【作者】 陈敏；

【导师】 张其武；

【作者基本信息】 武汉理工大学 ， 环境科学与工程， 2024， 博士

【摘要】 近年来,铅、镉等重金属污染及其稳定化治理仍然受到社会广泛关注。采用碱沉淀法使重金属固化为氢氧化物是相对简单、高效、应用广泛的重金属去除方法,但存在重金属氢氧化物沉淀稳定性差、溶液碱性过强导致的二次污染与处置以及CaO原料制备过程中方解石（CaCO₃）热分解产生的大量碳足迹等问题。相对而言,直接利用储量丰富、成本低廉、具有碳酸化反应活性的天然富镁硅酸盐矿物蛇纹石（Mg₃Si₂O₅（OH）₄）及其固废研发一类温和缓释型碱基材料,在反应过程中引入大气CO₂而诱导重金属矿化为更稳定的碳酸盐,可避免钙基强碱性沉淀剂应用的弊端,是一种可持续的重金属污染治理策略,具有重要的实用价值。因此,本论文聚焦于自然条件下废水中铅、镉的矿化稳定,以风化的蛇纹石废料（Mg₃Si₂O₅（OH）₄）为原料,采用机械力化学技术对蛇纹石晶体结构与表面性质进行调控,在此基础上提出了基于机械力化学界面反应的二元钙、镁基矿物协同活化方法,将蛇纹石与钙镁盐进行共同球磨活化从而构建了多组具有高反应活性的蛇纹石基复合矿物材料,大幅提升了铅、镉矿化效率。通过剖析矿物、重金属、大气CO₂多组分相互作用过程,阐明了机械力化学活化蛇纹石与CO₂协同强化重金属矿化的反应机理。主要研究结果如下:（1）一步高能球磨处理可提高蛇纹石晶格活性,促进其在水中溶解、释放Mg²⁺和OH^-而有效提升溶液pH至碱性,显示出“缓释碱性”。球磨活化的蛇纹石可在反应时间2 h内实现对水中Cu（II）、Pb（II）、Ni（II）、Fe（II）（～100 mg/L）的完全去除（>95%）,但对Cd（II）的矿化效率不足25%,这一差异与重金属碳酸盐、氢氧化物沉淀溶度积常数密切相关。通过研究铅固定反应机理可以发现,Pb（II）主要以碱式碳酸铅（Pb₃（CO₃）₂（OH）₂）形式沉淀于蛇纹石表面。这证实活化蛇纹石持续溶解形成的非均相碱性环境促进了大气CO₂的溶解并通过异质沉淀反应高效诱导铅碳酸盐矿化。该反应过程中活化蛇纹石显示出较好的耐酸性及对铅的持续矿化效果,为铅污染的长效矿化稳定治理提供了可能途径。（2）机械力化学活化蛇纹石可在24 h内实现对中低浓度镉的完全矿化固定,所得镉固定量（199.7 mg/g）分别是天然、热活化蛇纹石的23和13倍,显示出增强的镉固定性能,但反应效率远低于同等反应条件下的铅矿化效率。通过对蛇纹石、镉、大气CO₂相互作用过程进行深入剖析,确认了反应过程中CdCO₃和Mg-CdCO₃沉淀的逐渐形成,表明镉矿化产物主要为碳酸盐,且镉的矿化促进了镁组分的碳酸化反应。该研究结果整体体现了机械力活化蛇纹石、CO₂、镉的三元协同作用,由此在一定程度上提升了蛇纹石对镉的矿化效率及对大气CO₂的利用效率。（3）引入钙/镁盐与蛇纹石进行共同球磨处理,由此激发了两相界面反应而使部分钙镁盐负载于蛇纹石表面,得到了多组具有高反应活性的钙镁盐-蛇纹石基复合矿物材料。这使得蛇纹石表面活性碱土金属离子位点增加,从而进一步提升了镉碳酸盐矿化效率,使镉完全固定所需反应时间从蛇纹石单一矿物作用下的24 h大幅缩短至0.5～2 h内,同步实现对镉的高效矿化与对溶液碱度的温和调节（pH<9）,且在广泛酸性pH条件下具有稳健的镉矿化性能。（4）基于钙/镁硫酸盐与蛇纹石界面反应产生的表面硫酸盐活性位点可迅速诱导镉异质沉淀为碱式硫酸镉（CdSO₄·3.5Cd（OH）₂·x H₂O）。随着反应时间的延长,活性碱土金属离子介导可加速反应体系对大气CO₂的吸收而诱导CdSO₄·3.5Cd（OH）₂·xH₂O在短时间内进一步转化为CdCO₃沉淀。其中,蛇纹石-CaSO₄复合矿物对镉的矿化效率及对大气CO₂的利用效率高于蛇纹石-MgSO₄复合矿物,可在反应120 min内得到更多的CdCO₃沉淀,这一反应特性差异可归因于钙组分相对于镁组分更强的金属性及碳酸化反应活性性质。（5）蛇纹石与碳酸钙的界面反应下存在两相成分在界面上的相互扩散与掺杂现象,由此增强了两相协同反应活性并促进了镁硅酸盐组分对大气CO₂,最终实现了CdCO₃在矿物表面的高效原位矿化。与镁/钙硫酸盐-蛇纹石复合矿物相比,蛇纹石-碳酸钙复合矿物在溶液初始pH 3条件下对镉的矿化量进一步提高到241.8 mg/g,显示出基于两相界面掺杂的蛇纹石-碳酸钙复合矿物突出的镉矿化性能优势。综上所述,本论文开展的工作为多元钙镁基矿物的协同活化、蛇纹石基矿物材料与大气低浓度CO₂对重金属矿化的协同强化提供了一定的理论与实践基础,可潜在应用于自然条件下的重金属污染治理及蛇纹石、方解石、石膏等钙镁基固废的资源化利用等方面,有利于资源与环境的协调可持续发展。更多还原

【Abstract】 In recent years,heavy metal contamination such as lead and cadmium,as well as its stabilization treatment,has still been widely concerned by society.Generally,alkali precipitation method to immobilize heavy metals as hydroxides is relatively simple,cost-effective and widely applicable for the removal of heavy metals.However,depended on using lime（CaO）,conventional alkali method is confronted to some problems,including the secondary pollution risk and necessary treatment due to unstable metal hydroxide precipitates and highly alkali solution,as well as the carbon footprint during the preparation of CaO via thermal decomposition of calcite（CaCO₃）.Relatively,utilizing abundant and low-cost natural serpentine（Mg₃Si₂O₅（OH）₄）ore or solid waste,which is rich in magnesium silicate with carbonation reactivity,to develop a kind of mild alkaline mineral material,whereby to mineralize the heavy metals into stable carbonates with the presence of atmospheric CO₂ and avoid the drawbacks of calcium-based strong alkali precipitants,would be a sustainable strategy for the treatment of heavy metal contamination.Therefore,focusing on the mineralization of heavy metal（particularly lead and cadmium）in aqueous environment under ambient conditions,this thesis used the weathered serpentine waste as raw materials and employed mechanochemical methods to regulate the crystal structure and surface/interfacial properties of serpentine.On this basis,a collaborative activation method for calcium-and magnesium-based multiple mineral phases via mechanochemical interface reaction was proposed by co-milling calcium/magnesium salts with serpentine.In this way,several groups of serpentine-based composite mineral materials with high reactivity were constructed,with significant enhancement in heavy metal mineralization.The mechanism for heavy metal mineralization enhanced by the synergistic effect of mechanochemically activated serpentine and CO₂ was clarified based on the deep study on the interactions among minerals,heavy metal ions and atmospheric CO₂.The main conclusions were made as follows:（1）One-step high-energy ball-milling treatment on serpentine could increase its lattice reactivity and enhance its dissolution in water with Mg²⁺and OH^-releasing to boost solution pH to alkali conditions,namely called“slow-releasing alkaline”property.This new feature made it possible for activated serpentine to achieve the complete removal of copper（II）,lead（II）,nickel（II）and iron（II）（100 mg/L）in solution within 2h,while the Cd（II）removal rate was less than 25%.The different reaction levels could be explained by the internal property of each heavy metal.According to the in-depth study results on the immobilization mechanism of Pb（II）,it was found that Pb（II）was dominantly precipitated as stable Pb₃（CO₃）₂（OH）₂ on the surface of serpentine,which confirmed that the heterogeneous alkaline environment caused by the continuous dissolution of ball-milled serpentine in water promoted the dissolution of atmospheric CO_2,thus favoring the heterogeneous precipitation of considerable amount of Pb₃（CO₃）₂（OH）₂.Lead mineralization reaction by ball-milled serpentine was proved to be stable under a wide acidic pH range and exhibited durative effectiveness,which may be potentially applicable to the long-term treatment of lead pollution.（2）The complete mineralization of cadmium at low concentration（～100 mg/L）could be achieved by mechanochemically activated serpentine within 24 h,with a Cdfixation capacity of 199.7 mg/g which was approximately 23 and 13 times that of natural serpentine and thermally activated serpentine,suggesting the enhanced Cdfixation effectiveness of as-proposed serpentine in this study.However,the mineralization efficiency of cadmium was much lower than that of lead under the same operation conditions.Through the systematic study of the reaction among activated serpentine,Cd²⁺and atmospheric CO₂,the gradual formation of significant amount of CdCO₃ and Mg-CdCO₃ precipitate was confirmed,indicating the carbonation of cadmium as well as magnesium silicate along with Cdmineralization.These results generally demonstrated the synergistic effect among serpentine,CO₂ and cadmium,thereby boosting the mineralization rate of cadmium and utilization of atmospheric CO₂ by serpentine.（3）Co-milling of calcium/magnesium salts and serpentine induced the bi-phase interfacial reaction with partial calcium/magnesium salts loaded on serpentine surface,thus obtaining highly reactive serpentine-based combined mineral material.This resulted in the increase of active alkali metal sites on serpentine surface,thus further enhancing the mineralization efficiency of cadmium.In this way,the required reaction time for the complete fixation of cadmium ions could be dramatically reduced from 24h by using serpentine alone to 2 h by using serpentine-calcium/magnesium salt combined minerals,synchronously realizing the highly-efficient mineralization of cadmium and mild adjustment of solution alkalinity（pH<9）under a wide acidic pH range.（4）The active CaSO₄ or MgSO₄ sites generated on serpentine surface due to the interfacial reaction between serpentine and calcium/magnesium sulfates could induced the rapid heterogeneous precipitation of cadmium into basic Cdsulfate（CdSO₄·3.5Cd（OH）₂·x H₂O）.With the reaction time further prolonged,the basic Cdsulfate could be further transformed into CdCO₃ in a short time,favored by the enhanced dissolution of atmospheric CO₂ in presence of active Ca²⁺/Mg²⁺ion sites.Therein,the CaSO₄-serpentine combination showed a higher efficiency for Cdprecipitation,as well as the simultaneous utilization of atmospheric CO₂,than MgSO₄-serpentine combination,which could be ascribed to the stronger metallicity and corresponding carbonation performance of Caconstituent than those of Mgconstituent.（5）The interfacial reaction between serpentine and CaCO₃ induced the mutual diffusion and substitution of ion constituents at the bi-phase interface,which enhanced the reactivity of the two minerals and promoted the utilization rate of atmospheric CO₂by magnesium silicate,eventually realizing the efficient in-suit mineralization of cadmium into CdCO₃.At an initial pH of 3,a Cdfixation capacity as high as 241.8mg/g was achieved by using CaCO₃-serpentine combined mineral,higher than that by using MgSO₄-serpentine or CaSO₄-serpentine combined minerals,demonstrating the superiority of CaCO₃-serpentine combined mineral for enhancing the cadmium mineralization as a result of the synergy effect of serpentine and CaCO₃ triggered by their interfacial diffusion.In summary,this work provided certain theoretical and practical basis for the synergistic effect of serpentine-based mineral materials and low-concentration CO₂ in air to enhance heavy metal mineralization,as well as the synergistic activation of multiple calcium/magnesium mineral phases.The as-proposed strategy could possibly be applicable for the treatment of heavy metal contamination under ambient conditions,as well as the utilization of solid wastes including serpentine,calcite and gypsum,promoting the coordinated and sustainable development of resources and environment.更多还原