Abstract:

The enhanced efficiency of global wastewater treatment has led to a dramatic surge in sludge production. In China, the annual output of municipal sludge (calculated at 80% moisture content) has exceeded 60 million tons, with an annual growth rate of 5%‒10%, and the total output is projected to surpass 90 million tons by 2025. As a byproduct of wastewater treatment plants, sludge is characterized by high moisture content, large volume, and enrichment of hazardous substances such as heavy metals and persistent organic pollutants. Traditional disposal methods like land application, incineration, and landfilling tend to cause these hazardous substances to enter the ecological cycle, posing severe threats to ecosystem security and human health. Although pyrolysis technology can achieve sludge hazard mitigation and resource utilization by converting toxic heavy metals into relatively stable fractions, the biochar produced from single sludge pyrolysis has drawbacks such as unstable heavy metal speciation, low organic carbon content, and small specific surface area, leading to high ecological risks that restrict its practical application. To address this, co-pyrolysis technology involving the addition of auxiliary materials has emerged. It not only significantly enhances the immobilization efficiency of heavy metals but also optimizes the physicochemical properties of biochar, thus becoming a core technical pathway for the safe disposal and resource utilization of sludge. This paper systematically reviews the heavy metal immobilization effects and action mechanisms of three core additives (biomass, minerals and metal compounds, and industrial wastes) in sludge co-pyrolysis, elaborates on the diverse resource utilization scenarios of co-pyrolyzed biochar, and proposes future research directions. Regarding additive categories, co-pyrolysis of biomass and sludge can produce biochar with high carbon content, low ash content, and excellent pore structure, facilitating the conversion of heavy metals from unstable to stable speciation. Minerals and metal compounds immobilize heavy metals through chemical precipitation, ion exchange, and other processes: co-pyrolysis of CaSO₄ and sludge results in stable fraction proportions of Cr, Pb, and Cu reaching 96.99%, 89.23%, and 99.55%, respectively; K₂CO₃ can increase the specific surface area of biochar and promote the conversion of Pb, Zn, and other heavy metals to stable speciation; when the addition amount of CaSiO₃ is 9%, the stable fraction proportion of Zn reaches 79%. Moreover, pyrolysis temperature significantly affects the immobilization effect: at 700 ℃, the F4 fractions of Cr, Pb, and Zn are 1.28, 1.15, and 1.81 times higher than those at 300 ℃. Industrial wastes align with the concept of “treating waste with waste”: co-pyrolysis of municipal solid waste and sludge (≥550 ℃) leads to F4 fractions of Cr and Pb exceeding 70.29% and 97.65%, respectively; the biochar prepared by co-pyrolysis of waste tires and sludge (700 ℃, 10% mixing ratio) has a specific surface area of 49.71 m²·g⁻¹, with adsorption capacities of 50.25 mg·g⁻¹ for Cd and 90.05 mg·g⁻¹ for tetracycline. A multi-dimensional comparison of the three types of additives shows that biomass has significant advantages in pore structure and resource value, minerals and metal compounds excel in immobilization mechanisms and ecological risk control, while industrial wastes possess both multi-dimensional balance and synergistic disposal advantages. The immobilization of heavy metals by co-pyrolyzed biochar is the result of the synergistic effect of multiple mechanisms: physical immobilization restricts the migration of heavy metals through pore filling and carbon matrix encapsulation. Co-pyrolysis with coconut fiber increases the specific surface area and total pore volume of biochar by 140% and 47.5%, respectively, while the dense carbon matrix formed by co-pyrolysis with waste tires can reduce the mobility of Zn, Ni, and Cd by more than 80%; chemical immobilization is the core pathway. The ion exchange mechanism, accounting for 28.05%, achieves efficient immobilization through the “adsorption-desorption-readsorption” cycle between Ca²⁺ and heavy metal ions. The complexation of surface oxygen-containing and nitrogen-containing functional groups, as well as insoluble minerals such as sulfides and hydroxides formed by chemical precipitation, further enhance the stability of heavy metals; The cation-π interaction realizes synergistic improvement of immobilization effect through the electrostatic attraction between the delocalized π electron cloud of aromatic rings and heavy metal cations, and this effect is significantly enhanced above 550 ℃. In terms of resource utilization, co-pyrolyzed biochar exhibits cross-domain value: As sewage treatment adsorbents, tea residue-sludge co-pyrolyzed biochar removes Cd through mechanisms such as complexation and co-precipitation, and the adsorption capacity of coconut fiber-sludge co-pyrolyzed biochar for ciprofloxacin reaches 58.1 mg∙g⁻¹, which is superior to that of biochar produced by co-pyrolysis of various other biomasses; As soil remediation and amendment agents, FeSO₄-modified co-pyrolyzed biochar reduces the F1 fraction of vanadium in soil by 95%, decreases the vanadium concentration in plants by 81.5%‒96%, and simultaneously releases nitrogen, phosphorus, and potassium to improve soil fertility; As catalysts, iron-doped co-pyrolyzed biochar efficiently degrades refractory organic pollutants in heterogeneous Fenton-like reactions, and the g-C₃N₄ catalyst prepared by co-pyrolysis of sludge and melamine increases the removal rate of Eriochrome Black T (EBT) by 56%; In addition, it also shows good potential in scenarios such as enhancing the compressive strength of concrete, remediating marine oil pollution, and optimizing constructed wetland-microbial fuel cell (CW-MFC) systems. In summary, co-pyrolysis technology establishes a synergistic “physical-chemical” heavy metal immobilization system by adding multiple auxiliary materials, which not only reduces the ecological risk of sludge-based biochar but also expands the boundaries of resource utilization. Future research should focus on optimizing pyrolysis processes and equipment to reduce energy consumption and harmful byproducts, deepening the research on the synergistic mechanism of multiple substances, exploring the combined application of catalysts and co-pyrolysis materials, and conducting long-term assessments of heavy metal immobilization effects and ecological risks, so as to provide more solid theoretical support and data guarantee for the engineering and industrial application of this technology.

Key words: sludge-based biochar, co-pyrolysis, heavy metal, adsorption, immobilization

摘要：

污泥作为污水处理的副产物，往往富集重金属、持久性有机物等有害物质，处置不当会污染环境，威胁生态安全与人类健康。热解虽能实现污泥的风险减控与资源化，但热解产物重金属含量高、比表面积小，限制其利用。共热解通过添加辅助材料优化生物炭的性能。常用的添加物有3类：生物质类添加物通过高孔隙结构的物理截留、表面官能团络合及还原性组分诱导的价态转化，增强了重金属与碳基质的结合稳定性；矿物类添加物依托化学沉淀、离子交换及碱性调节作用，可将重金属转化为不溶性硫化物、磷酸盐或氢氧化物等稳定矿物相；工业废弃物类添加物则通过碳基质包裹、S/Cl化学反应及组分协同效应，优化生物炭的孔隙结构与表面活性位点，促进稳定化合物生成并强化金属固定。此3类添加物在共热解体系中相互作用，表现出明显的协同效应，能够多途径削弱重金属的迁移性与生物可利用性，其固定效率普遍高于单一污泥热解产物。该文综述污泥共热解固定重金属的常用添加物，解析其固定效果与机理，介绍资源化利用场景并提出未来的研究方向，为共热解生物炭在重金属固定与实际应用中的发展提供理论依据。

关键词: 污泥基生物炭, 共热解, 重金属, 吸附机理, 固定化