Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 17 (2017) 161 – 164 15th Water-Rock Interaction International Symposium, WRI-15 Removal of harmful constituents from geothermal water by selected anion clays Yaowu Caoa, Qinghai Guoa,1, Yaqin Zhuanga, Zhengyan Yua, Wei Guoa, Canhai Zhangb, Mingcheng Zhub, Qian Zhaoa, Tianlin Rena a State Key Laboratory of Biogeology and Environmental Geology & School of Environmental Studies, China University of Geosciences, 430074 Wuhan, Hubei, P R China b Huanghe Hydropower Development CO., LTD., State Power Investment Corporation, 810008 Xining, Qinghai, P R China Abstract Four kinds of anion clays (hydrocalumite, hydrotalcite, iowaite and takovite) synthesized in this study were used for removing harmful constituents from the geothermal waters collected from three hydrothermal areas located in Qinghai Province, northwestern China Iowaite performed better than the other anion clays in term of water dearsenication, while hydrotalcite exhibited higher efficiencies on removal of fluoride and sulphate The increase in dosage of iowaite dramatically elevated the chloride concentrations of the treated geothermal waters as the enhanced release of original interlayer chloride in the iowaite duo to the anion exchange Therefore, hydrotalcite is the optimal sorbent for uptake of undesirable constituents in Qinghai geothermal waters and is promising for wider use in treatment of geothermal waters in the future © TheAuthors Authors.Published Published Elsevier © 2017 2017 The by by Elsevier B.V.B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Anion clay; geothermal water; harmful constituent; sorption; Qinghai Introduction Sorption reactions at mineral-water interfaces have a profound impact on the solid-water treatment of harmful constituents and thus play an important role in controlling the mobility and fate of these species in aqueous environments Anion clays (or layered double hydroxides) belong to the family of clay minerals, which have been widely applied in sorption reactions in view of their special lamellar microstructure Although the use of these minerals in contaminant treatment is hampered by their limited natural distribution, the relatively simple and * Corresponding author Tel.: +86-13554116793; fax: +86-27-87436235 E-mail address: qhguo2006@gmail.com 1878-5220 © 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.041 162 Yaowu Cao et al / Procedia Earth and Planetary Science 17 (2017) 161 – 164 economical synthesis at an industrial scale provides a possibility for their practical applications In addition, taking into account of their tailor-made structures and chemical compositions, the option of the appropriate anion clays which adjust measures to local conditions is a central aspect of particular water treatment engineering Generally in high-temperature geothermal systems, as the longer time of fluid-rock interaction, the geothermal waters could dissolve large amounts of chemical species According to our research, F, As and other harmful contents in geothermal waters of Qinghai province exceed the World Health Organization (WTO) guideline values by dozens of times However, some geothermal areas in Qinghai province are close to residential areas and surface geothermal waters discharge into local rivers directly Thus to investigate the effective environmental management is necessary for future geothermal resource utilization We collected three geothermal water samples from three geothermal regions in the Qinghai province and parts of their chemical composition are shown in Table In this study, four kinds of anion clays are synthesized in the laboratory, such as hydrocalumite (HC), hydrotalcite (HT), iowaite (I) and takovite (T) Through sorption experiments, we can optimize the most appropriate sorbent to purify the Qinghai geothermal waters Table Chemical composition of the water samples from the Qinghai province Sample No pH GH 7.3 GD 8.0 XH 7.2 As F Cl SO4 (µg/L) (mg/L) (mg/L) (mg/L) 92.81 5.16 715.31 241.15 21.80 11.94 363.36 621.55 16.48 7.36 222.66 472.95 Fig XRD patterns of synthesized anion clays Materials 2.1 Synthesis of sorbents and characteristic The coprecipitation method1 was used to synthesize the four anion clays in this study The target products were mainly controlled by initial salt solutions In the case of hydrotalcite (Mg6Al2(OH)16Cl2·4H2O) for example, salt solutions were prepared by mixing magnesium chloride and ferric chloride with the molar ratio of 3, which was added dropwise into a beaker along with a sodium hydroxide solution under substantial stir The obtained suspension was then heated at 80°C to favour the crystallization The solid products were filtered, washed and dried at 60°C Likewise, hydrocalumite (Ca4Al2(OH)12Cl2·6H2O), iowaite (Mg6Fe2(OH)16Cl2·4H2O) and takovite (Ni6Al2(OH)12Cl2·4H2O) were prepared using this method The four solid samples were analyzed by X-ray Diffraction (XRD) using Cu Kα radiation (X’Pert PRO DY2198 diffractometer) The XRD patterns were collected from 5° to 65° with a 2θ step of 0.033° 2.2 Sorption experiments and analysis 25 ml of three water samples were taken into 100 ml plastic bottles for sorption experiments Three groups of sorbent dosages were prepared in equal molar mass of 0.082 mmol, 0.82 mmol and 4.08 mmol After 24 h reaction time at room temperature, the solution samples were decanted and filtered through a 0.45 μm cellulose acetate membrane The arsenic concentrations in the reacted solutions were analyzed by an AFS-2202 duel-channel atomic fluorescence spectrophotometer The fluoride, chloride and sulphate concentrations were measured using DIONEX CO ICS-900 ion chromatograph Results and discussion 3.1 Characteristic of solid phase The XRD patterns of the solid phase are shown in Fig The reflections of the anion clays were in agreement with those reported in the literature2, which indicated that the solid materials had a regular layered structure, and no characteristic peaks were from impurities Yaowu Cao et al / Procedia Earth and Planetary Science 17 (2017) 161 – 164 3.2 Sorption of F, As and sulphate with selected anion clays Three geothermal water samples contained some harmful species that surpassed the WTO drinking water standards, such as F, As and sulphate As the initial arsenic concentrations of three water samples were relatively low, the arsenic in solutions could be all removed by anion clays at the sorbent dosages of 0.82 mmol and 4.08 mmol (the removal percentage could come up to 100%) Thus the Fig only showed dearsenication efficiency at the least sorbent dosage The Fe-based anion clay, iowaite had high affinity to arsenic duo to the Fe-As complexation occurring on its surface so that iowaite had great advantages on arsenic removal Hydrocalumite took the second place in arsenic removal, while hydrotalcite exhibited the undesirable performance at the smaller sorbent concentration The removal efficiency of fluoride in geothermal waters was shown in Fig In general as the sorbent dosages increased, the removal efficiencies increased The removal of fluoride by hydrocalumite improved with the increased dosages due to the formation of fluorite (CaF2), which was distinct from the other anion clays Since hydrocalumite possessed a smaller solubility, the dissolved Ca2+ easily captured the F- of the geothermal waters to precipitate less soluble fluorite, which increased the removal efficiencies of fluoride by hydrocalumite In addition, hydrotalcite and iowaite also exhibited the desirable performance of the fluoride removal Fig.2 Efficiency of arsenic removal at the dosage of 0.082 mmol from three geothermal water samples (GH, GD, XH) Fig.3 Efficiency of fluoride removal at different dosages from three geothermal water samples (GH, GD, XH) Fig.4 Efficiency of sulphate removal at different dosages from three geothermal water samples (GH, GD, XH) In the upper portion of geothermal systems, H2S originated from magmatic assimilation of crustal material containing sulfides may oxidize to form sulphate in geothermal fluid Although sulphate has no toxic effect on humans except at very high concentrations, large amounts of sulphate will increase the water total dissolved solids (TDS) and cause catharsis, dehydration and gastrointestinal irritation3 Fig showed different removal efficiences of sulphate in the Qinghai geothermal waters by four kinds of anion clays Obviously hydrotalcite and takovite presented higher removal efficiencies on sulphate reduction and the final concentrations of the solutions met the WHO drinking water standard (< 250 mg/L) 163 164 Yaowu Cao et al / Procedia Earth and Planetary Science 17 (2017) 161 – 164 3.3 Chloride variation after the sorption Chloride was the main intercalation anion of four kinds of anion clays (HC, HT, I and T), which was reported as relatively weak interlayer bonding4 The free anions in solutions easily occupied the interlayer-binding site leading to the release of interlayer chloride into the solutions This anion exchange, exhibited excellent ability to capture hazardous anions, also gave rise to the secondary pollution with the excess chloride Fig shows the variation trends of chloride concentrations in solutions treated with the anion clays The positive values represented the increase of chloride concentration, while the negative ones showed the reduction of that The addition of iowaite into water samples dramatically elevated the chloride concentrations, which exceeded the drinking water standard by more than 10 times at the highest dosage due to the anion exchange With the increased dosages of hydrocalumite and takovite the chloride concentrations made a slight growth However, the variations of the chloride concentrations were negative values with the addition of hydrotalcite at the dosage of 4.04 mmol The negative growth in the chloride concentrations illustrated that abundant hydrotalcite as a sorbent could uptake hazardous anions and excessive chloride from water samples simultaneously Fig.5 Variation of chloride concentration in three geothermal water samples (GH, GD, XH) at different dosages Conclusion The batch sorption experiments to study the removal efficiency of harmful anions from geothermal waters showed that iowaite performed better than the other anion clays in terms of water dearsenication, while hydrotalcite exhibited higher efficiencies on removal of fluoride and sulphate and both of efficiencies were above 90% at the dosage of 4.08mmol However the increasing the dosage of iowaite increased the chloride concentration in treated solutions duo to anion exchange Therefore hydrotalcite was an effective sorbent to purify the Qinghai geothermal waters and also did not cause any secondary pollution during the treatment Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No 41572335) and the Science and Technology Item of State Power Investment Corporation (No 2015-138-HHS-KJ-X) References Vaccari A Clays and catalysis: a promising future Applied Clay Science 1999; 14, 161-198 Cavani F, Trifirò F, Vaccari A Hydrotalcite-type anionic clays: Preparation, properties and applications Catalysis today 1991; 11, 173-301 Runtti H et al Sulphate removal over barium-modified blast-furnace-slag geopolymer Journal of Hazardous Materials 2016;317, 373-384 Goh K H, Lim T T, Dong Z Application of layered double hydroxides for removal of oxyanions: a review Water Res 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