Modification Of Wheat Straw (Cellulose) And Mechanism Of Pb(Ⅱ) And As(Ⅴ) Adsorption

Water environment pollution has become a serious problem in worldwide,partly due to the excess emissions of heavy metals,even at trace level,which may pose a sever threat to human,aquatic ecosystems and other biological systems.Numerous treatment technologies have been proposed for the removal of efficient heavy metal from waters.Among all these treatment processes,adsorption is one of the most widely used and promising techniques due to its high efficiency,low cost,easy operation,eco-friendly,and the availability of sorts of adsorbents.In China,wheat stalk(WS)is one of the most abundant natural biopolymers.Most of the WS was abandoned or burned,causing wasting of resources and severe air contamination via the emission of carbon dioxide,volatile organic compounds,nitrogen oxides,methane and sulfur dioxide.Hence,it is imperative to develop new applications of straw for environmental protection.WS can be modified into heavy metals adsorbent.These not only can solve pollution problems in aqueous,but also provide a new approach for corn stalk.Lately,a wide variety of attempts have been in finding effective and economic natural adsorbents from agricultural residues.Many studies have reported that straw modified by various methods as an adsorbent showed up outstanding performance for pollutants owing to its structure features and functional groups such as amino,thiol,carboxyl.However,for some heavy metal ions,they exist in the form of anionic metals in water.Adsorbent with only one type of functional group is usually incapable of eliminating various heavy metals.It is a little difficult for most adsorbents to remove both anionic metals and cationic metals in water.To our best knowledge,there are only a few quantitative data on the amphoteric resins based on agriculture adsorbent materials.However,chloroacetic acid and simple quaternary ammonium salts as functional groups are grafted directly on those adsorbents.The compound with a pincer structure has more adsorption sites,which can be easier latched more ions through chelating and ion-exchange properties.Therefore,in the current study,a cationic compound(IA)reagent and a anionic reagent(IM)with pincer structures were synthesized.Then,IA and IA as modifier,anionic,cationic and nonionic adsorbents and a ampholytic hydrogel was synthesized and characterized by SEM,FTIR and XPS analysis.Furthermore,the adsorption properties for Pb(Ⅱ)and As(Ⅴ)were evaluated in batch experiments.The main results are as follow:(1)A response surface methodology(RSM)with a Box-Behnken design(BBD)was used to optimize the synthesis process of adsorbents.According to the results,the optimal conditions for WS-IA preparation were NaOH 11.4 mL g-1,reaction temperature 82.3°C,reaction time 7.5 h and dose of IA 0.4 g g-1,respectively;the optimal conditions for WS-IA-IM preparation were NaOH 5.6 mL g-1,reaction temperature 62.4 °C,reaction time 5.7 h and dose of IM 0.32 g g-1,respectively;the optimal conditions for WS-IAIM production were NaOH 7.0 mL g-1,reaction temperature 64.0 °C,reaction time 6.0 h and dose of IM 0.33 g g-1,respectively;Under the optimal conditions,the maximum predicted qe were as well as the actual values.(2)The results of SEM,Zeta potential,FTIR and XPS showed that the structures of WS-IA,WS-IM,WS-IAIM and WSC-IAIM-ECH are correct.Additionally,IA and IM groups were in the form of sodium carboxylate and chlorine salt respectively,on WS surface.(3)Batch adsorption experiments results suggested that the optimized pH values for removal of Pb(Ⅱ)and As(Ⅴ)were 5.0 and 1.0,respectively.The adsorption processes were better to be described by Langmuir equation,which indicated that the adsorption followed the mechanism of monolayer adsorption.The Adsorption kinetics was fitted well with the pseudo-second order model.Adsorption thermodynamic tests implied that the adsorption process was a spontaneous and endothermic reaction.Furthermore,the experimental results showed that the adsorption mechanism followed the monolayer chemical adsorption with an ion-exchange process.Based on the Langmuir equation,the maximum adsorption capacity(Qmax)of WS-IA,WS-IAIM and WSC-IAIM-ECH for Pb(Ⅱ)were 180.48 mg g-1,180.12 mg g-1and 835.26 mg g-1,respevtivly;the maximum adsorption capacity(Qmax)of WS-IM,WS-IAIM and WSC-IAIM-ECH for As(Ⅴ)were 28.36 mg g-1,27.48 mg g-1 and 88.35 mg g-1,respevtivly.Reusability studies confirmed that the WS-IAIM could be easily regenerated and repeatedly utilized in the wastewater treatment.