Preparation Of Micro-sand/modified Starch Composite Adsorbents And Their Adsorption Performance For Removal Of Dyes And Heavy Metal Ions From Water

Starch(ST)is one of the high-performance materials with advantages of widespread availability,environmental friendliness,and biodegradability.Moreover,starch has been also considered as an ideal adsorbent because of abundant free hydroxyl groups distributed on its backbone.Various pollutants such as numerous metal ions and many dyes could be easily bound to these groups to further purify the water.However,different kinds of pollutants usually coexist in real wastewater.While most traditional adsorbents,including starch,have no evident selectivity to certain individual pollutant.Evidently,it is significant to remove and separate the target pollutants in their individual pure form efficiently through the selectivity of certain adsorbent for recycling use and avoiding secondary pollution.Accordingly,different kinds of effective functional groups have been introduced onto adsorbents by modification to improve the selectivity.In addition,most of starch-based adsorbents could not be separated from water rapidly and easily after achieving adsorption equilibrium in practical applications because of their extremely small size and lower density.In this work,in view of the above problems of starch-based adsorbent,two types of silica sand composite modified starch adsorbents were successfully prepared by a simple method:one is a cationic composite adsorbent(ST-CTA-SS)composed of starch-3-chloro-2-hydroxypropyl triethyl ammonium chloride(ST-CTA)and micro silica-sand(SS),the other is an anionic composite adsorbent(CMS-SS)composed of carboxymethyl starch(CMS)and micro silica-sand.The former adsorbent showed enhanced selective removal of anionic dye(acid green 25(AG25)and methyl orange(MO))in dye and As(V)binary mixed system;while the latter one could selective removal of cationic dyes(methyl blue(MB)and crystal violet(CV))from dye and Cu(II)binary mixture at suitable pHs.The composite adsorbents not only retain the starch good adsorption capacity,but also could be separated quickly from the water because of the high density of silica sand.Moreover,the composite adsorbents also have good recycling efficiency so that it has good practical potentials in the water treatment.In short,several interesting results have been drawn as shown below:(1)A novel composite adsorbent composed of starch-3-chloro-2-hydroxypropyl triethyl ammonium chloride and micro silica-sand was prepared simply,with the characteristics of environmental friendliness and low cost.A variety of techniques were used for characterizing the surface morphology,molecular structure,and settleability of this composite adsorbent.ST-CTA-SS was applied to the removal of As(V)and two anionic dyes,AG25 and MO,from water in respective single and two metal-dye binary systems.ST-CTA-SS shows a marked improvement of the adsorption capacities for the aforementioned three pollutants,which is attributed to the prominent composited structure compared with ST-CTA without SS.The adsorption capacities of ST-CTA-SS for AG25,MO,and As(V)are up to approximately 912.6±44.6,458.7±21.3,and 76.63±3.53 mg g-1,respectively,and the adsorption saturations for those three pollutants are all achieved within 20 min.Moreover,this composite adsorbent always exhibited highly selective adsorption of dyes,either MO or AG25,over As(V)from their aqueous mixtures.After saturated adsorption,ST-CTA-SS could be rapidly settled and separated from water due to the imbedded SS,which could be easily regenerated by dilute NaOH solutions.This starch-based composite adsorbent thus owns significant application potentials in water treatment.(2)Besides,a chloroacetic modified starch compositing adsorbent(CMS-SS)was prepared by combination of silica sand.CMS-SS was employed for the removal of MB,CV,and Cu(?)from water in respective single and binary systems.Compared with CMS without.silica sand,CMS-SS shows evidently improved adsorption capacity for these three pollutants ascribed to the additional chloroacetic groups.The isotherms and kinetics study for these pollutants adsorption on CMS-SS indicated that the Langmuir model and the pseudo-second-order model were more suitable.The adsorption process is thus a homogeneous monolayer chemisorption manner.The adsorptions of MB,CV,and Cu(?)by CMS-SS are all spontaneous and exothermal processes driven by increasing entropy from the calculated thermodynamic parameters.The adsorption behaviors of ST-CTA-SS have a high pH dependence,and electrostatic attractions play a more important role in this adsorption.Based on the adsorption equilibrium study,dyes showed higher affinity to CMS-SS than Cu(?)causing a considerable extent of preferential adsorption of dye over metal ions in their aqueous mixture.This adsorbent after saturated adsorption could be rapidly separated from water due to embedded silica sand.In summary,these starch-based adsorbents due to the significant characteristics of a wide sources and environmental-friendliness have been received much more attentions.Moreover,they have a good adsorption effect for the removal of dyes and heavy metal ions from water.After mixing with silica sand,these adsorbent not only exhibited enhanced adsorption performances but also showed evidently selectivity for target pollutants,which is significant in real applications.Therefore,it believed that these micro-sand/modified starch composites should have a great application prospect as adsorbents in the field of water treatment.