Using Anionic Clays To Remove Diverse Harmful Constituents Of Fluid From Geothermal System

Currently,fossil fuels such as coal,oil and natural gas are in short supply.Geothermal energy is being paid more and more attention as a new kind of renewable energy,and its development and utilization is becoming a global hotspot.Compared to the underground cold water system,the water-rock interaction in the geothermal system is carried out at a higher reaction temperature.So the hot water can leach a large number of chemical components from the surrounding rock medium,including various harmful components,such as fluorine,arsenic,boron and sulfate,etc.When the geothermal water is discharged into the environment in the form of hot spring or geothermal wastewater?after utilization?,it often pollutes other types of natural water bodies that are used as drinking water in the geothermal area or the surrounding area.During the development and utilization of dry hot rock geothermal resources,fracturing liquid from the process of reservoir stimulation also contains a variety of harmful components,such as chlorine and fluorine,etc.Their contents are far higher than that of the highest allowable emission concentration standard for geothermal water.If it is not processed and discharged into the environment,it will also cause serious pollution problems.Thus,geothermal energy,while renewable,is not absolutely clean.The effective treatment of multiple harmful components in the fluids from geothermal systems?including natural geothermal fluid and fracturing liquid from Enhanced Geothermal Systems?,in order to avoid polluting other environmental media after entering the environment around the geothermal area,is of great practical significance.In view of that the typically harmful components?fluorite,arsenic,boron,sulfate and chloride,etc?in the fluids from geothermal systems are usually in the form of anions(for example F^-,HAsO₄^2-/As O₄^3-,H₂AsO₃^-/HAsO₃^2-,B?OH?₄^-,Cl^-,etc)in the liquid phase,this article selected anionic clays with good treatment effect on anionic components as adsorption material to carry out the research on the effective treatment of fluorine,arsenic,boron,chlorine and other harmful components of natural geothermal fluids and fracturing liquid from Enhanced Geothermal Systems in different hydrochemical types.Anionic clay is a kind of Layered Double Hydroxides?LDHs?,and of layered structure and structural memory effect.In addition to these,it is of large specific surface area and high anionic exchange capacity.The calcinated product of anionic clay under certain temperature conditions can reabsorb water molecules and anions in liquid phase to restore the layered structure.This material is easily producted and low cost.So,it has been widely used in effectively treating different types of harmful ions in wastewater or contaminated water effective treatment.According to the previous batch of experimental results,all kinds of anionic clays are similar in structure,but the removal effect and mechanism of different anions are different.Comparing to the adsorption capacity of all kinds of anionic clays,calcined hydrocalumite,iowaite and hydrotalcite respectively show a good effect on removing fluorine,arsenic and boron.Therefore,this article selected these three anionic clays to carry out the experimental study on the removal of fluorine,arsenic,boron and chloride,etc.Conclusions made in this research are as follows:?1?The natural geothermal water samples from Yangbajain of Damxung County in Tibet?ZK359?and Banglazhang of Longling County in Yunnan?DFQ?and Zhacangsi of Guide basin in Qinghai?ZCS?were selected as the actual treatment objects.Meanwhile,calcined hydrocalumite,iowaite and hydrotalcite were particularly selected as filling material to carry out the experimental study on the removal fluorine,arsenic and boron.When the inner diameter of the reaction column is 30 mm and the length is 40 cm,and the seepage rate of geothermal water is 2.7 mL/min,the best ratio of anionic clays?after calcining?to effectively remove fluorine,arsenic and boron from ZK359,DFQ and ZCS for attaining the standard for drinking water quality?GB5749-2006?are:hydrocalumite:iowaite:hydrotalcite=15:1:20 and 40:3:20 and 40:1:40.?2?When the seepage rate of geothermal water increases in the reaction column,the residence time of geothermal water in the filter layer is shortened.For these reasons,the ability of the anionic clays to remove harmful anions in water has not been fully utilized,thus reducing the efficiency of water purification.However,reducing the seepage rate can improve the removal rate of harmful anions,but also reduce the water treatment quantity in a given time.In practical application,it should be based on the content of harmful components in water and water demand to choose the appropriate seepage rate through trial and error,and ensure that the content of the harmful components after water treatment is lower than the standard for drinking water quality,and the water treatment quantity should be enough.When the inner diameter of the reaction column changes,the seepage path length of geothermal water is changed simultaneously.The increase of seepage section area does not affect the reaction time of anionic clay and geothermal water,but the decrease of the seepage path length means that the residence and reaction time of geothermal water in the filter layer of anionic clay are shortened,which is not conducive to the removal of harmful components.Therefore,increasing the inner diameter of the reaction column reduces its water treatment effect.In practical application,the inner diameter of the reaction column should be minimized under the permits of the reaction column manufacturing process,so as to maximize the removal effect of the harmful components under the same conditions.?3?The correct order of exchange ability between the common anions in the anionic clay layer is CO₃^2->SO₄^2->HPO₄^2->F^->Cl^->B?OH?₄^->NO₃^-.Therefore,compared with the main existing forms of fluorine,arsenic and boron after adding anionic clay to geothermal water,CO₃^2-and SO₄^2-have the advantage of competitive adsorption on anionic clay.However,the bicarbonate and sulfate in geothermal water are generally components with high content.When using anionic clays to treat geothermal water,the pH value of the reaction system will increase significantly with the addition of anionic clays,which will convert the bicarbonate to carbonate in water.The adsorption of anionic clays to carbonate in water has also increased this conversion.As a result,primary anionic components such as bicarbonate and sulfate in geothermal water are easier to be adsorbed by the surface of anionic clays or occupy the interlayer positions,thereby reducing the removal rate of the anionic clays on fluorine,arsenic and boron.In practical geothermal water treatment,the amount of anionic clays obtained according to the content of fluorine,arsenic and boron in water should be further increased to counteract the effect of bicarbonate and sulphate as the main anions.?4?Fracturing fluid generated in the process of hydraulic fracturing and chemical stimulation from Enhanced Geothermal Systems also contains a large number of harmful components in the form of anions,including chloride,sulfate,fluoride,arsenate,arsenite and borate,etc.According to the chemical composition of fracturing fluid generated from Enhanced Geothermal Systems of Fenton Hill in America and Soultz-sous-Forêts in France,the main chemical characteristics of the solution were prepared in the laboratory.Simultaneously,the granite sampled in Zhacangsi of Gong-he Basin in Qinghai province was selected as the dry hot rock objects to simulate the chemical stimulation test and obtain the fracturing fluid sample to carry out the experimental study of removing the harmful components by anionic clays.It showed that hydrotalcite with CO₃^2-as interlayer anions can be used to effectively treat the fracturing fluid with chloride as the main harmful component,such as Soultz and Zhacangsi samples.However,when fluoride,arsenate and borate are the main harmful components,such as the fracturing fluid from Enhanced Geothermal System in Fenton Hill,hydrocalumite,iowaite and hydrotalcite still are effective combination of anionic clays for removing harmful components.