

A biochar composite (SSBC) was prepared with sorghum straw and sewage sludge as raw materials by modification with Na₂CO₃ for the removal of sulfamethoxazole (SMX) from water.

Soil washing is an important method for the remediation of contaminated soil. This research presents the optimization of soil washing conditions in the remediation of Zn-contaminated soils with water-soluble chitosan (WSCS). Response surface methodology (RSM) was used to optimized the washing conditions after single factor experiments. The central composite design (CCD) with three factors and five levels was applied to the optimization of the removal efficiency of Zn from soils, and WSCS concentration, pH value, and washing time were evaluated variables in the washing process. Results indicated that the pH value (p < 0.0001) was the most significant factor which mainly affected the distribution and content of metal species in aqueous solution, ion exchange and adsorption/desorption behavior of metals, solubility of chelating agent, as well as readsorption of metal complexes. The optimal conditions for the Zn removal from soils were WSCS concentration of 1.5%, pH of 3.3, and washing time of 72 min. The removal efficiency could reach 65.4% under the optimized conditions, which was close to the predicted value of 68.3% by the response surface method. Therefore, it could be found that the response surface methodology was an effective method to determine the optimal conditions for the removal of metals from contaminated soils by soil washing.

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.