Adsorption Of Fluoride Ions In Aqueous Water By The Modiifed Portland Cement

Fluoride contamination in groundwater has been found popular in a lot of placesin China and the fluoride concentration in many regions is higher than prescribed byStandards for Drinking Water Quality, which is a serious threat to the flora and faunaand human health. At present, the main treatment methods include adsorption,membrane and precipitation, and adsorption method is an attractive alternative toothers because of its stability of effluent, ease of operation and environmentalrespectability. This study focused on the production method of adsorbent by usingPortland cement and discussed the fluoride removal characteristics by static test andcolumn test. This research will provide the theoretical basis and technical support forthe adsorbent development and fluoride removal from groundwater and drinkingwater.The production methods of adsorbent and fluoride adsorption characteristicswere investigated by batch experiment. It was found that the granular cement can beobtained by solidified cement paste which was cured by mixing the Portland cementand deionized water, the granular cement modified by0.1mol/L ofAl₂（SO₄）₃18H₂O（CAL） has the higher removal efficiency than others. Theadsorption of fluoride from aqueous water on CAL mainly occurred during the first6h, and achieved equilibrium gradually at about24h. The adsorption capacity basicallykept around91%when the initial pH is between5.0and11.0, however, decreased to86%and82%when the initial pH is at3.0and12.0, respectively. The experimentaldata revealed that the Langmuir isotherm models fitted well with the fluoride sorptionprocess on CAL, the monolayer adsorption capacity was7.52mg/g at25℃.Thermodynamical study revealed that the process of fluoride adsorption on CAL wasspontaneous and endothermic and the standard enthalpy change （H⁰） was3.343kJ/mol. The adsorption process was well described by pseudo-second orderkinetic model, and the correlation coefficient was0.999. In addition, the presence ofchloridion ions, sulfate ions and nitrate ions showed the relative independent of the fluoride removal, but the presence of phosphate ions and carbonate ions showednegative effect on the adsorption of fluoride. The concentration of fluoride in practicalgroundwater decreased to0.18mg/L from2.1mg/L under the adsorbent dosage of5g/L, and removal efficiency received90%.The experiments were applied to quantify bed length and influent fluoride ionsconcentration on breakthrough volume during the removal of fluoride from aqueouswater using CAL column. Results indicated that the breakthrough volume appeared toincrease with increase of the bed length and decrease with increase of the influentfluoride ions concentration. The Thomas model was found suitable for describing theadsorption process of the dynamic behavior of the CAL column. When the bed lengthwere5cm、10cm、15cm, the breakthrough volume were5.6L,10.3L and16.12L,and the exhaust volume were8.4L,17.1L and23.7L, and the saturation capacitywere1.749mg/g、1.730mg/g、1.682mg/g, respectively. The saturation CAL can beregenerated by10%of NaOH, and the regeneration efficiency was more than92%after2cycles of desorption and regeneration. So, CAL is an idea defluoridationmaterial...