The Environmental Fate And Bioavailability Of Lead In Shooting Range Soils

Exposure to Lead(Pb)is of global concern due to its potential risks to human health.Shooting range sites are a major contributor of anthropogenic Pb to the envi-ronment.Shooting ranges may contain lead(Pb),antimony(Sb),copper(Cu),and ar-senic(As)that leach from lead-based bullets,fragments,and bullet jackets.Spent bul-lets accumulate in the stop butt of shooting ranges and are subject to weathering over time.This releases heavy metal(loid)s into the soil,posing a potential risk to employ-ees,shooters,and the surrounding environment.In China a large amounts of bullet are consumed annually at shooting range sites resulting in extremely elevated Pb concen-tration in soils.However,these sites have been insufficiently investigated.What's more,previous studies have concentrated mainly on the extent of lead contamination,the mobility of lead and lead ecotoxicology.Little effort has been made to determine Pb bioavailability based on human health risk assessment and to establish its relation-ship with Pb speciation and physicochemical characterization of shooting range soilsIn order to investigate contaminant levels,and shed light on the fate and bioa-vailability of Pb in different types of shooting range soils,as well as providing valua-ble fundamental information for risk based assessment and/or remediation,21 con-taminated soils and weathered bullets from 5 different shooting ranges of China were collected and analysed for:1)total heavy metal content including Pb,Cu,As,Sb;2)speciation of Pb in soils using selective sequential extraction(SSE),X-ray power dif-fraction(XRD)and X-ray absorption near edge structure(XANES);3)mineral com-pounds in weathered bullet using scanning electron microscopy(SEM)/energy disper-sive X-ray spectroscopy(EDS)and electron microprobe analysis(EMPA);4)Pb bio-availability using an in vivo mouse-kidney/liver assay and two in vitro digestion mod-els:Unified BARGE method(UBM)and relative bioavailability leaching procedure(RBALP).Soil concentrations were highly elevated for Pb(ranging from 847.2 to 15,550.0 mg/kg,averaged at 4,796 mg/kg)and Cu(ranging from 41.0 to 1319.3 mg/kg,aver-aged at 330.1 mg/kg).Sb was only significantly increased in the most contaminated Chengdu(CD)range,while As was below measurement thresholds.The order of con-tamination level of these 5 shooting ranges is:CD>FY>HF>CZ>BC.XRD results showed mineral compounds were varied in different shooting rang-es according to the soil properties and climate condition.While hydrocerussite(Pb3(CO3)2(OH)2)and pyromorphite(Pb5(PO4)3Cl)were predominant in alkaline Baicheng(BC)soils with P present,anglesite(PbSO4)were the dominating crystal phased of Pb in acidic soil of Chuzhou(CZ)and acid rain affected CD range.Shan-nonite(Pb2O(CO3))was present in all five ranges,likely due to special high-temperature conditions common at shooting ranges.SEM/EDS showed that weather-ing products on the lead layer inside bullets were cerussite and hydrocerussite sug-gesting a conspicuous chemical transfer of Pb from the bullets to the ambient soils.On the corroded lead layer surface,unevenness,microcracks,and spallation were usually present.Adjacent to the microcracks,various euhedral and subhedral crystals of the secondary products of lead were formed.Cerrusite crystals have laminated,double cone,and snow fake shapes,while hydrocerussite crystal has a hexagonal ap-pearance.Visual Minteq speciation of solution Pb reported that Pb(OH)+and Pb2+ were the predominant Pb species in soil solutions.Selective sequential extraction results indi-cated that iron/manganese oxide-bound,carbonate-bound and humid acid bound were the major fractions of Pb and Cu,with predominant phase varying in different ranges.Only acidic CZ soil had a relatively high portion of ion-exchangeable Pb(14%).Re-sidual fraction of Pb account for less than 5%in all ranges while it is the dominant phase of Sb and As.Correlation analysis showed that pH and ion exchangeable Pb were significantly negatively correlated.Finer particle percentage(<63 ?m)were pos-itively correlated with the total concentration of each metal and negatively correlated with their migration ability indicating a greater retention of metals in finer particles.The order of migration ability of metals in soils was Pb>Cu>Sb>As.Once bullets are discharged and undergo weathering,cracks form on the surface of the lead layer,allowing exposure to air,water,and soil.Consequently,this process can intensify bullet weathering and Pb migration.Weathering crusts from the steel-core bullets were 0.8-1 mm wide with two rings.The inner ring primarily consisted of FeOx,a corrosion product of steel.Heavy metals,such as Cu,Zn,and Pb,from the inner bullet,were distributed as stains on the FeOx matrix.In the outer ring,quartz,feldspar,and calcite from the surrounding soils were embedded in the FeOx matrix.Bioaccessible Pb concentration varied significantly among ranges,but the vari-ance of Pb bioaccessibility(Pb BAc)was very small,with all shooting ranges over 90%of Pb were bioaccessible by in vitro assays.Pb BAc result of RBALP and gastric phase of UBM were similar and highly correlated(R2 = 0.9889),with an average of 98%(PbRBALP)and 95%(PbUBM-G).Pb relative bioavailabilities(Pb RBA)using an in vivo mouse assay had a range of 85?100%and 66?115%using kidney and liver as target organs respectively.Compared to other contamination sources,shooting range soil has a high Pb RBA,which is in accordance with the Pb BAc result.Previous stud-ies on bioavailability of shooting range soils using both mouse model and RBALP were added to the results from this study to create a linear comparison between both methods,showing a strong linear correlation:y = 1.14 x-25.74,R2=0.75,p<0.05,indicating simple and straightforward RBALP assay can be a reliable method to pre-dict Pb bioavailability.Pb in water-soluble fraction + ion exchangeable fraction +Carbonate bound fitted best with the bioaccessible lead using linear regression model(y = 0.39 x + 28.732,R2=0.89,P<0.001).However,no significant relationship was found between the single soil properties or Pb fractions and Pb BAc.To summarise,the shooting range soils tested showed high concentrations of bi-oavailable lead.Subsequently lead from these soils was also found to have a relatively high mobility.Therefore risk-based management and/or remediation strategies should also assume high bioavailability of lead in shooting range soils.