Polypyrrole-grafted Peanut Shell Biochar As A Potential Sorbent For Fluoride Removal From Aqueous Solution

Fluoride contamination in groundwater has become a worldwide problem in recent years,which is still a serious health hazard for human beings.In China,fluoride contaminated groundwater is mainly distributed in economically backward rural areas and remote areas.For the lack of water resources,economic backwardness and lack of funds,it is very difficult for people who live in the above-mentioned areas to find alternative drinking water sources,leading to the difficulty to solve the problem of endemic fluorosis.In order to help people to drink more clean water,the cost-effective defluoridation treatment technology is urgently developed.In consideration of environmental respectability and ease of operation,adsorption method is an attractive and alternative comparing with other treatment.In this study,some kinds of low-cost adsorbents have been explored for defluoridation.Comparing the fluoride adsorption capacity of these adsorbents with each other,polypyrrole-grafted peanut shell biochar(PPy/BC)performed the best fluoride removal efficiency.Fourier transform infrared(FTIR)spectroscopy results showed that the reason why PPy/BC performed well was the conjugated structure between the nitrogen functional groups on the surface of biochar and PPy.Variables of defluoridation process(i.e.,adsorbent dosage,fluoride solution pH,and anionic interference)were tested.It showed that PPy/BC composite performed commendably from pH 2.0 to 10.0,and exhibited high selectivity for fluoride in the presence of several co-existing anions.Temperature had little effect on the adsorption of fluoride at low initial fluoride concentration ranging from 0 to 31.7 mg·L-1.Langmuir isotherm fitting curves showed the maximum adsorption capacity of PPy/BC was 17.153 mg·L-1.The results showed that PPy/BC had more advantages in economical efficiency,environmental adaptability and fluoride removal efficiency compared with other similar adsorbents.The morphology and chemical composition of PPy/BC was determined by scanning electronic microscopy(SEM),transmission electron microscope(TEM),Brunauer-Emmett-Teller(BET)method,X-ray photoelectron spectroscopy(XPS),Fourier transform infrared(FTIR)spectroscopy and automatic titration.We inferred the mechanisms of fluoride removal by PPy/BC by combining the above-mentioned results with kinetic,isotherm and thermodynamic studies.It showed the process completed by chemical adsorption relating to inner-sphere adsorption.Ion exchange played an important role in the removal of fluoride ions.Meanwhile,the electrostatic attraction of hydroxyl groups on the surface of biochar played a secondary role.Surface adsorption and mesoporous diffusion were the main rate controlling steps.Further computation of XPS data suggested the formulation of a three-layer-deep hypothesis for PPy.