Cadmium Isotope Fractionations And Its Mechanisms During Photodissolution And Adsorption Onto Calcite

Determining isotope fractionation factors in specific geochemical processes and corresponding fractionation mechanisms are not only the keys to dissecting natural isotope signals,but also the footstone for stable isotope geochemistry.The cadmium（Cd）isotope system has emerged as an effective tracer for studying Cd geochemical cycles,and has made impressive progresses in paleo-environment reconstruction,Cd pollution tracing,etc.As Cd is a dispersed element,it’s difficult to collect enough Cd isotope signatures in each geological end-members.At present,Cd isotope research is still in the stage of Cd isotope data accumulation and fractionation factors determination for various processes.To date,Cd isotope behaviors during evaporation/condensation,adsorption,precipitation,biological,and weathering processes have been studied tentatively.However,Cd usually undergoes very complex processes during its geochemical cycles,among which dissolution of solid Cd and adsorption of dissolved Cd are very common.For example,Cd could be released to aqueous from Cd sulphides due to not only weathering,but also photo-irradiation,and subsequently,dissolved Cd can be sorbed onto minerals’surfaces which is related to mineral facies（such as calcite and birnessite,etc.）and aquatic chemistry（co-existed compounds）.It’s prerequisite to determine Cd isotope fractionation factors and the fractionation mechanism in relevant conditions for the natural Cd isotope signals interpretation.Hence,in this work,based on the development of high precision Cd isotope analysis method,Cd isotope behaviors during Cd sulphides photodissolution and dissolved Cd sorbed onto calcite are studied,and the main results are as follows:1.Developed a high precision Cd isotope analysis method using double spike MC-ICP-MS.Cd was separated from samples using AG-MP-1M resin and TRU resin,of which recovery was>95%and blank was<0.1 ng.The effect of the double spike ratio in the double spike-sample mixture（p）on the measuredδ^114/110Cd was evaluated,and the appropriate p ranged between 0.1-0.8.In addition,the observed shift/deviation of the measuredδ^114/110Cd _{NIST 3108} from 0 was discussed on a theoretical basis and a correction approach is developed.Potential polyatomic ions have no significant effect onδ^114/110Cd measurement,while isobaric ions such as Sn,Pd and In can seriously affect the accuracy of measurements.The tolerances of isobaric interference can be improved to 0.1,0.05and 0.02 for Sn/Cd,Pd/Cd and In/Cd,respectively,after mass discrimination correction.Long term monitoring of the measured NIST 3108 and BGEG-Cd showed repeatability forδ^114/110Cd of 0.00±0.04‰and-1.00±0.04‰（2sd）,respectively.Finally,we measured Cd isotope compositions of several geological standard materials,and the measuredδ^114/110Cd values were in good agreement with the reported values.2.Geochemical cycle of Cd sulphides occurs widely in sulfide deposits,soils,and estuaries,etc.Although the impacts of photo-irradiation on Cd release from cadmium sulphide in natural settings were demonstrated,no quantitative constraints exist on the associated isotope fractionations.In this work,we studied Cd isotope behaviors during photodissolution of Cd sulphides(Cd S and Cd S_xSe_1-x)and identified Cd isotope signatures during photodissolution under different conditions.The photodissolution in MQ water appeared to follow the Rayleigh model withΔ^114/110Cd_{aqueous-remained} of+0.27‰(Cd S_xSe_1-x)and+0.19‰（Cd S）,respectively.The presence of Na Cl significantly promoted the dissolution and resulted in enlarged Cd isotope fractionation,which was better fitted by the equilibrium fractionation model withΔ^114/110Cd_{aqueous-remained} of+0.82‰(Cd S_xSe_1-x)and+0.61‰（Cd S）,respectively.However,when humic acid involved,photo-dissolved Cd was analytically indistinguishable from raw solid sulphides despite with the highest release kinetics.Dissolved Cd species were determined using extended X-ray absorption fine structure（EXAFS）.We speculated that Cd-HA complex played a key role during Cd sulphides photodissolution with HA.Different photodissolution pathways resulted in the diverse Cd isotope fractionation patterns and kinetics.Photodissolution of Cd sulphides is similar to weathering（simulate acid dissolution）,two-dimension information on dissolution rates and isotope behaviors could potentially distinguish photodissolution from the acidic dissolutions.This work can be considered as a reference for evaluating the contribution of photoirradiation on the dissolution of Cd sulfide under relevant natural conditions and as well as for other photosensitive minerals.Additionally,Cd isotope might give insight into cadmium-sulphide photocorrosion processes and mechanisms in different settings.3.Calcite,which widely spreads in soil,and ocean etc,is an important sink for dissolved Cd.Determining Cd isotope fractionation factors during Cd sink into calcite is the key to interpret natural carbonate Cd signals.Sinking Cd into calcite generally involves two steps,fast adsorption and followed slower uptake（e.g.,co-precipitation）.However,the available publications are only focused on fractionation factors during Cd co-precipitate into calcite.In this work,we studied Cd isotope behaviors during adsorption onto calcite and examined influences of several co-existed compounds on Cd isotope fractionation factor.Cd was determined to be enriched in light isotopes at calcite surface in all experiments.And the collected isotope data are consistent with a closed system equilibrium exchange between the aqueous and the sorbed phases.In MQ water,Cd isotope fractionation factor is(α_{sorbed-aqueous})0.99934,and the presence of（in）organic compounds would alter thisαin different degrees.α_{sorbed-aqueous} is altered to 0.99926 and0.99960 in the presence of 1,3-propanedithiol and citrate,respectively.The most impressive result could be the observation ofα_{sorbed-aqueous}=0.99984 influenced by PO₄^3-,which may emphasize the role of PO₄^3-in oceanic Cd isotope signatures interpretation.Coordination environments of sorbed Cd were determined by EXAFS.And based on the results,the preferential adsorption of lighter Cd could be interpreted from the change of Cd aqueous speciation and the structure of Cd complexes formed at calcite surfaces.Octahedral Cd-O would be distorted in different degree when aqueous Cd sorbed as inner-sphere complex in the cases of MQ water,-thiol,and citrate since the increased coordination number of O atoms and subtly lengthened Cd-O bond length.And the very small Cd isotope fractionation in the case of phosphate may result from the barely changed Cd coordination environment between aqueous and sorbed species.Our results give an alternative consideration in addition to co-precipitation when applying carbonated Cd isotope compositions in paleo-geology studies.