【作者】 张鹏；

【导师】 袁松虎；

【作者基本信息】 中国地质大学 ， 环境科学与工程， 2018， 博士

【摘要】 黄铁矿是地球表生环境中含量最丰富的硫化矿物,广泛分布于江海河口沉积物和热液矿床中。虽然黄铁矿在无氧条件下形成,但是人类活动和自然过程会导致黄铁矿暴露于有氧环境发生氧化。根据黄铁矿赋存环境的不同,黄铁矿氧化可以发生在酸性（如:酸性硫酸盐土壤）和中性条件下（如:盐碱沼泽）。黄铁矿的形成与氧化影响着铁,硫,氧,碳和氮等元素的全球循环。此外,黄铁矿氧化产生含有硫酸,溶解态二价铁(Fe²⁺)以及其他金属离子的酸液,可以导致严重的环境污染。近年来研究发现黄铁矿在非生物氧化过程中除了产生酸液外,还可以产生过氧化氢（H₂O₂）和羟自由基（·OH）等活性氧物种,并且产生的·OH可以有效地氧化转化污染物。在黄铁矿氧化产生·OH的机理方面,虽然酸性条件下黄铁矿被O₂和H₂O₂氧化产生·OH已经有了一定的认识,但是O₂和H₂O₂在黄铁矿表面还原产生·OH的机理,以及不同反应途径对产生·OH的相对贡献仍不清楚;中性条件下黄铁矿被O₂氧化产生·OH已经被关注,但是·OH产生机理以及土壤孔隙水中广泛存在的小分子有机酸对产生·OH的影响鲜有报道。在黄铁矿氧化产生·OH的环境效应方面,虽然含砷黄铁矿氧化释放砷的机制已经被大量研究,但是对于黄铁矿氧化产生·OH对As（III）氧化与迁移的影响研究较为薄弱;酸性硫酸盐土壤区域曝氧氧化过程中黄铁矿产酸以及重金属离子的释放已经广泛的研究,但是对于过程中产生·OH的规律以及氧化效应尚不清楚。本研究中选择苯甲酸作为·OH的探针化合物,以定量反应过程中·OH的累积浓度。为了探究酸性条件下黄铁矿无氧和有氧氧化产生·OH的机理,将10 g/L黄铁矿加入到pH 2.6且含10 mM苯甲酸的溶液中。实验结果显示无氧和有氧条件下黄铁矿氧化10 h时间内累积·OH浓度分别线性增加到7.5和52.2μM。无氧条件下,·OH是黄铁矿表面硫缺陷位点空位（表面空位）氧化吸附态水（H₂O）产生,并且·OH产生量随着表面暴露程度的增加而增加。有氧条件下,·OH主要由氧气（O₂）在黄铁矿表面还原产生,次要地由表面空位氧化H₂O以及溶解态Fe²⁺与硫中间体氧化产生。O₂还原途径中,O₂通过两电子转移产生H₂O₂,其进一步与溶解态Fe²⁺通过Fenton机理反应产生·OH。表面空位氧化H₂O途径中,产生的H₂O₂同样与溶解态Fe²⁺反应产生·OH。为了探究酸性条件下黄铁矿被H₂O₂氧化产生·OH的机理,实验检测不同条件下（无氧,2–12 g/L黄铁矿,0.025–1 mM H₂O₂,pH 2–4）黄铁矿被H₂O₂氧化过程中·OH和溶解态Fe²⁺/Fe³⁺的产生以及H₂O₂的分解。结果显示,产生·OH的速率常数随着黄铁矿浓度和H₂O₂浓度的增加而增加,随着pH的增加而降低。为了评估表面反应对产生·OH的作用,将1 mM 2,2’-联吡啶（BPY）加入到H₂O₂/黄铁矿体系以络合溶解态Fe²⁺从而抑制H₂O₂分解。当加入1 mM BPY后,不同条件下·OH的产生速率常数降低了44.4%–65.6%,说明表面反应和溶液反应共同贡献·OH的产生。对于表面反应,密度泛函理论计算显示H₂O₂可能首先吸附到黄铁矿表面Fe（II）位点,随后转化为吸附态·OH,最后吸附态·OH与H₂O形成氢键释放到溶液中。对于溶液反应,溶解态Fe²⁺与H₂O₂通过Fenton机理反应主要产生·OH。基于上述机理发现,进一步建立了H₂O₂氧化黄铁矿产生·OH的动力学模型,用于评估表面反应和溶液反应产生·OH的相对贡献。表面反应对产生·OH的相对贡献,随着反应时间的增加而降低,并且依赖于黄铁矿表面浓度与溶解态Fe²⁺浓度的比值。当比值低于临界值1.6×10³ m²/mM时,表面反应主要产生·OH。除了酸性条件外,黄铁矿氧化也可以发生在中性条件下,如缓冲较好的滨海和河口沉积物,在这些区域小分子有机酸广泛地存在于土壤孔隙水中。为了探究中性条件下黄铁矿氧化产生·OH,将50 g/L黄铁矿加入到pH 6–8且含20 mM苯甲酸溶液中,检测反应过程中·OH和溶解态Fe（II）/Fe（III）的产生。pH 7的条件下,不存在小分子有机酸时,黄铁矿有氧氧化420 min时间内累积·OH浓度为7.5μM,而存在1 mM乙二胺二乙酸钠,柠檬酸或草酸时,420 min时间内累积·OH浓度分别达到14.8,12.0或11.2μM。为了探究小分子有机酸对黄铁矿氧化产生·OH的影响机制,本研究选择柠檬酸作为小分子有机酸的代表。pH 7的条件下,随着柠檬酸浓度由0.5增加到5 mM,420 min时间内累积·OH浓度由10.3增加到27.3μM。随着pH由8降低到6,不存在小分子有机酸时,420 min时间内累积·OH浓度由2.1增加到23.3μM;而存在3 mM柠檬酸时,420 min时间内累积·OH浓度由8.8增加到134.9μM。中性条件下,不存在小分子有机酸时,·OH主要由黄铁矿表面吸附态Fe（II）有氧氧化产生;而存在柠檬酸时,·OH主要由柠檬酸亚铁（Fe（II）-citrate^-）有氧氧化产生和次要由表面空位氧化H₂O产生。柠檬酸促进·OH产生的主要机理为,柠檬酸对Fe（II）的络合作用提高了H₂O₂与Fe（II）反应产生·OH的效率和柠檬酸铁（Fe（III）-citrate）加速黄铁矿氧化。黄铁矿氧化过程中,Fe（II）-citrate^-主要由柠檬酸络合吸附态Fe（II）和黄铁矿还原Fe（III）-citrate形成,Fe（III）-citrate主要由Fe（II）-citrate^-氧化形成。基于上述机理发现,进一步探究了黄铁矿氧化产生·OH对砷（As）迁移转化影响以及酸性硫酸盐土壤曝氧氧化过程中黄铁矿氧化产生·OH以及氧化效应。为了检测酸性条件下黄铁矿氧化产生·OH对As（III）迁移转化的影响,将6.67μM As（III）加入到pH 2.6且含10 g/L黄铁矿的悬浊液中。有氧和无氧条件下,10 h时间内As（III）被氧化的比例分别为55.4%和13.9%,氧化转化对As在黄铁矿表面吸附作用不明显。为了探究中性条件下黄铁矿氧化产生·OH对As（III）迁移转化的影响,首先在无氧条件下将2.67μM As（III）加入到pH 7.4且含10 g/L黄铁矿的悬浊液预平衡30 min时间至溶解态As（III）浓度降低为1μM,随后曝氧氧化。不存在柠檬酸时,有氧氧化6.5 h时间内溶解态总As浓度由1逐渐降低至<0.001μM。溶解态As减少的主要原因是氧化过程中黄铁矿氧化产生的·OH氧化As（III）为As（V）,随后As（V）被吸附到新生的Fe（III）氢氧化物上。为了探究柠檬酸对As（III）迁移转化的影响,曝氧氧化阶段将0.1–1 mM柠檬酸加入到上述溶液中。随着柠檬酸浓度由0.1增加到1 mM,有氧氧化6.5 h时间后溶解态As浓度由0.3增加到2.67μM,并且柠檬酸可以促进As（III）被氧化为As（V）。柠檬酸促进黄铁矿氧化释放砷的主要机理是柠檬酸与黄铁矿表面的砷发生配体交换,与As（V）竞争吸附黄铁矿与Fe（III）氢氧化物以及与Fe（III）氢氧化物反应形成胶体促进砷向溶液相转移。为了探究酸性硫酸盐土壤曝氧氧化产生·OH以及氧化效应,选择在广东省台山市八家村一块酸性硫酸盐土壤场地采集了4口深度为0–150 cm土壤剖面,每10cm取样一份,共60个样品。典型实验中将2 g原始土壤加入到10 mL初始pH 4.4的溶液中,随后曝氧氧化。结果显示曝氧氧化24 h时间内,每克干酸性硫酸盐土壤可以产生0.001–3.581μmol·OH,平均产量为0.705μmol/g。相关性分析显示·OH产生量与黄铁矿中Fe（II）和吸附态Fe（II）含量之间呈正相关,而与有机质和其他Fe（II）物种含量之间无相关性,说明黄铁矿与吸附态Fe（II）氧化主要贡献·OH的产生。XRD结果证实黄铁矿存在于土壤中。进一步探究了酸性硫酸盐土壤曝氧氧化过程中CO₂产生,24 h时间内CO₂产量为0.76–54.5μmol/g,平均产量为8.93μmol/g。将5%乙醇（·OH淬灭剂）加入到原始土壤悬浊液后,大部分情况下CO₂产生量有所降低,平均降低幅度估算为19.8%,说明·OH可以氧化有机碳产生CO₂。将0.1%HgCl₂（用于杀灭微生物）到原始土壤悬浊液后,CO₂产量显著降低,说明微生物呼吸为CO₂产生的主要途径。为了探究无机碳溶解对CO₂产生的作用,选择将2 g灭菌的土壤加入到10 mL初始pH 4.4且存在5%乙醇的溶液中。结果显示随着初始H⁺浓度的增加,CO₂产量随着增加,说明酸性条件下无机碳溶解可以贡献CO₂产生。本研究揭示了黄铁矿非生物氧化产生·OH机理,尤其是界面电子转移机制,不同反应途径的相对贡献和小分子有机酸的影响,以及产生的·OH对砷迁移转化和CO₂排放的影响。研究结果补充了黄铁矿氧化的基本理论,为理解物质在自然界中的迁移转化提供了理论基础。考虑到·OH的强氧化性,因此酸性矿山废水修复策略与方法评估中,需要考虑黄铁矿非生物氧化产生·OH对缓释材料的氧化。更多还原

【Abstract】 Pyrite is the most common sulfide mineral on supergene environments and is frequently found in hydrothermal deposits and marine sediments.Although pyrite is formed in anoxic environments,it is often exposed to oxic conditions due to natural and anthropogenic processes upon weathering.Basing on the formation environment for pyrite,Pyrite oxidation can occur under acidic（e.g.,acidic sulfate soil）and neutral conditions（e.g.,salt marshes）.Formation and oxidation of pyrite affect the global cycles of iron,sulfur,oxygen,carbon and nitrogen.In addition,pyrite oxidation can produce sulfuric acid,dissolved ferrous iron(Fe²⁺)and other metal ions,causing the serious environmental pollution.In the last years,studies have shown that the hydrogen peroxide（H2O2）and hydroxyl radicals（·OH）are produced from pyrite abiotic oxidation.The·OH produced can oxidize pollutants.For the mechanisms of·OH production from pyrite oxidation,although·OH production from pyrite oxidation by O₂ and H₂O₂ under acidic conditions has been recognized,the mechanisms of·OH production from O₂ and H2O2 reduction on pyrite surface and the relative contribution of each pathway are still not clear.Production of·OH from pyrite abiotic oxidation under circumneutral conditions has also been noted,while the mechanims of·OH produciton and the influences of low-molecular-weight organic acids（LMWOAs）,which prevails in soil pore water,are rarely studied.For the oxidative impact of·OH,although the mechanisms of arsenic（As）release from the oxidative dissolution of As-rich pyrite have been widely studied,the influence of·OH that was produced from pyrite oxidation on the transformation and release of As（III）is not clear.Previous studies have widely studied the production of acidity and the release of heavy metal ions from acid sulfate soils during oxygenation,while·OH production from pyrite oxidaton and its related environmental impacts are not clear.In this study,the oxidation of benzoic acid（BA）to p-hydroxybenzoic acid（p-HBA）is used as a probe reaction for quantifying the production of·OH from abiotic pyrite oxidation.To explore the mechanism of·OH production from pyrite oxidation by oxygen（O₂）under acidic conditions,10 g/L pyrite was added into 10 mM BA solution at pH 2.6.Production of·OH,H₂O₂,Fe²⁺and SO₄^2-were measured during oxidation course.The cumulative·OH within 10 h under anoxic and oxic conditions increased to7.5 and 52.2μM,respectively.Under anoxic conditions,·OH was produced from the oxidation of adsorbed H₂O on the sulfur-deficient sites on pyrite surface（surface sulfur-defects）,showing an increased production with the increase of pyrite surface exposure.Under oxic conditions,the formation of·OH proceeds predominantly via the two-electron reduction of O₂ on pyrite surface along with a minor contribution from the oxidation of H₂O on surface sulfur-defects and the reactions of Fe²⁺/sulfur intermediates with O₂.For both O₂ reduction and H₂O oxidation on the surface sulfur-defects,H₂O₂was the predominant intermediate,which subsequently transformed to·OH through Fenton mechanism.To explore the mechanism of·OH production from pyrite oxidation by H₂O₂ under acidic conditions,pyrite oxidation by H₂O₂ under anoxic conditions was performed under different conditions（2–12 g/L pyrite,0.025–1 mM H₂O₂ and pH 2–4）,and·OH and aqueous Fe²⁺/Fe³⁺production as well as H₂O₂ consumption were measured during the oxidation.Rate constants of·OH production increased with the increase in pyrite dosage and H₂O₂ concentrations,while decreased with the increase in pH.In order to evaluate the contribution of surface reaction to·OH production,1 mM 2,2’-bipyridine（BPY）was added to inhibit H₂O₂ decomposition by aqueous Fe²⁺.When 1 mM BPY was added into H₂O₂/pyrite systems,the rate constants of·OH production decreased by44.4%–65.6%,which suggests that both surface and aqueous reactions contributed to·OH production.Regarding the surface reaction,density functional theory（DFT）calculation reveals that H₂O₂ was adsorbed onto the Fe（II）sites on pyrite surface and transformed to surface adsorbed·OH which desorbed subsequently into the aqueous solution.On the basis of mechanistic understanding,a kinetic model was developed to assess the relative contributions of surface and aqueous reactions to·OH production.The relative contribution of surface reaction is dependent on the ratio of pyrite surface concentration to aqueous Fe²⁺concentration,which decreases with the progress of pyrite oxidation.When the ratio is higher than the threshold value of 1.6×10³ m²/mM,the surface reaction becomes predominant for·OH production.Besides acidic environments,pyrite oxidation also occurs in circumneutral environments,such as well-buffered marine and estuarine sediments and salt marshes where LMWOAs（e.g.,citrate and oxalate）prevail.To explore·OH production from pyrite oxidation at circumneutral conditions,50 g/L pyrite was added into 20 mM BA solution at pH 6–8.Productions of·OH and dissolved Fe²⁺/Fe³⁺were measured during oxidation course.For the oxidation of 50 g/L pyrite at pH 7 under oxic conditions,the cumulative·OH reached 7.5μM within 420 min in the absence of LMWOAs,whereas it increased to 14.8,12 and 11.2μM in the presence of 1 mM ethylenediaminotetraacetate,citrate and oxalate,respectively.Here,citrate was chosen to serve as an LMWOAs model to explore the influences of LMWOAs at different pH on·OH production.When the citrate concentration was increased from 0.5 to 5 mM,the cumulative·OH increased from 10.3 to 27.3μM within 420 min at pH 7.With the decrease in pH from 8 to 6,the cumulative·OH increased from 2.1 to 23.3μM in the absence of LMWOAs,but it increased from 8.8 to 134.9μM in the presence of 3 mM citrate.In the absence of LMOWAs,·OH is produced mostly from the oxidation of adsorbed Fe（II）by O₂.In the presence of citrate,·OH production is attributed mainly to the oxidation of Fe（II）-citrate^-by O₂ and secondarily to the oxidation of H₂O on surface-sulfur defects.Mechanisms of citrate enhanced·OH production were ascribed to the complexation of Fe（II）by citrate increased the yield of·OH production from H₂O₂ decomposition by Fe（II）and the acceleration of pyrite oxidation by Fe（III）-citrate.Fe（II）-citrate^-is generated mainly from the complexation of adsorbed Fe（II）by citrate and the reduction of Fe（III）-citrate,and the generation is suppressed by the oxidation of adsorbed Fe（II）.Fe（III）-citrate is generated predominantly from Fe（II）-citrate^-oxidation.Basing on these finding,we further explore the oxidative transformation of arsenic（As）during pyrite oxidation and the·OH production and CO₂ emission from oxygenation of acid sulfate soils.To explore the influence of·OH that was produced from pyrite oxidation under acidic conditions on the fate and transport of As（III）,6.67μM As（III）was added into 10 g/L pyrite suspensions at pH 2.6.The oxidation percentages of As（III）under oxic and anoxic conditions were 55.4%and 13.9%within10 h,respectively.Oxidation of As（III）to As（V）had negligible influence on As adsorption on pyrite surface.To explore the influence of·OH that was produced from pyrite oxidation under circumneutral conditions on the fate and transport of As（III）,2.67μM As（III）was added into 10 g/L pyrite under anoxic conditions at pH 7.4.Prior to exposing to oxic condition,the As-loaded pyrite suspension was mixed 30 min with the decrease in dissolved As（III）concentration to 1μM.For the oxygenation of As-loaded pyrite,the dissolved As concentration gradually decreased from 1 to<0.001μM.During the oxidation course,the As（III）was oxidized to As（V）,which was re-adsorbed into the generated Fe（III）oxyhydroxides.To explore the influence of citrate on the fate and transport of As（III）,0.1–1 mM citrate was added into above suspensions.With the citrate concentration increased from 0.1 to 1 mM,the As partitioned in the solution increased from 0.3 to 2.67μM.The addition of citrate can accelerate As（III）oxidation.The mechanisms of citrate-enhanced release of As were mainly attributed to the ligand exchange of citrate with As for pyrite surface sites,the competitive adsorption of citrate with As on Fe（III）oxyhydroxides and pyrite,and the partitioning of As on the newly formed Fe（III）colloids.To explore·OH production from oxygenation of acid sulfate soils and its oxidative impact,we collected 60 soil samples in four soil core profile with the depth at 0–150 cm in Bajia,Taishan city,Guangdong province.The typical experiments were mixed 2gram pristine soil with 10-mL solution（initial pH 4.4）under oxic conditions.For oxygenation of all of 60 soil samples,the cumulative·OH within 24 h reached by0.001–3.581μmol per gram,produced an average of 0.705μmol/g.Correlation analysis showed that the cumulative·OH was positively correlated with the contents of Fe（II）contained in pyrite and of adsorbed Fe（II）,while it was uncorrelated with the contents of organic carbon and of the other Fe（II）species,which suggested that the oxidation of pyrite and adsorbed Fe（II）mainly contributed to·OH production.Result of X-ray diffraction（XRD）proved that pyrite exist in pristine soils.Further,we explored the CO₂production from the oxygenation of acid sulfate soils,the cumulative CO₂ within 24 h was 0.76–54.5μmol/g,giving a mean production of 8.93μmol/g.When 5%ethanol,serving as an·OH scavenger,was added into pristine soil suspensions,the CO₂produced decreased in most case.The decrease of percentage was estimated to be a mean of 19.8%,which suggested that·OH can oxidize organic carbon to CO₂.When0.1%HgCl₂ that was used to kill microbes was added into pristine soil suspensions,the CO₂ produced significantly decreased,which suggested that the microbial respiration is an important pathway for CO₂ production.To explore the role of inorganic carbon dissolution on CO₂ production,2 gram sterilized soils was added into 10-mL solution that was contained 5%ethanol.Results showed that production of CO₂ increased with the increase in initial H⁺concentration,which suggested that the decomposition of inorganic carbon by H⁺also contributed to CO₂ production from oxygenation of acid sulfate soils.This study reveals the mechanisms of·OH production from pyrite abiotic oxidation especially for the electron transfers on interface and the relative contribution of each pathway and its impact on the fate and transport of As and CO₂ emission,which can supplement the fundamentals for pyrite oxidation and provides a knowledge for understanding the fate and transport of a substance in the natural system.Given the the strong oxidative property of·OH,it is necessary to consider the oxidation of·OH on the inhibited materials for the remediation of acid mine drainage,更多还原