| In China,the people within a 2.2 million km2 area suffer from endemic fluorosis due to high fluoride levels in the drinking water.Adsorption technology is considered as one of the most promising technologies and widely used to remove fluoride from drinking water due to its low cost and ease of operation.In this study,a series of available fluoride biosorbents were prepared.The preparation,physicochemical characteristics,adsorption properties and mechanism of biosorbents were investigated and the drawing conclusions as follows:1.A low-cost and highly efficient biosorbent was prepared by loading Al/Fe oxides onto tea waste and was tested for the ability to remove fluoride from drinking water.Key factors,including adsorbent dosage,initial fluoride concentration,contact time and initial pH of the biosorbent,were investigated.It was found that the removal of fluoride(%)increased with an increase in the biosorbent dosage from 0.4 to 8 g/L.For Tea-Al and Tea-Al-Fe,the maximum level of fluoride removal occurred in the range of 0.4 to 2 g/L biosorbent.Thereafter,there was no significant change in the percent of removed fluoride as the quantity of biosorbent increased.Adsorption of fluoride increased rapidly in the first thirty minutes,then increased gradually until reaching equilibrium within 120 min for Raw tea waste,Tea-Fe,Tea-Al and Tea-Al-Fe.The adsorption capacity of Raw tea waste,Tea-Fe,Tea-Al and Tea-Al-Fe rose with increasing initial fluoride concentration and leveled off at high F-concentration.The solution pH played an important role in the removal of fluoride,andtea waste loaded with Al or Al/Fe showed the most potential for use as a biosorbent to remove fluoride from drinking water through the wide pH range of 4.0 to 8.0.2.In addition,the residual concentrations of Al and Fe in the drinking water after Tea-Al-Fe treatment were below the standards set by WHO when treatment was conducted at pH values ranging from 5.0 to 10.0.The experimental data were analyzed using two-parameter theoretical models.The maximum fluoride adsorption capacities for the Raw tea,Tea-Fe,Tea-Al and Tea-Al-Febiosorbents were 3.83,10.47,13.79 and 18.52 mg/g,respectively.Lagergren pseudo-second-order kinetic models proved to be more suitable in describing the adsorption kinetics of fluoride based on the correlation coefficient(R2>0.99),suggesting that a chemisorptions step might be rate determining in the fluoride adsorption process.SEM,EDS,XRD and XPS were performed to elucidate the mechanism of fluoride adsorption.The results showed that bimetallic Fe-O-Al bonds might form on loading progress and a synergistic interaction between Fe and Al oxide occurred during the defluorination process.3.In order to improve fluoride adsorption capacity,a novel and low-cost biosorbent of tea waste supported hydrous aluminium oxide(Tea-APAM-Al)was prepared with help of anionic polyacrylamide(APAM).Batch adsorption studies were carried out on key factors of adsorbent dosage,initial fluoride concentration,contact time,initial pH and co-existing ions to evaluate the efficiency of fluoride removal.It was found that Tea-APAM-Al performed well over a considerable wide pH range of 4.0-9.0.With increasing the Tea-APAM-Al dosage from 0.4 to 1.2 g/L,the fluoride removal increased from 71.5% to 89.7%.The adsorption capacity of Tea-APAM-Al rose with increasing initial fluoride concentration and leveled off at high F-concentration.It was found that the fluoride removal increased with the contact time,and Tea-APAM-Al removed nearly 90% of the fluoride in the first 20 min at fluoride concentrations of 10,20 and 30 mg/L.In the following period,the adsorption process slowed down substantially and became steady after 180 min.4.With the exception of bicarbonate,other co-existing ions(nitrate,chloride and sulphate)did not have significant effect on the defluoridation process of Tea-APAM-Al.The adsorption process could be well described by Lagergern pseudo-second-order kinetic model.Adsorption data could be well fitted by Langmuir isotherm model and the maximum fluoride adsorption capacity for Tea-APAM-Al was 42.14 mg/g.SEM,EDS,XRD,FTIR and XPS were performed to elucidate the mechanism of fluoride adsorption.The results showed that the fluoride adsorption mechanism was likely to involve hydroxyl and sulfate ions exchange with fluoride.Moreover,anion exchange with sulfate ions was the main mechanism for fluoride adsorption at low initial fluoride concentration.In this study the reuse of tea waste biomass as a sorbent of fluoride from drinking water showed high adsorption capacity,low dissolution of Al and Fe and environmental friendly.Biosorbents of tea waste in purifying drinking water will reduce the negative environmental effects from tea processing plants which provideda new method for comprehensive utilization of tea waste.These findings demonstrate these fluoride biosorbents synthesized in this paper to remove fluoride,could be beneficial to economicand environment. |
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