Mechanisms Of Hydroxyl Radical Production From Abiotic Oxidation Of Pyrite And Its Environmental Impact

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 52.2?M,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 7.5?M within 420 min in the absence of LMWOAs,whereas it increased to 14.8,12 and 11.2?M 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 27.3?M within 420 min at pH 7.With the decrease in pH from 8 to 6,the cumulative·OH increased from 2.1 to 23.3?M in the absence of LMWOAs,but it increased from 8.8 to 134.9?M 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?,6.67?M 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?,2.67?M 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 1?M.For the oxygenation of As-loaded pyrite,the dissolved As concentration gradually decreased from 1 to<0.001?M.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 2.67?M.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 by0.001–3.581?mol per gram,produced an average of 0.705?mol/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 0.76–54.5?mol/g,giving a mean production of 8.93?mol/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,...