Sorptive Removal Of Perfluorooctane Sulfonate From Aqueous Solution By Layered Double Hydroxides

Perfluorooctane sulfonate(PFOS)has been used worldwide since the 1950s in a variety of applications,such as surfactants,coatings and firefighting foams,due to its unique physicochemical properties of hydro-oleophobicity and chemical stability.PFOS in the environment has been known to be extremely persistent and toxic to wildlife and human beings because of its immunotoxicity,neurotoxicity,reproductive toxicity,endocrine disruption as well as developmental toxicity.Unfortunately,PFOS can be universally detected in the environmental media,wildlife and even in the serum of human.Therefore,it has been defined as an emerging contaminant and is listed in Annex B of the Stockholm Convention on Persistent Organic Pollutants.The serious contamination of PFOS in the environment has drawn a lot of concern from environmental scientists and regulators and there is an urgent call for its sustainable and effective remediation in water.In this study,we investigated the sorptive removal of PFOS from aqueous solution by three different forms of layered double hydroxides(LDHs),namely,nitrate-,carbonate-and chloride-intercalated LDHs.The basal spacing of the synthesized LDHs using coprecipitation method are in the sequence of nitrate>carbonate>chloride form of LDHs.At the initial concentration of 1 mg/L,the PFOS removal rates were 95.5%,50.3%and 90.6%for the nitrate,carbonate and chloride forms of LDHs,respectively.Batch experiments showed that the sorption process was very fast with an equilibrium time of less than 60 min.The sorption kinetics were best described by the pseudo-second-order model,emphasizing that chemisorption occurs in the sorption process.The intraparticle diffusion model reveals diffusion of relatively large PFOS molecules into the space-limited interlayer region.The good fitting results of the Langmuir model to the sorption isotherms suggested a monolayer sorption of PFOS by the LDHs.The calculated maximum PFOS uptake capacities of the nitrate,carbonate and chloride forms of LDHs were 865,329 and 211 mg/g,respectively.The solution chemistry parameters of the pH,ionic strength and coexisting ions had different impacts on the sorption of PFOS by different forms of LDHs as different mechanisms including surace electrostatic interaction and interlayer anion exchange can be involved.The sorption process governed by surface electrostatic interaction is susceptive to the solution pH and ionic strength.The sorption of PFOS increases with the decrease in pH,and the increase in ionic strength can reduce the electrostatic interaction between LDHs surface and PFOS through electrical double layer compression When the sorption process is dominated by the interlayer anion exchange,coexisting sulfate and carbonate anions can significantly reduce the sorption of PFOS as a result of their competing effects.The anion exchangeabilities of the synthesized LDHs are in the order of nitrate>chloride>carbonate form of LDHs.Generally,the sorption of PFOS by nitrate form of LDHs is mainly through interlayer anion exchange while the surface adsorption dominates the PFOS sorption by the carbonate form of LDHs.However,both interlayer anion exchange and surface adsorption are involved in the mechanism of PFOS sorption by the chloride form of LDHs due to its moderate anion exchangeability.