Stabilization of heavy metals in biochar derived from plants in antimony mining area and its environmental implications

Increased oxygen content in biochar lowered bioavailability of polycyclic aromatic hydrocarbons-related toxicity to various organisms

Agricultural or environmental application of biochar (BC) is connected with the introduction of biochar-derived components among which polycyclic aromatic hydrocarbons (PAHs) and heavy metals are the most toxic. Their presence and bioavailability are crucial considering biochar toxicity. The effect of feedstock and pyrolysis temperature on the physicochemical properties of produced biochar and contaminant content was established and combined with toxicity to a broad range of living organisms. The obtained data revealed that predicting the bioavailability of PAHs using the total content is misleading. The toxicity was influenced by factors in the following way: the bioavailable PAHs > ash > total PAHs content in BC stressing the role of BC physicochemical characteristics. Among tested BC properties, surface functionalization, e.g. presence of oxygen-containing functional groups was crucial in revealing the toxicity. The data clearly indicate that additional research is required to determine BC's impact on various organisms and performing one ecotoxicity test is not sufficient.

Environmental remediation potential of a pioneer plant (Miscanthus sp.) from abandoned mine into biochar: Heavy metal stabilization and environmental application

Pyrolysis stands out as an effective method for the disposal of phytoremediation residues in abandoned mines, yielding a valuable by-product, biochar. However, the environmental application of biochar derived from such residues is limited by the potential environmental risks of heavy metals. Herein, Miscanthus sp. residues from abandoned mines were pyrolyzed into biochars at varied pyrolysis temperatures (300-700 °C) to facilitate the safe reuse of phytoremediation residues. The results showed that pyrolysis significantly stabilizes heavy metals in biomass, with Cd exhibiting the most notable stabilization effect. Acid-soluble/exchangeable and reducible fractions of Cd decreased significantly from 69.91 % to 2.52 %, and oxidizable and residue fractions increased approximately 3.24 times at 700 °C. The environmental risk assessment indicated that biochar pyrolyzed over 500 °C pose lower environmental risk (RI < 30), making them optimal for the safe utilization of phytoremediation residues. Additionally, adsorption experiments suggested that biochars prepared at higher temperature (500-700 °C) exhibit superior adsorption capacity, attributed to alkalinity and precipitation effect. This study highlights that biochars produced by pyrolyzing Miscanthus sp. from abandoned mines above 500 °C hold promise for environmental remediation, offering novel insight into the reutilization of metal-rich biomass.

Formation mechanisms and degradation methods of polycyclic aromatic hydrocarbons in biochar: A review

Biochar has been widely used in soil amendment and environmental remediation. Polycyclic aromatic hydrocarbons (PAHs) could be produced in preparation of biochar, which may pose potential risks to the environment and human health. At present, most studies focus on the ecotoxicity potential of biochar, while there are few systematic reviews on the formation mechanisms and mitigation strategies of PAHs in biochar. Therefore, a systematical understanding of the distribution, formation mechanisms, risk assessment, and degradation approaches of PAHs in biochar is highly needed. In this paper, the distribution and content of the total and bioavailable PAHs in biochar are reviewed. Then the formation mechanisms, influencing factors, and potential risk assessment of PAHs in biochar are systematically explored. After that, the effective strategies to alleviate PAHs in biochar are summarized. Finally, suggestions and perspectives for future studies are proposed. This review provides a guide for reducing the formation of biochar-associated PAHs and their toxicity, which is beneficial for the development and large-scale safe use of environmentally friendly biochar.

Biochar from phytoremediation plant residues: a review of its characteristics and potential applications

Phytoremediation is a cost-effective and eco-friendly plant-based approach promising technique to repair heavy metal-contaminated soils. However, a significant quantity of plant residues needs to be properly treated and utilized. Pyrolysis is an effective technology for converting residues to biochar, which can solve the problem and avoid secondary contamination. This paper reviews the generation, and physicochemical properties of biochar from phytoremediation residues, and its application in soil improvement, environmental remediation, and carbon sequestration. In spite of this, it is important to be aware of the potential toxicity of heavy metals in biochar and the environmental risks of biochar before applying it to practical applications. Future challenges in the production and application of residue-derived biochar include the rational selection of pyrolysis parameters and proper handling of potentially hazardous components in the biochar.

Comparison of Adsorption Performance of Biochar Derived from Urban Biowaste Materials for Removal of Heavy Metals

This study investigated the adsorption performance of biochar produced from different types of urban biowaste material viz., sugarcane bagasse (SB), brinjal stem (BS), and citrus peel (CP) for removal of heavy metal ions (Pb, Cu, Cr, and Cd) from aqueous solution. The effects of biowaste material, dosage of biochar, solution pH, and initial concentration of heavy metal ions and isotherm models were performed to understand the possible adsorption mechanisms. The results showed that the biochar derived from BS and SB removes Cu (99.94%), Cr (99.57%), and Cd (99.77%) whereas biochar derived from CP removes Pb (99.59%) and Cu (99.90%) more efficiently from the aqueous solution. Biochar derived from BS showed maximum adsorption capacity for Cu (246.31 mg g-1), Pb (183.15 mg g-1), and Cr (71.89 mg g-1) while the biochar derived from CP showed highest for Cd (15.46 mg g-1). Moreover, biochar derived from BS and SB has more polar functional groups and less hydrophobicity than the biochar derived from CP. This study reveals that solution pH and biochar doses play a major role in removal of heavy metal ions from aqueous solution. The results of Langmuir model fitted well for Pb and Cu while the Freundlich model for Cr and Cd. Our study concludes that the biochar derived from different biowaste materials adsorbs heavy metal ions majorly through surface complexation and precipitation processes. The results of this study will be very useful in selecting the effective urban biowaste material for making biochar for heavy metal removal from the aqueous environment.

Mitigation of arsenic release by calcium peroxide (CaO2) and rice straw biochar in paddy soil

Biochar has a great potential in the stabilization of soil heavy metals; however, the application can actually enhance the mobility of Arsenic (As) in soil. Here, a biochar-coupled calcium peroxide system was proposed to control the increase in As mobility caused by biochar amendment in paddy soil environment. The capability of rice straw biochar pyrolyzed at 500 °C (RB) and CaO₂ to control As mobility was evaluated by incubation for 91 days. CaO₂ encapsulation was performed for pH control of CaO₂, and As mobility was evaluated using a mixture of RB + CaO₂ powder (CaO₂-p), and RB + CaO₂ bead (CaO₂-b), respectively. The control soil solely and RB alone were included for comparison. The combination of RB with CaO₂ exhibited remarkable performance in controlling As mobility in soil, and As mobility decreased by 40.2% (RB + CaO₂-p) and 58.9% (RB + CaO₂-b) compared to RB alone. The result was due to high dissolved oxygen (6 mg L^-1 in RB + CaO₂-p and RB + CaO₂-b) and calcium concentrations (296.3 mg L^-1 in RB + CaO₂-b); oxygen (O₂) and Ca²⁺ derived from CaO₂ is able to prevent the reductive dissolution and chelate-promoted dissolution of As bound to iron (Fe) oxide by biochar. This study revealed that the simultaneous application of CaO₂ and biochar could be a promising way to mitigate the environmental risk of As.

Preparation of esterified biomass waste hydrogels and their removal of Pb2+, Cu2+ and Cd2+ from aqueous solution

The treatment of polluted water is a serious environmental problem in the world. Biomass is easily modified and can be prepared into adsorbent materials, which is expected to solve the problem of heavy metal ion adsorption in sewage. In this paper, esterified tobacco straw based hydrogels (ETS-PAA) were synthesized from waste tobacco straw biomass. The structure and thermal stability of these hydrogels were characterized by FTIR, SEM, EDS, XPS and TG. The adsorption of metal ions by the hydrogel was measured by ICP-MS. The effects of initial ion concentration, adsorption time, pH, and temperature on the heavy metal adsorption were investigated. The results showed that ETS-PAA possessed more pores, which led to a better adsorption capacity. The maximum adsorption amounts of Pb²⁺, Cu²⁺ and Cd²⁺ were 2.41 mmol·g^-1, 1.93 mmol·g^-1 and 1.77 mmol·g^-1, respectively. Finally, the adsorption mechanism and kinetics were analyzed. The adsorption was mainly accomplished by ion exchange of -COOK on the monomer chain with heavy metal ions, coordination of -OH and -CONH with heavy metal ions and interaction of ester bond, -COOH with heavy metal ions. The adsorption process was in accordance with the pseudo-second-order kinetic model and Freundlich model. The adsorption process belonged to multilayer chemisorption. This work shows that ETS-PAA was a promising material for the removal of heavy metal pollutants from aqueous solution.

Potential of Lemna minor and Eichhornia crassipes for the phytoremediation of water contaminated with Nickel (II)

Phytoextraction of Nickel (II) in water by two types of aquatic macrophytes (Lemna minor and Eichhornia crassipes) was investigated using synthetic aqueous solutions of NiSO₄ at concentrations of 0.5, 1.5 and 2.5 mg/L. The toxic effects of nickel salt in plants were evaluated through the presence of necrosis and chlorosis. The bioconcentration factor, Nickel (II) removal efficiency and kinetics of removal were also calculated. Results of this study show bioconcentration factors higher than 1000, which categorize L. minor and E. crassipes as hyperaccumulators. Besides, L. minor presented a removal percentage higher than 68%, compared to E. crassipes that did not exceed 50% in any of the three concentrations studied. However, E. crassipes showed better resistance to the effects of nickel and obtained a greater removal capacity during the phytoremediation process that lasted for 10 days. In contrast, L. minor suffered necrosis and chlorosis in a concentration-dependent way. Consequently, both macrophytes are sustainable alternatives for nickel removal from contaminated water.