Study On The Adsorption Method For Desalination Of Saline Soil Washing Wastewater

Soil salinization is one of the serious problems in China’s ecological environment and agricultural development. There are about four hundred million acres of saline soil in China. A important method for soil remediation is “ irrigation desalting”. However, during the process of irrigation desalting, it often produces a large amount of wastewater. The discharge of wastewater may not only cause the waste of water, but also lead to environmental problem such as secondary salinization. Therefore, desalting of wastewater in very important. In this study, clay minerals, which with broad source, low price, environmental benefits were used as basic materials to prepare adsorption materials through the synthesis and modification. Subsequently, the laboratory simulation wastewater and Xinjiang saline soil washing wastewater were used as the research object, the properties of adsorption materials for salt ions in the water were studied. Furthermore, simulation study was carried on in the adsorption of water migration by using adsorption materials as interlayer materials to investigate oil salt(water and salt movement) inhibitory effect. The experimental results could provide a theoretical basis for the treatment of saline soil washing wastewater.The main results are listed as follows:1) Preparation of Mg-Al hydrotalcite and its adsorption properties for Cl- and SO42-. Best preparation conditions of nitrate hydrotalcite(NO3--LDH) and calcinated carbonate hydrotalcite(CO32--LDO) were studied.The best conditions are that the molar ratio of NO3--LDH was 2:1, CO32--LDO was 3:1, calcination temperature was 400 ℃. The materials were characterized by XRD, FT-IR, SEM and TG-DSC,which shows that the hydrotalcite has good layered structure and thermal stability. The adsorption characteristics of Cl- or SO42- by NO3--LDH and CO32--LDO showed that the adsorption capacity of adsorbents for Cl- and SO42- changed slightly when the p H varied in the range of 4~10; The adsorption speed of anions of the adsorption materials changed from fast to slow, and reached the saturation adsorption in about 4 hours. Kinetics and adsorption isotherm analysis results showed that: the dynamic datas can be fitted by the quasi two level rate equation, the correlation coefficient was above 0.99 and the adsorption process can be described by Langmuir adsorption isotherm model, which appeared as a single molecular layer adsorption. The maximum adsorption capacity of NO3--LDH on Cl- and SO42- was 92.05, 151.89 mg?g-1, respectively. The maximum adsorption capacity of CO32--LDO on Cl- and SO42- was 51.23, 233.38 mg?g-1. In the binary system, NO3--LDH, CO32--LDO showed a selective adsorption to SO42-, CO32--LDO performance better. Both of NO3--LDH and CO32--LDO exhibited good reusability, after 3 times, the adsorption capacity still reached 80.35% and 52.42% of the initial adsorption capacity, respectively. These results showed that NO3--LDH and CO32--LDO had good adsorption properties for Cl- and SO42-, but the difference is that CO32--LDO exchange adsorp anion by OH-, while NO3--LDH by NO3-.2) Preparation of modified bentonite and its adsorption properties for Na+, Mg2+, K+, Ca2+. The natural bentonite was roasted for 2h under 200℃ for thermal activation, and then treated with various concentrations of(NH4)2CO3 and H2SO4 to prepare ammonium bentonite(Am B) and acidation bentonite(Ac B).The modified materials were characterized by XRD, FT-IR and SEM, and they have a larger specific surface area. The simulation of Xinjiang saline soil washing wastewater was used as the object to study the adsorption characters of the Am B, Ac B for Na+、Mg2+、K+、Ca2+ in the water. The results showed that, with the increase of p H, the adsorption capacity of the two materials for Na+、Mg2+、K+、Ca2+ increased. Kinetics and adsorption isotherm datas analysis and study showed that the kinetic datas conformed to the two level rate equation,the adsorption process fitted with the law described by Freundlich adsorption isotherm model. The maximum adsorption capacity of Am B on Na+, Mg2+, K+, Ca2+ were 53.67, 25.79, 17.77, 25.59 mg?g-1, respectively. The maximum adsorption capacity of Ac B on Na+, Mg2+, K+, Ca2+ were 46.32, 20.15, 22.17, 25.72 mg?g-1. In the binary system, both Am B and Ac B showed a selective adsorption, the order was Ca2+>K+>Mg2+>Na+. The main difference between the two materials is that Am B and Ac B are NH4+ and H+ cation exchange adsorption.3) In order to achieve the purpose of desalination, other salt ions should be avoided in the adsorption, so CO32--LDO and Ac B wre selected to treat anions and cations of Xinjiang saline soil salt waste water(salinity 0.47%), thus OH- and H+ can combine to form water, then achieve the purpose of desalination. The effect of the adsorbent dosage, adsorption material composition, the order of adding, adsorption time, concentration of wastewater and the amount of C-PAM on desalination were investigated. The results showed that: When the mass ratio was 2:1, the total dosage was 4 g, adsorption time was 4 h, the dosage of PAM was 0.3 mg?L-1, the desalting result was the best, desalination rate could reach 27.21%.4) The CO32--LDO and Ac B mixture as the interlayer is arranged in the soil layer to explore its inhibitory effects on soil water and salt movement, include thickness of adsorption residue, location of the adsorption residue and the mineralization degree of simulation groundwater. The results showed that, inhibitory effect of sandwich CO32--LDO and Ac B of the water and salt movement is mainly based on two aspects: the adsorption effect of salt ions by salt slag in groundwater and the interlayer can slow down the speed of groundwater. The dual role makes the thicker adsorption residue layer and the higher, inhibition of soil salt effect is more obvious.