In the field of environmental science and engineering, microorganisms, enzymes and algae are promising biomass materials that can effectively degrade pollutants. However, problems such as poor environmental adaptability, recycling difficulties, and secondary pollution exist in the practical application of non-immobilized biomass materials. Biomass immobilization is a novel environmental remediation technology that can effectively solve these problems. Compared with non-immobilized biomass, immobilized biomass materials have the advantages of reusability and stability in terms of pH, temperature, handling, and storage. Many researchers have studied immobilization technology (i.e., methods, carriers, and biomass types) and its applications for removing refractory organic pollutants. Based on this, this paper reviews biomass immobilization technology, outlines the mechanisms and factors affecting the removal of refractory organic pollutants, and introduces the application of immobilized biomass materials as fillers for reactors in water purification. This review provides some practical references for the preparation and application of immobilized biomass materials and promotes further research and development to expand the application range of this material for water purification.

Mg/Fe layered bimetallic oxide mulberry rod biochar composites (MFBCs) were prepared from mulberry rods and characterized using electron microscopy scanning (SEM), X-ray diffraction (XRD), Fourier infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). We investigated the adsorption properties of MFBCs for phosphorus, which was recovered via crystallization using calcium chloride as a precipitant. According to the findings, the MFBC is a layered bimetallic oxide with a specific surface area of 70.93 m²·g⁻¹. Its point of zero charge values, or pHzpc, was 7.66. The removal of phosphorus usingMFBCs gradually decreased with increasing pH, and the optimum pH for phosphorus removal was 4.0. The maximum phosphorus adsorption by MFBCs at 298 K was 29.682 mg·g⁻¹ for MFBCs. The adsorption process of phosphorus onto MFBCs is a heat absorption process, and the adsorption isothermal data of phosphorus onto MFBCs fit with the Langmuir adsorption isothermal model. Phosphorus recovery is achieved when calcium chloride is added to the phosphate-enriched desorption solution at a Ca/P molar ratio of 2.2. The phosphorus product obtained from this process is very pure hydroxyphospapatite. The recovery rate of phosphorus in the desorption solution is 99.64%.

Magnetite-loaded biochar has recently received attention owing to its ability to remove arsenic from contaminated soil. In this study, mulberry stem biochar (MBC) and Fe₃O₄-loaded mulberry stem biochar (Fe₃O₄@MBC) were produced and used in a 100-day incubation experiment to investigate their performance in the stabilization of arsenic in paddy soil severely polluted by the As (237.68 mg·kg⁻¹) mechanism. Incubation experiments showed that Fe₃O₄@MBC was more effective in immobilizing As after incubation for 100 days. Moreover, adding Fe₃O₄@MBC facilitated the transformation of exchangeable heavy metals into organic-bound and residual forms, thereby reducing As available concentrations, mobility, and bioavailability in the soil, and elevating slightly the soil pH and dissolved organic carbon (DOC). The concentration of TCLP-extractable As (As_TCLP) in contaminated soil was reduced from 93.85 to 7.64 μg·L⁻¹ within 10 d, below the safety limit for drinking water set by the World Health Organization (WHO). The characterization results of Fe₃O₄@MBC after incubation indicated that the mechanisms for As passivation are linked to redox reactions, complexation, electrostatic attraction, surface adsorption, and coprecipitation. Conclusively, Fe₃O₄@MBC is a promising amendment in highly As-contaminated soil and provides a theoretical reference in such polluted paddy soil remediation.

In this study, two biochar composites, namely hydroxyapatite/mulberry stem biochar (HMp) and magnesium-doped HMp (Mg0.1-HMp), were prepared using mulberry stem as the major raw material using the sol–gel process. Characterization and batch experiments were carried out on HMp and Mg0.1-HMp to investigate the Pb(II) adsorption mechanism and the factors affecting the adsorption, respectively. The results indicated that carboxylic compounds, phenols, and carbonyl functional groups were formed on the surfaces of HMp and Mg0.1-HMp. At an optimal pH of 5, an adsorption period of 6 h was achieved at an initial Pb(II) concentration of 100 mg/L and adsorbent quantity of 2 g/L. The maximum Pb(II) adsorption capacities of the HMp and Mg0.1-HMp were 303.03 and 312.50 mg/g, respectively, at 25 °C. The maximum Pb(II) adsorption capacity of Mg0.1-HMp was 2.55 times more than that of mulberry stem biochar (MBC). The adsorption of Pb(II) by HMp and Mg0.1-HMp is consistent with the Langmuir isotherm and pseudo-second-order kinetic models, demonstrating a spontaneous, endothermic, and irreversible process dominated by monolayer chemical adsorption. These results show that the mechanisms of Pb(II) by Mg0.1-HMp mainly involved electrostatic interaction, complexation, precipitation, and ion exchange.

In this study, mulberry tree stems were used as raw material to prepare magnetic modified biochar, Fe-BC-500, using the co-precipitation method. The structure of Fe-BC-500 was systematically characterized and tested for arsenic (As) adsorption in batch experiments by varying parameters such as solution pH (3–11), the concentrations of co-existing anions (2–20 mg/L), and ionic strength (0–0.5 mol/L NaNO₃). The results indicate that Fe-BC-500 exhibited optimal adsorption capacity at pH 4 and an initial As(V) concentration of 20 mg/L. The influence of co-existing anions on As(V) adsorption followed the order PO₄³⁻ > SO4²⁻ > NO₃⁻. Kinetic analysis showed that adsorption of Fe-BC-500 on As(V) followed a pseudo-second-order kinetic model, with a correlation coefficient of 1.00, indicating chemical adsorption. The Langmuir model accurately described the isothermal adsorption results, indicating monolayer adsorption. Mechanistic analysis showed that As(V) was fixed on the Fe-BC-500 surface through complexation reactions, demonstrating adsorption specificity. This study provides a theoretical basis and highlights the application potential of magnetically modified biochar for removing As(V) from water.

Arsenic (As) contamination in paddy soils poses a serious risk to rice safety and human health. To mitigate this issue, we developed a low-temperature, partially pyrolyzed Fe/Mn/Mg-modified biochar (FMM-BC) and evaluated its performance and mechanisms for remediating As-contaminated soil through a pakchoi–rice rotation pot experiment, aiming to reduce As accumulation in rice grains and pakchoi. The results indicated that FMM-BC application altered soil physicochemical properties and As speciation, reducing both water-soluble and bioavailable As and promoting its transformation from exchangeable to more stable organic-bound and residual fractions. Compared with the control, FMM-BC application reduced arsenic content in rice stems, leaves, and brown rice to 1.94 mg∙kg⁻¹, 5.24 mg∙kg⁻¹, and 1.21 mg∙kg⁻¹, respectively. In contrast, unmodified biochar (BC) increased As bioavailability and plant uptake, underscoring the importance of Fe/Mn/Mg modification. FMM-BC also enhanced the translocation of Fe, Mn, and Mg within rice plants, thereby modifying internal As transport dynamics and suppressing its accumulation in aboveground tissues. Under FMM-BC treatment, arsenic content in pakchoi stems and leaves decreased to 1.19 mg∙kg⁻¹ (vs. 1.96 mg∙kg⁻¹ in the control), and brown rice declined to 0.27 mg∙kg⁻¹ (vs. 1.49 mg∙kg⁻¹ in the control)—well below the national food safety threshold (0.35 mg∙kg⁻¹). These findings demonstrate that FMM-BC effectively stabilizes As in contaminated soils and reduces its transfer to edible plant parts, with Fe/Mn/Mg playing a key role in enhancing As immobilization and limiting its mobility within the soil–plant system.