Formation Mechanism And Treatment Of Poor Quality Groundwater In Xiji County In The Southern Ningxia

Xiji County in southern Ningxia, locates in Loess Plateau in NorthwesternChina. However, the Tertiary aquifer as the major water-supplying layer in thisregion is affected by the high-salinity sediment, which leads to seriously excessivepoor quality components in groundwater. There is even up to9000mg/L of totaldissolved solids （TDS） in groundwater in some regions, in which fluoride is alsoexcessive, which is very harmful to human health of local residents.With an aim to meet the normal demand of domestic and production water oflocal residents, according to the hydrogeological conditions and chemicalcomposition characteristics of groundwater in this region, the formation mechanismsof the poor quality groundwater in Tertiary aquifer in this researched region arestudied by the combination method with hydrochemical analysis, isotopic analysisand inverse hydrogeochemical modeling based on analysis of the distribution andinfluencing factors of poor quality groundwater in researched region; and accordingto the high fluoride and salinity in groundwater caused by different reasons, themodified hydroxy-aluminum lanthanum composite material as a high efficientadsorbent to fluoride is prepared, furthermore, the combined technology ofmembrane-desalination and adsorbent-defluorination is formed by combining thedesalination ability of membrane filter and the defluorination ability of modifieddefluorination adsorbents, finally the economic and effective removal technology isdeveloped for the poor quality groundwater with high fluoride and mineralization.The obtained results of this study as follows:1. Distribution of poor quality groundwater in researched regionThe best groundwater quality is corresponding to Cretaceous in this researched region, and the groundwater quality in Quaternary is a little worse,whereas the as the major studying Tertiary aquifer whose groundwater quality is theworst. All of the content of F-, Cl-and SO₄^2-is seriously excessive except for TDS.The content of TDS is controlled by groundwater flow due to the groundwatermovement effect in Tertiary, which is consistent with the basis phases ofhydrochemical type change, and the content of TDS increases from north to south,and the hydrochemical type change from HCO₃· SO₄· Cl-Na intoSO₄·Cl-Na·Ca·Mg.2. Formation mechanisms of poor quality groundwaterThe formation of hydrochemical components in Tertiary poor qualitygroundwater in Xiji County is mainly affected by lixiviation, on which the effect ofhalite, mirabilite and gypsum dissolution is the greatest, subsequently is the effect ofdissolution of dolomite, albite and so on. The influence of the exchange adsorptionof cation on the groundwater chemistry formation is next to lixiviation, and thecontent of N⁺in groundwater increases rapidly because of a large amount of haliteand mirabilite dissolution, thereby cation exchange adsorbing strongest with Ca²⁺.The distribution of δ³⁴S_SO₄and SO₄^2-content shows that the value of δ³⁴S_SO₄inTertiary groundwater generally increases significantly with the concentration ofSO₄^2-increasing and within the range of sulfate mineral δ³⁴S_SO₄value, whichindicates SO₄^2-in Tertiary groundwater mainly sources from the dissolution ofsulfate minerals.It is found that the groundwater chemical components on this path are mainlyaffected by dissolution of halite, mirabilite and so on, and increase rapidly with N⁺concentration increasing, and the tendency of the cation exchange adsorption of N⁺is becoming more and more obvious, which leads to cation exchange adsorptionbetween N⁺and Ca²⁺by studying the inverse hydrogeochemical modeling of thetypical groundwater flow path in Tertiary; the concentration of Ca²⁺in groundwaterincreases rapidly in the effect of dissolution of dolomite and gypsum and the ionexchange, which makes calcite be saturated to be precipitated; and the concentration of F-on this flow path increases because of dissolution reaction of fluorite.3. Preparation of modified hydroxy-aluminum lanthanum compositedefluorination adsorbentPalygorskite, scoria and clay with high modified values are selected to beadsorption materials based on comparing the performances of a large number ofadsorbents, whose adsorption abilities can be enhanced significantly after they aremodified by lanthanum nitrate and aluminum hydroxide.The best ratio of palygorskite, scoria, clay, and lanthanum nitrate, aluminumhydroxide is1:6:1,1:2:1and1:2:1.5; the best thermal treatment temperature is300℃; and the saturated adsorption amount is0.159,0.128and0.155mg/L at themoment, respectively.2.5%of the KAl（SO₄）2·12H₂O is determined the best regeneration fluid, andquality volume ratio between adsorbent and regeneration fluid is1:7.5, and theregeneration time is24h, and all of the regeneration efficiencies are above102%after six regeneration experiments, which would validate that the adsorbent has goodadsorption ability to fluoride.It can be concluded that the best quality ratio among modified palygorskite,scoria and clay in the modified composite fluoride adsorbent is0.5:1.5:1, andthe dynamic adsorption amount of the fluoride adsorbent after complexing is0.14mg/g by the operation of the defluoridation simulator of actual site.4. Performance of modified fluoride adsorbent and adsorption mechanismsIt is found that the adsorption kinetics of modified palygorskite, scoria and clayadsorbents to fluoride in groundwater can be fitted to pseudo-second order kineticmodel by studying the adsorption kinetics and isotherms of the selected bestmodified palygorskite, scoria and clay adsorbents; their isotherm adsorption modelsall fits to Langmuir adsorption model, and the largest theoretical adsorption amountis0.51,0.39and0.61mg/g, respectrively.It is proved that a great number of La-Al-O composite oxides form at thetreatment temperature of300℃to remove fluoride most by studying the physical characterizations and mechanisms of the different modified adsorbents through XRD,SEM, EDX and XPS.5. Removal of salts in groundwater by reverse osmosis technologyThe effects of different inlet pressures, concentrations of TDS on the treatmentefficiency are investigated by studying removal of Cl-and SO₄^2-in groundwater withthe primary one-stage cycle reverse osmosis technology, it is proved that cation canbe removed well when the high concentrations of Cl-and SO₄^2-are removed ingroundwater simultaneously by the technology, thereby softening groundwater.The needed osmosis pressure in the reverse osmosis process increases withincrease of TDS content, if the inlet pressure is kept, the clean water effluent willdecrease and the membrane pollution increase, therefore the inlet pressure and TDScontent should be controlled in the actual operation process.The salts in poor quality groundwater in the researched simulation site areremoved well in the operating pressure range from0.65to0.7Mpa by selecting lowpressure reverse osmosis membrane components, and the groundwater quality in thisregion can accord with the national drinking water standard. Although the removalefficiency of low concentration of Cl-is relatively low, it still can be up to95%, butthe removal efficiency of SO₄^2-is significant that can be up to99%.