| Heavy metals in the lead-zinc smelting slag stored in the open air will be dissolved and released under the rainfall and surface runoff,and migrate and penetrate into the surrounding water and soil environment,resulting in huge environmental risks.Therefore,the study on the release and migration characteristics of heavy metals in zinc smelting slag is of great significance for its environmental impact assessment and pollution control.Taking the zinc volatile kiln slag as the research object,the dynamic leaching test was carried out to study the release characteristics of As,Cd,Cu,Pb and Zn in slag under the conditions of acid rain and dry wet alternation,the process of soil heavy metal pollution induced by waste slag under the conditions of soil covering and open storage,and the migration,enrichment and retention characteristics of heavy metals in waste slag leachate in site profile soil.The main results are as follows:（1）A 50 days column leaching experiment was carried out to study the release characteristics and chemical speciation change of heavy metals in rotary kiln slag after zinc extraction under simulated acid rain with different p H values（p H4.0,p H5.6,p H7.0）under dry-wet alternation and continuous leaching（CLp H4.0）.The results showed that strong acidity rain and dry-wet alternation promoted the release of Pb in the slag during the whole leaching period,and the release of Cd,Cu and Zn was promoted during the late leaching period（35-50 d）.Acid solution promoted the rapid release of heavy metals from waste residue at the initial stage of leaching.The concentration of heavy metals in leachate under p H4.0 and CLp H4.0leaching reached the peak value on the 5th day,while the peak value was reached on the 10th day under p H5.6 and 7.0 treatments.The concentration of As,Cd,Cu,Pb and Zn in leachate was in the range of 0.008-0.197,150-1438,1.88-234,1.30-5.96 and 15.2-120 mg·L-1,respectively.The concentration of Cd,Cu,Pb and Zn was 1500-14380,3.76-468,1.30-5.96and 7.6-60 times of the threshold limit of emission standard of pollutants for lead and zinc industry（GB25466-2010）,respectively.The higher leachate concentration of Cd was in consistent with their higher content of active forms in slag based on BCR sequential extraction analysis.The transformation of residual fraction As and Cu to more active fraction was promoted by acid rain leaching.The mineral dissolution of slag controls the dissolution and release of heavy metals.XRD analysis showed that the diffraction peak of Pb SO3 in leached slag was disappeared,and the diffraction peak intensity of Cu and Zn minerals decreased.The results suggested that Cd,Cu,Pb and Zn in slag after leached by acid rain could pose great environmental risks and need pollution control.（2）The leaching test of slag and soil was carried out to study the influence characteristics of heavy metals on subsoil pollution under the soil covering treatment of zinc volatilization kiln residue and open-air storage under the scenarios of dry-wet alternation and continuous leaching of simulated rainfall for 50 days.After leaching treatment,the contents of Cd,Cu,Pb and Zn in the subsoil of slag increased by 165～185,2.72～6.56,3.11～4.67 and 1.31～1.52 times respectively.The release and accumulation of heavy metals in the slag were aggravated by the soil covering treatment and the alternate wetting and drying leaching of slag.The increase of heavy metals in covering soil treatments were greater than that in open-air stacking treatments,and the dry-wet alternate treatments were greater than that in continuous leaching treatments.The concentration of Cd and Pb in soil TCLP leaching solution exceeds 2109-2319 and 3.90-13.3 times of the class IV standard limit in the quality standard for groundwater（GBT14848-2017）.The heavy metals in the slag were mainly deposited in the soil with the form of acid extractable.The maximum increase of acid extractable Cd and Zn in the soil was 33.4%～37.2%and 20.5%～26.4%,followed by Cu（5.15%～8.65%）and Pb（2.40%～8.49%）after leaching.And the content of organic matter,urease and catalase activities decreased significantly（P<0.05）.Among them,the decline range of slag covering soil treatments were greater than that of open-air storage treatments,and the dry-wet alternative treatments were greater than that of continuous leaching treatment,which was related to the high retention of heavy metals in soil.After leaching,The environmental quality of the soil has decreased.The richness and diversity of microbial communities in soil decreased,and the abundance of Proteobacteria,Chloroflexi,Actinobacteriota and Acidobacteriota increased,which were the dominant bacteria.Bacteroidota,Spirochaetota,Desulfobacterota and cyanobacteria were significantly negatively correlated with the contents of Cd,Cu,Pb and Zn.Generally,the soil covering treatment of slag will promote the migration and accumulation of heavy metals in the soil and increase the risk of soil heavy metal pollution.（3）A 90 day dynamic leaching experiment was carried out to study the leaching and migration characteristics of heavy metals in slag leachate in the soil of smelting site.The results showed that heavy metals in topsoil would migrate downward with small particle size soil.The content of heavy metals and the proportion of acid extractable heavy metals in 0-10cm topsoil increased significantly,showing the characteristics of surface enrichment.The content and activity of heavy metals decreased with the increase of profile depth.Under continuous leaching treatment,the enrichment of heavy metals in 0-5 cm soil was significantly higher than that in dry wet alternative treatment,on the contrary in 5-10 cm,it was related to the fact that dry wet alternative treatment promoted the downward leaching of organic carbon,which was conducive to the downward migration of heavy metals.Cd,Cu,Pb and Zn in the soil of 0-10 and 10-15 cm profile soils were enriched in particle size less than 1.0mm,and the content of heavy metals in the soil with particle size of 2.0～1.0 mm in 10-15 cm profile was higher,which was related to the specific mineral composition of the profile soil.The change of acid extractable state of heavy metals in particle size soil was the same as that of heavy metals.The contents of Cd,Cu and Pb in the profile soil far exceed the class II screening value of China’s standard for management and control of soil pollution risk of construction land（GB36600-2018）.The soil has strong adsorption capacity for heavy metals.In general,heavy metals remain on the surface after leaching,and are mainly concentrated in smaller particle size particles in the form of acid extractable state,indicating that the potential ecological risk of small particle size soil particles is greaterGenerally,As,Cd,Cu,Pb and Zn in zinc smelting volatile kiln slag will be released and migrated to soil and groundwater under rainwater leaching,resulting in great environmental risks,especially Cd.It is necessary to strengthen the prevention and control of rainfall leaching pollution. |
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