Studies On The Modification Of Tea Waste And Starch And The Mechanism Of Defluoridation Adsorbents From Drinking Water

Fluoride is one of essential trace elements for humans body's normal growth,appropriate amount of fluoride is benefit for detal cavities,but high concentrations can lead to dental fluorosis andbone disease.Fluoride is a widespread mineral,naturally occurring in the ores,but it will be dissolved out by the rain,which causes fluoride pollution of drinking water.Nowadays,adsorption stands out due to its simplicity.But,low adsorption capacities,slow adsorption processes,narrow optimum pH ranges and high residue of the most materials,limit their practical applications.Hence,development of effective,safe and inexpensive adsorbents for the treatment of large volumes of water containing high concentrations of fluoride is timely challenge for environmental chemists.In order to solve the above-mentioned technique problems,in this study,a series of available fluoride biosorbents based on tea weast and starch were prepared.The adsorption properties and mechanism of biosorbents were investigated.The main contents of this paper are including:?1?A low-cost and highly efficient biosorbent was prepared by loading zirconium?IV?onto ball-milled,ultra-fine tea powder?UTP-Zr?for removal of fluoride from drinking water.To evaluate the fluoride adsorption capacities of UTP and UTP-Zr over a wide range of conditions,the biosorbent dosage?0.4-4.8 g/L?,contact time?1-360min?,initial pH?3.0-12.0?,initial fluoride concentration?5-200 mg/L?and presence of other ions(NO₃^-,Cl^-,SO₄^2-,HCO₃^-)were varied.It was found that increasing UTP-Zr from 0.4 to 3.2 g/L increased the fluoride removal from 52%to 93%.Adsorption of fluoride increased rapidly in the first forty minutes.After theinitial period,the fluoride removal reached equilibrium at about120 min for UTP and UTP-Zr.The equilibrium absorption capacity of UTP-Zr was similar over the initial pH range of 3.0-9.0,which can be conveniently used for water treatment without pH adjustment.The increasing the solution pH had no effect on absorption capacity of UTP.The equilibrium adsorption capacity significantly increased with increasing initial fluoride concentration andnearly leveled off when the initial concentrations were from 5 to200 mg/L.?2?The residual concentration of Zr in the water after passing through the UTP-Zr was below the limit of detection?0.01 mg/L?across the pH range of 3.0-11.0.With the exception of bicarbonate,other co-existing ions?including nitrate,chloride and sulphate?did not have significant effect on the fluorideremoval of UTP-Zr.The Langmuir isotherm model fit the equilibrium data well and was used to calculate a maximum adsorption capacity of 12.43 mg/g for UTP-Zr.The kinetics of fluoride adsorption by the biosorbent fit the pseudo-second-order kinetic model for UTP and UTP-Zr.SEM,EDS and XPS were performed to investigate the mechanism of fluoride adsorption.The results showed that UTP-Zr was through chemical ion exchange and electrostatic attraction between UTP-Zr and fluoride during the defluorination process.?3?Corn starchwas treated by glucoamylase and?-amylase to obtain porous starch.Then a novel biosorbent of porous starchmodified byZrOCl₂,AlCl₃,FeCl₃ and La?NO₃?₃ was prepared to produce the composites PS-Zr,PS-Al,PS-Fe and PS-La,respectively.Fluoride adsorption from water was tested at different the biosorbent dosage?0.4-4.8 g/L?,contact time?1-360 min?,initial pH?3.0-12.0?,initial fluoride concentration?5-400 mg/L?for four biosorbents.It was found that fluoride adsorption increased with the increasedbiosorbent dosages for four biosorbents andthe PS-Zr composite was provedto be the most suitable biosorbent for the treatment of fluoride-contaminated water.Increasing PS-Zr from 0.4 to 1.6 g/Lincreased the fluoride removal from 30%to 96%.The adsorption capacity increased quickly in the first 40 min for the four biosorbents and gradually reached steady state with further increasing contact time.Notably,the absorption capacity of PS-Zr remained fairly constant over the pH range of 3.0-9.0,with a maximum adsorption capacity of 6.16mg/g at pH of 4.The adsorption capacities of the biosorbents increased along with the increase ininitial fluoride concentration,and nearly leveled off at the higher concentrations.PS-Zr showed the highest adsorption capacities across all fluoride concentrations.?4?The residual concentration of Zr after treatment of PS-Zr was always below the detection limit of ICP spectrometry?0.01 mg/L?in solutions with pH ranging from3.0 to 11.0.The Langmuir model well described the PS-Zr and PS-Al experimental fluoride removal data and the calculated maximum adsorption capacities?q⁰?obtained were 25.41 mg/g and 16.40 mg/L,respectively.In addition,the Freundlich isotherm models better fit the PS-Fe and PS-La fluoride adsorption data than the Langmuir isotherm model based on R² values and experiment datas.The adsorption process could be well described by Lagergern pseudo-second-order kinetic model?R²>0.97?for PS-Zr,PS-Al,PS-Fe and PS-La.SEM,EDS,FTIR and XPS were performed to elucidate the mechanism of fluoride adsorption.The results showed that the strong interactions between the fluoride ions and the metal cations of the surface layers of each biosorbent,ion exchange between the fluoride ions and the-OH groups on the surface of the metal may also occur at the biosorbent's surface layers.In this paper the reuse of tea waste and starch biomass as a sorbent of fluoride from drinking water showed low cost and less preparation steps,but high adsorption capacity,low dissolution of metal ions and environmental friendly.