Simultaneous removal of fluoride and arsenic in geothermal water in Tibet using modified yak dung biochar as an adsorbent

Fluoride (F) and arsenic (As) are two typical and harmful elements that are found in high concentrations in geothermal water in Tibet. In the present work, yak dung, an abundant source of biomass energy in Tibet, was made into biochars (BC1, BC2 and BC3) by pyrolysis under different conditions, and the better biochar was modified by FeCl2 (Fe-BC3). The adsorption conditions were optimized to adsorb F and As in geothermal water. The results showed that BC3 can remove 90% F- and 20% As(V), which is the best effect of the three initial biochars. Fe-BC3 could remove 94% F- and 99.45% As(V) under the same conditions as BC3, which was an adsorbent dosage 10 g/L, pH 5-6 and temperature of 25 °C. It was also demonstrated that the removal rate did not decrease at 80 °C. A quasi-second-order kinetic model best described the adsorption behavior of ions on the surface of the biochar. The maximum adsorption capacity of F- and As(V) on Fe-BC3 was 3.928 mg/g and 2.926 mg/g, respectively. The features of Fe-BC3 were characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR), Brunauer-Emmett-Teller (BET), Energy Dispersive Spectrometer (EDS), and Scanning Electron Microscopy (SEM) to understand the adsorption process.

氟（Fluoride, F）与砷（Arsenic, As）为两类典型有害元素，在西藏地热水中常呈高浓度赋存。本研究以西藏本地储量丰富的生物质能源原料牦牛粪为原料，通过不同热解工艺制备得到三种生物炭（BC1、BC2、BC3），并选取性能最优的BC3经氯化亚铁（FeCl₂）改性得到改性生物炭Fe-BC3。以吸附西藏地热水中的氟、砷为目标，本研究优化了吸附反应条件。实验结果显示，三种原始生物炭中BC3的吸附性能最优，对F⁻的去除率可达90%，对As(V)的去除率为20%。在吸附剂投加量10 g/L、pH值5~6、反应温度25℃的相同条件下，Fe-BC3对F⁻和As(V)的去除率分别可达94%和99.45%。进一步研究表明，该吸附体系在80℃下仍未出现去除率下降的情况。吸附动力学拟合结果表明，准二级动力学模型最契合该离子在生物炭表面的吸附行为。Fe-BC3对F⁻和As(V)的最大吸附容量分别为3.928 mg/g和2.926 mg/g。为深入解析其吸附机制，本研究通过X射线衍射（X-Ray Diffraction, XRD）、傅里叶变换红外光谱（Fourier Transform Infrared, FTIR）、Brunauer-Emmett-Teller比表面积分析法（BET）、能量色散光谱（Energy Dispersive Spectrometer, EDS）以及扫描电子显微镜（Scanning Electron Microscopy, SEM）对Fe-BC3的物化特性进行了表征。