Sources,Migration,Morphological Transformation And Removal Of Tungsten From Typical Geothermal Water Environment In Tengchong

Tungsten is a transition metal in Group VIb of the Periodic Table of Elements and is widely found in rocks and natural waters.Because of its excellent metallic properties,tungsten has long been considered a non-toxic and harmless element and used as a substitute for toxic heavy metals,such as lead,and has been widely involved in industrial and military applications.Until the last two decades,reports of health and safety incidents involving tungsten have made the toxicology of tungsten and its health and environmental effects a hot topic of research.At present,studies on the sources of tungsten in the environment have focused on natural processes,such as dissolution of tungsten-bearing minerals in groundwater systems,volcanic activity,and evaporation concentration in arid alkaline lakes,etc.,while the research degree of tungsten geochemistry from geothermal system is relatively low.In this study,tungsten of geothermal water from typical hydrothermal geothermal area in Tengchong(Rehai,Ruidian,and Banglazhang)was selected as the object,and the source and speciation of tungsten in Tengchong geothermal waters were analyzed.Based on the systematic study of the geochemical behavior of tungsten at the water-mineral interface,migration,transformation and accumulation processes of tungsten in the geothermal environment are also discussed.In response to the potential contamination of tungsten-rich geothermal water in the discharge process to other types of natural water bodies that are sources of drinking water in the areas surrounding the geothermal zone,an iron-based anionic clay(pyroaurite,Fe-Mg-NO₃-LDH)that facilitates the effective removal of tungsten from water in terms of chemical structure was prepared.Its effectiveness in removing tungsten from geothermal water was objectively evaluated by laboratory experiments.It is expected to contribute to the elimination or minimization of the negative environmental effects of tungsten from geothermal sources in the future.The main research contents and results of the thesis include:1.The water chemistry and isotopic characteristics of geothermal waters in study area were analyzed,and the genesis of geothermal water and the source of the dissolved components were discussed.Neutral/weakly alkaline hot springs were distributed in Rehai,Ruidian and Banglazhang geothermal area,and acidic hot springs only found in Rehai geothermal area.The hydrochemical types of acidic geothermal waters in Rehai geothermal area are mainly Na(�Ca)-SO₄ and Na-SO₄�Cl types.The hydrochemical types of neutral/weakly alkaline geothermal waters in Rehai and Ruidian geothermal area are mainly Na-Cl,Na-Cl�HCO₃and Na-HCO₃�Cl types,and the geothermal waters in Banglazhang geothermal area are mainly Na-HCO₃�SO₄and Na-HCO₃ types.In the major components,acidic geothermal waters in Rehai had higher contents of Ca²⁺and SO₄^2-and lower contents of Na⁺and Cl^-,and do not contain HCO₃^-.Neutral/weakly alkaline geothermal waters in Rehai and Ruidian had higher contents of Na⁺,Cl^-and HCO₃^-and lower contents of Ca²⁺and SO₄^2-.Geothermal waters in Banglazhang had higher contents of Na⁺and HCO₃^-and lower contents of Ca²⁺and Cl^-.The concentration of Mg²⁺did not change significantly in waters of geothermal areas.Among the characteristic components,contents of As,F,Li,Rb,and Cs in acidic geothermal waters from Rehai were the lowest,and contents of Li,Rb,and Cs in neutral/weakly alkaline geothermal waters from Rehai and Ruidian were highest.The sample with the highest content of Li was found in Ruidian,and the content of F in geothermal waters of Banglazhang was the highest.The content of Si did not change significantly in waters of geothermal areas.The characteristics of hydrogen and oxygen isotopes show that the geothermal water is mainly derived from local meteoric waters recharge,and some samples of neutral/weakly alkaline geothermal waters in Rehai show significant oxygen drift,indicating that geothermal water is also supplied by magmatic fluids.The sulfur isotope features indicate that sulfur species in the Banglazhang geothermal waters are not from magmatic fluid.The correlation analysis between the components of geothermal water and Cl-shows that there is a parent geothermal liquid rich in Cl^-,Na⁺,K⁺,F^-,As,Li,Rb,Cs,etc.,and the neutral/weakly alkaline springs of Rehai at the surface are formed by adiabatic cooling,conductive cooling,or dilution by waters derived from shallow sources of this parent geothermal liquid.If the geothermal fluid cools adiabatically during upward flow,the H₂S-rich geothermal vapor separated from it can heat the upper stagnant water near the surface,and at the same time,the H₂S in the geothermal vapor is oxidized in the shallow oxidizing environment to produce SO₄^2-and H⁺,which is the reason for the formation of acidic springs in Rehai.In the case of the Banglazhang geothermal water,there is no magmatic heat source or there is a magmatic heat source but it is not influenced by magmatic fluids,and therefore no parent geothermal fluid is formed in the deep part of the geothermal system.Thus,the chemical components in the Bangla palm geothermal water are the result of the strong leaching of the thermal storage envelope by the deeply circulating infiltrating genesis water under the background of high heat flow(of course,the content of each chemical component is inevitably influenced by the mixing of shallow groundwater).2.The geochemical origin of tungsten in geothermal water was identified,and its migration and accumulation patterns in the geothermal water environment were analyzed.The concentration of tungsten in geothermal water samples in the study area varied from0.2 to 265.9 mg/L(average:66.1 mg/L),which was much higher than the tungsten content in shallow surface water and cold groundwater.Compared with acidic geothermal water,neutral and alkaline geothermal water are more favorable for tungsten enrichment.The geothermal fluid source and its formation mechanism are decisive factors for the distribution pattern of tungsten in geothermal water.The Rehai geothermal system has a magmatic heat source,and its geothermal water has a significant positive correlation between Na,Cl,As,Li,Rb and Cs components and tungsten,indicating that tungsten and the above components are the same conservative components and share similar geochemical behaviors.In contrast,there is no significant correlation between Mg,Ca,SO₄^2-,Si and other components and tungsten,which is related to the precipitation of relevant hydrothermally altered minerals within the thermal reservoir or the mixing of Mg-and Ca-rich shallow groundwater.There is no obvious correlation between tungsten and other chemical components in Banglazhang geothermal water,which is due to the fact that the main source of each chemical component is the water-rock interaction between geothermal water and surrounding rocks during deep circulation,and its concentration is influenced by multiple factors such as surrounding lithology and its porosity/fracture rate and mineral composition,the degree of water-rock interaction,and the mixing ratio with shallow cold water.Hydrogeochemical studies of tungsten and related chemical fractions suggest that magmatic water recharge may be one of the main sources of tungsten in the geothermal waters in Rehai.But the positive correlation between tungsten content and geothermal reservoir temperature suggests that water-rock interactions between geothermal waters and surrounding rocks in the Rehai,Ruidian and Banglazhang contribute generally to tungsten in geothermal waters.The W/Mo ratios of geothermal water in the study area range from 42 to 218,which are much higher than those in general natural waters,indicating that tungsten is more conserved in geothermal water than molybdenum and its concentration is generally not controlled by the solubility product constants of tungsten-containing minerals during migration.However,the tungsten content in the sediments of hot spring in the study area varied from 0.3 to 990.5ppm(average:94.5 ppm),and the tungsten content in the sediments of acidic hot spring was generally much higher than that of neutral and alkaline hot spring,because the tungsten in acidic geothermal water tends to precipitate as tungsten oxide or be sorbed by iron-bearing minerals in the hot spring sediments.In contrast,tungsten in geothermal water under neutral and alkaline conditions is relatively conservative(relatively difficult to precipitate or be sorbed),and therefore can flow with the hot springs to longer distances or even sink into the surface waters of the geothermal zone.3.The form and influencing factors of tungsten in geothermal water were identified,and the transformation between different forms of tungsten were explored.The forms of tungsten in geothermal water include monomeric state of tungstate,polytungstate and thiotungstate,and them interconvert with changes in p H,total tungsten concentration,ionic strength,sulfide concentration and other aqueous environmental conditions.p H is the main controlling factor for the distribution of polytungstate in geothermal water.When it is less than 7,monomeric tungstate is likely to polymerize.As the p H value decreases,WO₄^2-,HW₆O₂₁^5-and W₁₀O₃₂^4-successively become the major forms of tungsten in geothermal water.While at very low p H(?1),tungsten precipitates as tungsten oxide(WO₃�H₂O).The low concentration of total tungsten in geothermal water is not conducive to the polymerization of monomeric tungstate,and W₁₀O₃₂^4-can become the primary form of tungsten only at low p H;as the concentration of total tungsten increases,the species of polytungstate and its percentage in total tungsten also increase.The effect of ionic strength of geothermal water on polytungstate is relatively small,as shown by the variation in the percentage of different types of polytungstate in total tungsten in a specific p H range.Sulfide concentration and S(II)/W ratio are the key factors controlling the formation of thiotungstate in geothermal water.Only when the S(II)/W ratio in geothermal water is greater than 100 will the monomeric tungstate undergo sulfation and form thiotungstate,and as the S(II)/W ratio increases,both the percentage of thiotungstate in total tungsten and its degree of sulfation increase accordingly.In addition,p H will also affect thiotungstate in geothermal water.Tungstate is only converted to thiotungstate within a p H range of 2?10,and the optimal p H range for thiotungstate formation is 4?6.4.The geochemical behavior of tungstate and thiotungstate at the interface between water and iron-bearing minerals was investigated,and the influence of typical iron-bearing minerals in sediments of hot springs and their outflow channels on the environmental geochemical fate of tungsten in geothermal system was analyzed.The distribution characteristics of tungsten along the hot spring outflow channels(both in the hot spring water and in the hot spring sediments)indicate that tungsten of geothermal system origin is easily sorbed by iron-bearing minerals in the springs and their sediments after discharging from the surface with the hot springs,and thus accumulates in the surface environment of the geothermal zone.However,different forms of tungsten in geothermal water have different affinities for sorption on various types of iron-bearing minerals.Tungsten in the form that is relatively difficult to sorb by iron-bearing minerals,can be retained in geothermal water and may be discharged with it into other types of natural water bodies downstream of the hot spring area as a source of drinking water for the population.In this study,batch experiments on the sorption of tungstenate(WO₄^2-)and thiotungstate(WS₄^2-)in water by typical sulfide minerals of iron(pyrite)and typical oxide minerals(goethite)were designed and carried out in this study to gain insight into the geochemical behavior of different forms of tungsten within the surface environment of the geothermal zone.The results show that goethite sorbs different forms of tungsten in water more strongly than pyrite,and both minerals sorb WS₄^2-more strongly than WO₄^2-.The results of the sorption kinetics of WO₄^2-and WS₄^2-onto the two minerals showed that the sorption rate was mainly controlled by the chemisorption process,and the sorption isothermal experiments showed that the higher temperature was favorable for the sorption.The p H has a certain effect on the adsorption of tungsten by both minerals.The sorption efficiency of WO₄^2-and WS₄^2-by the two minerals increases and then decreases with increasing p H,reaching a peak at p H=5.The form transformation of WO₄^2-and WS₄^2-at lower p H and the competitive sorption of OH^-at higher p H are the main reasons why p H affects the sorption of tungsten by iron-bearing minerals,and the latter effect is more significant.In summary,for neutral/alkaline thermal springs with low sulfide content and S(-II)/W,the geothermal system source tungsten possesses a greater potential to contaminate surface waters downstream of the geothermal zone if the iron-bearing minerals in the hot spring sediments on the outflow channel are dominated by iron sulfide minerals.5.Systematic tungsten removal experiments were carried out using a nitrate intercalated pyroaurite-like anion exchangeable clay(Fe-Mg-NO₃-LDH),and the effects of p H and the main coexisting anions in geothermal water on its tungsten removal performance were analyzed,then the mechanism of tungsten removal by Fe-Mg-NO₃-LDH was discussed.The migration and accumulation characteristics of tungsten in geothermal water indicate that tungsten from geothermal systems can contaminate the sediments and soils near the hot springs and the surface water bodies into which the hot springs feed,and the negative environmental-health effects cannot be ignored when the contaminated surface water bodies are the drinking water sources for the residents in the geothermal area or its surrounding areas.In this study,a new iron-based anionic clay(Fe-Mg-NO₃-LDH)was prepared for the removal of tungsten from geothermal water by taking advantage of the strong affinity between iron and tungsten.The batch experiments of tungsten removal by Fe-Mg-NO₃-LDH showed that the maximum sorption of tungsten in solution could reach69.0 mg/g.The kinetics data can be better fitted to the pseudo-second-order model taking chemical sorption into account than the pseudo-first-order model,indicating that the sorption rate in this reaction system is mainly controlled by the chemisorption process.The isothermal adsorption experiments showed that the tungsten in solution was mainly sorbed by the monolayer of Fe-Mg-NO₃-LDH in the form of complexation with the laminate Fe,and the high temperature was favorable to promote the sorption reaction.Due to the high p H buffering capacity of Fe-Mg-NO₃-LDH,it can maintain a stable tungsten removal capacity within a p H range of 3?11.The coexisting anion competition adsorption experiments showed that the prevalence of SO₄^2-in geothermal water had the most obvious effect on tungsten removal efficiency,while HCO₃^-and Cl^-had little effect on tungsten removal.The characterization of Fe-Mg-NO₃-LDH before and after the reaction indicates the mechanisms for the efficient removal of tungsten from water by Fe-Mg-NO₃-LDH include primarily the anion exchange of aqueous tungstate with interlayer nitrate and possibly the stronger inner-sphere complexation of tungstate with iron in the layers.