Preparation And Application Of Heavy Metal Capturing Agents

As the development of industry, the emission of heavy metal contaminated waterbecomes more and more serious, that is a serious threat to environment and human health.Traditional separation methods of heavy metal ion in wastewater such as chemicalprecipitation separation method, usually needs a lot of chemical reagents, produces a largenumber of sludge difficult to dispose, with the shortages of secondary pollution and theresulting effluent which can’t meet the requirements. However, on the other hand, ionexchange or electrochemical separation has a better effect of separation, it’s processing cost istoo high. As a result, research and development of a kind of special capturing agent with highefficiency, economy and environmental protection will have significantly social, economicand environmental value.The preparation of DTCT heavy metal capturing agent, DTC （dithiocarbamic） modifiedchitosan heavy metal capturing agent （DTC-CS）, and chitosan-polyvingakohol adsorptionmembrane （CS-PVA）, and their separation effects of Cu²⁺and Cd²⁺in the wastewater werestudied in this work. The main research results are as follows:1. DTCT: A macromolecular skeleton with the branched structure of amino wassynthesized by using ethylenediamine and triethylene tetramine as monomers andepichlorohydrin as the crosslinking agent. The final product a water-soluble andmacromolecular heavy metal capturing agent was obtained by grafting CS2on the aminogroup of the macromolecular skeleton. The synthetic conditions were optimized by theorthogonal test. DTCT was used for the separation of Cu²⁺and Cd²⁺from wastewater. Theoptimum capturing effect is gained when disposing of1000mg/L Cu²⁺solution, the dosage ofDTCT is25mg/mL, pH3, and reaction time is1min; when disposing of1000mg/L Cd²⁺solution, the dosage of DTCT is35mg/mL, pH6, and reaction time is5min, the removalrates of Cu²⁺and Cd²⁺are both more than99.9%, which can meet the requirement of thenational emission standards well. When DTCT is used to the actural wastewater, its separationeffect is also remarkable. The stability of the resulting chelating precipitate and the chelatingmechanism are studied, which prove the DTCT can effectively remove the heavy metal ionsfrom wastewater.2. DTC-CS: The synthesis of DTC-CS was in the medium of methanol and water, usingchitosan and CS2as the raw material,35wt%NaOH as the catalyzer. The structure containingDTC group was confirmed by means of FT-IR, elemental analysis, XRD and TG. Theoptimum condition of capturing Cu²⁺（dosage is33mg/mL, pH is4.5-5, reaction time is30min） and the optimum condition of capturing Cd²⁺（dosage is60mg/mL, pH is5.5-7, reactiontime is50min） are determined, and the removal rates of Cu²⁺and Cd²⁺can both reach morethan99.9%. The desorption and recovery experiments of DTC-CS show that the mixedsolution of1mol/L thiourea and0.05mol/L HCl give the best desorption effect of Cu²⁺andCd²⁺, the maximum desorption rates of which were respectively87.13%and89.64%.Compared with DTCT, DTC-CS can be reused and cost-efficient.3. CS-PVA membrane: The CS-PVA membrane was prepared by two means,immersion-precipitation phase inversion technique and casting technique. The morphology of the prepared membrane was described by scanning electronic microscopy. The surface of theimmersion-precipitation phase inversion technique and casting technique. The morphology ofthe prepared membrane was described by scanning electronic microscopy. The surface of themembrane prepared by immersion-precipitation phase inversion technique is rough, porousand loose, quite different from the membrane prepared by means of casting, which has asmooth and dense structure, and its adsorption capacity of heavy metal ion is higher than thatof the dense membrane. In addition, the adsorption time, pH and the dosage of the membraneprepared by immersion-precipitation phase inversion technique are investigated. The resultsshow that when the the dosage is5g/100mL, pH of Cu²⁺and Cd²⁺solution respectively are4.5and6, adsorption time respectively are270min and300min, the effect of separation andadsorption are the best. The changes of the membrane structure before and after adsorption areanalysed by FT-IR and XRD, and the reused times of the membrane is researched, whichindicates that the membrane is stable and recycled.In conclusion, DTCT has the best removal effect for heavy metal ions. However, it hassome drawbacks, such as unrecycled, to some extent toxic, and residual in water. DTC-CS canbe reused, and has lower toxicity to creatures due to the combining the biodegradable chitosan.CS-PVA membrane has a well effect of adsorbing the heavy metal ions. Moreover, it iscompletely non-toxic, environmental friendly, and can be circularly utilized, it is a kind ofgreen and highly efficient water treatment materials, and will become the promising trend forthe development of heavy metal capturing materials.