Fluoride is essential to human health and is beneficial in moderation,but excessive intake can lead to serious consequences such as dental fluorosis and bone disease.The erosion of fluoridated minerals in groundwater and the discharge of fluoridated wastewater have resulted in some drinking water sources exceeding the fluoride ion standard and threatening the safety of drinking water.Adsorption has become an effective means of fluoridation of drinking water due to its effectiveness,ease of operation,high removal rate and recyclability.The commonly used fluoride adsorbent activated alumina has problems such as low adsorption capacity and regeneration difficulties.In this study,biochar was prepared from agricultural waste walnut shells as a raw material,and its porous structure was used as a backbone to load different forms of aluminium oxides to investigate the effect of aluminium form on the adsorption and fluoride removal.In order to further improve the adsorption and de-fluorination performance of the material,Al/Ca bimetallic modification was introduced to improve the adsorption stability and capacity of the material.In this study,the effects of the initial dosage,p H,temperature and coexisting ions on the adsorption of fluoride ions were investigated by static adsorption experiments.Meanwhile,the stability and adsorption mechanism of biomass carbon were investigated by using SEM,FTIR and XPS characterisation techniques.To be specific:(1)Al-WBC-1 and Al-WBC-2 were obtained by modifying biomass charcoal with thin hydrous alumina form(Al OOH-n H2O)andγ-Al2O3.The WBC fluorine adsorption was highest around p H 3~4,and both Al-WBC-1 and Al-WBC-2 were effective in removing fluorine from the water column between p H 4 and 8.The presence of HCO3-and PO43-had a greater effect on fluoride removal.The two biomass carbon species reached equilibrium at around 300 and 400min,respectively,in accordance with the quasi-secondary kinetic model,with maximum adsorption amounts of 30.65 mg·g-1and 24.30 mg·g-1,respectively,fitted by the Langmuir model.The fluorine removal rate of the two adsorbent materials could be maintained at about 60%after five regenerations with 0.2 mol·L-1aluminium sulphate as regenerant.The analysis of surface functional groups,elemental composition,p Hpzc,adsorption kinetics and X-ray photoelectron spectroscopy showed that the fluorine removal mechanism of Al-WBC-1 was dominated by electron adsorption and ion exchange,while Al-WBC-2 mainly carried out ion exchange through surface hydroxyl groups,and due to the low adsorption capacity and small specific area ofγ-Al2O3,Al-WBC-1 had a better fluorine adsorption effect than Al-WBC-2 has a better fluorine adsorption effect than Al-WBC-2.(2)Amorphous alumina supported biochar(Al-WBC-3)and its adsorption and de-fluorination efficiency was investigated.SEM results showed that the surface of the modified biochar was rough and some pores were filled with aluminum hydroxide.XRD diffraction pattern analysis shows that the adsorbent of biochar before and after modification is amorphous,indicating that Al is loaded on the surface of biochar in amorphous form.In the range of p H 5~7,Al-WBC-3 showed a good fluorine adsorption effect,but when HCO3-and PO43-co-existed,the fluorine adsorption was inhibited to a certain extent.Al-WBC-3 reached equilibrium after adsorption for 400 min,conforming to the second-order kinetics and Langmuir isotherm model,and the maximum adsorption capacity was 37.11 mg·g-1.Fluorine ion adsorption is mainly achieved by single layer chemisorption,and ion exchange plays a major role in the adsorption process.The results of continuous flow fixed bed dynamic simulation test show that under the condition of influent concentration of fluoride ion is 5mg·L-1,the effluent concentration can be stably controlled below 1mg/L,and the load of aluminum in the material is stable,and no obvious dissolution is observed.(3)On the basis of(2),Ca ions were introduced to modify Al/Ca bimetallic biochar(Al/Ca-WBC).SEM analysis showed that the carbon surface was coated with Al/Ca hydroxide,and the Al/Ca element distribution was agglomerated.XRD diffraction pattern analysis shows that Ca2Al(OH)6Cl·10H2O lattice exists on the surface of the material.Boehm titration and Spearman rank correlation analysis showed that Al/Ca-WBC had more phenolic hydroxyl sites and alkaline sites than other modified biochar,which was more favorable for fluorine ion adsorption and ion exchange.Al/Ca-WBC reached equilibrium after adsorption for 420 min,in line with the quasi-second-order kinetics,and the maximum adsorption capacity was 42.89 mg·g-1by Langmuir fitting.In the range of p H 5~7,the adsorption effect of SO42-,HCO3-and PO43-was better,and the adsorption capacity decreased significantly when the concentration of organic matter was higher.After 5 times of regeneration,the removal rate of fluorine of the adsorbent can still remain at about 60%.The dynamic simulation test in continuous flow fixed bed showed that the stability of aluminum ion and calcium ion supported by biochar was high,and no obvious precipitation was observed.The main adsorption mechanisms of Al/Ca-WBC are intra-granular diffusion,electron adsorption,ion exchange and metal bonding bridge. |