Desorption Of Adsorbed Pb(?) On Graphene Oxide Under Alkaline Conditions

Lead is widely used in industrial field because of its excellent chemical stability and low price.The unreasonable discharge of heavy metal wastewater has caused serious pollution,particularly in water and soil.Graphene Oxide?GO?contains many oxygen-containing functional groups,i.e.carboxyl?—COOH?,hydroxyl?—OH?,and epoxy?—O—?,that have been detected at the edges and surfaces of GO nanosheets.The various oxygen-containing functional groups ensure that GO has good hydrophilicity in aqueous solutions and provides active adsorption sites for many kinds of contaminants,especially heavy metal ions.In addition to the well-defined oxygen-containing functional groups,some oxidative debris?OD?,also exist on GO surface.The structure and characteristcs of OD are similar to fulic acid,and OD can be separated from GO under alkaline condition.However,a paucity of data exist regarding the the influence of OD on GO adsorption performance,as well as the potential escape of Pb???from spent GO-based adsorbent when discharged into natural groundwater if not safely disposed.Therefore,it's helpful to study the structure and composition of GO,particularly the effect of OD stripping under alkaline conditions on Pb???transport in saturated porous media.The study of environmental stability of GO-based materials as adsorbents for heavy metal ions removal has become a contemporary challenge.Based on the environmental application and characteristics of GO materials,this paper studied the variation of oxygen-containing groups on the surfaces of GO nanosheets and OD stripping characteristics as well as the release of adsorbed Pb???on spent GO under alkaline conditions by preparing alkalinity with sodium bicarbonate.We combined FTIR and fluorescence spectra analysis with the morphological forms of the released Pb???under alkaline conditions,the environmental stability of GO adsorbent with Pb???attached was explored.The details are as follows:?1?OD produced in oxidation process of GO played a critical role in Pb???adsorption on GO,and the adsorption process of GO and OD was in accord with the preudo-second-order kinetic model.The maximum adsorption capacities of GO and OD on Pb???were 562.98 mg·g^-11 and 2974.40 mg·g^-1,respectively.?2?Alkalinity was the key to separate OD from the surface of GO.OD could be stripped gradually from GO surfaces under 0.01—1.0 mol·L^-1 NaHCO₃ conditions although the stripping kinetic process was very slow,and the exfoliation process reached equilibrium after 20 h.After 80 h,the stripped OD content with NaHCO₃treatment was around 1.57%—2.94%of GO.?3?The deposition and detachment of the OD in the quartz sand media were affected dramatically by the ion strength of the solution.The recovery rate of OD in saturated sand columns decreased from 95.67%to 70.14%when the ionic strength increased from 0.001 mol·L^-1 to 0.1 mol·L^-1.The higher mobility of OD than GO apparently improved the transportation of adsorbed Pb???through porous media.OD as a carrier could significantly promoted the escape and release of Pb???in sand layer.?4?When GO-Pb???entered into the artificial groundwater environment,5.52%—26.68%of adsorbed Pb???on spent GO desorbed with OD stripping from GO and released to water as an OD-Pb???complex form which could disperse steadily in water.The particle size of OD-Pb???was only about 10 nm,which enhanced the mobility of Pb???in the sand layer and further exacerbated the risk of repeated pollution of GO-Pb???in the aqueous environment.