Migration And Transformation Of Sulfur In Paddy Soil And Dissolution Of Mechanism Of Heavy Metal In Secondary Iron Sulfate Mineral In Metal Sulfide Mine Area

Acid mine drainage?AMD?is formed during mining activities and is a typical environmental problem around the world.In addition,many mine areas have a low and variable rainfall,which result in water-deficiency.Therefore,AMD is often used for irrigation of agriculture crops,and this resulted in the contamination of soil,reflecting their acidity,toxicity and iron sulfate mineral in the environment.The distribution,transport and toxicity of heavy metals in soils contaminated by AMD has been studied intensively by researchers over all the time.However,the information regarding the speciation,spatial distribution,migration and transformation of associated concentrations of sulfur have been scarcely reported.Although sulfur is relatively nontoxic,its migration and transformation play a significant role in soil acidity and mobilization of toxic metals.In the present work,we have surveyed the farmland affected by AMD in the Dabaoshan mine area,Guangdong province to characterize 1)species and distribution,2)migration and transformation,3)fractionation of sulfur isotopes of sulfur in paddy soil and 4)mechanism of dissolution of heavy metal in iron sulfate mineral.The detailed informations were summarized as followes:?1?Spatial and temporal distributions of sulfur speciations in paddy soils affected by AMD: From Liangqiao?S1?to Xuwu?S5?,concentrations of exchange sulfate or total sulfur?TS?in soil did not exhibit clear trends along downstream of the Hengshi river where the TS in river were previously found to gradually decrease from upstream to downstream.Although the paddy soils in S1-S5 has not been irrigated by Hengshi river in recent years,the concentrations of exchange sulfate total sulfer in these soils were much higher than that in control site soil?S6 in Shuangshi village?.Hence,exchange sulfate and total sulfer may be considered as the evidence of AMD irrigation.Furthermore,the largest amount of exchange sulfate was measured in 20³0 cm depth in Shangba village?S3??934 mg/kg?in 2012,this is because the large surface areas of soil particles and the abundant amorphous iron hydroxides or fine clay could greatly enhance the sulfate adsorption.In addition,cyclic variation of exchange sulfate was not significant from 2012 to 2014.In soil profile in S3,the highest concentrations of reduced inorganic sulfur?RIS?was found at depths of 0-20 cm and was significantly positively correlated with total soil C and N contents.Exchange sulfate was the dominating sulfur species in 20-30 cm depth.Furthermore,ester sulfate?E-S?was the dominating sulfur species in every layer.Therefore,the exchangeable sulfur and ether bond sulfur were mainly formed by AMD irrigation.?2?The migration and transformation of SO₄^2-in AMD contaminated soil: additional adsorption of SO₄^2-?0¹000 mg/L?showed that no SO₄^2-was adsorpted in the soils of AMD in Shangba village?5-layer soil profile at 0⁸0 cm depth?,but a large amount of SO₄^2-was desorbed.Specifically,52%-86% of SO₄^2-was desorbed in the surface soil?0²0 cm?.The reason is theforms of SO₄^2-in the soil profile were different.In surface soil,the organic matter and PO43-?450 mg/kg?with high concentration occupy the sites of SO₄^2-on soil particles,which result in watersoluted sulfate?WS??⁸7% total sulfate?being the dominant species.With increasing depth,there was a peak of adsorption by inner-sphere complexes at 20-30 cm soil depth where SO₄^2-mainly formed special adsorption with amorphous iron oxides and hydroxyl??H-O-H?.Moreover,in each layer soil has reached saturation and contains WS which was not adsorbed.The SO₄^2-came from the mineralization of organic sulfur?ester sulfate?.Isotope analysis results showed that the ?34S value of WS and AS was negative-4.60‰ and-5.67‰,which was lower than the ?34S lowest value?-1.03‰?in Hengshi river water.The SO₄^2-utilized by plant or microorganisms was mainly SO₄^2--?32S,which result in the higher concentration of SO₄^2--?34S in the soil.Therefore,the main migration and transformation pathway of SO₄^2-is due to the exchange sulfate desorption and E-S mineralization.?3?Fractionation characteristics of sulfur in river and farmland: Due to the formation of secondary iron sulfate minerals and their adsorption to SO₄^2-,the sulfate in river was significantly reduced?2250 mg/kg²50 mg/kg?from the Liwu mud impoundment?RS1?to Shuilouxia?RS5?.However,SO₄^2-in the river and sediment reaches ?34S varied from-1.24‰ to-1.16‰,indicating that fractionation of sulfur were not obviously affected by the formation and adsorption of secondary iron sulphate minerals..In addition,?34S of exchange sulfate in farmland?S1^S5?varied from-4.23‰ to +4.66‰,which did not show regular distribution,and cultivation method?dry land and paddy field?did not affect the fraction of sulfur isotope.?4?Mechanism of dissolution of heavy metals in secondary iron sulfate minerals: in the AMD environment,Fe3+ and SO₄^2-are easily replaced by other ions?such as Cu2+ and Cr O42-?in the chemical structure of the secondary iron sulfate minerals.Fe3+ and SO₄^2-were substituted by Cu2+ and Cr O42-,respectively.We found that p H had a great influence on the stability of secondary iron sulfate minerals and the dissolution of heavy metals.The dissolution of Cu2+ and Cr O42-in p H 1 was significantly higher than that at p H = 10.The dissolution of Cr reached the dissolution equilibrium in the 4d in both p H conditions.At p H 1 and 10,the dissolved Cr O42-was adsorbed on the mineral surface by complexation with the Fe3+ with high concentration?1.74 mmol/L?and maghemite,respectively,which prevented the further dissolution of minerals.The dissolution rate of Cr O42-in jarosite was significantly higher than that of Cu2+,which was mainly due to the formation of highly stable Cu O6 octahedra after copper dissolution.Moreover,Cr O42-can be quickly dissolved from minerals,but it was easily adsorbed or complexed.With less ability of protonation and complex,Cu2+ will continue dissolve out from jarosite.Therefore,the dissolution mechanism of heavy metals in the jarosite was mainly affected by the p H,the substituted ions and the structure of the substituted sites.