Preparation Of Heavy Metal Adsorbent Dithiocarboxyl Corn Stalk And Its Performance For Cadmium Removal

Wastewater containing cadmium has become one of the industrial wastewater with the greatest harm to human and environment.Various traditional methods like chemical precipitation,ion-exchange and membrane filtration have been utilized,but they show the defect of high costs and secondary pollution in practical application.Adsorption is widely utilized in removal of cadmium ions from wastewater owing to the advantage of high selectivity and good efficiency.Traditional adsorbents with drawbacks of costliness or unsuitable recycling property have applied for removing heavy metal ions including inorganic nanomaterials,adsorption resins,and activated carbon.However,raw crop stalks have shown certain adsorption capacities for heavy metal ions due to their functional groups like hydroxyl,carboxyl and amino groups,which often needed to be modified to improve their adsorption performance.At present,the research about the introduction of heavy metal ion coordination groups onto crop stalks focuses on N-containing and O-containing groups,while there are few reports on the introduction of S-containing groups.Therefore,corn stalk(CS)was chemically modified to prepare dithiocarboxyl corn stalk(DTCS)considering its strong chelating property of dithiocarboxyl groups for heavy metal ions in this paper.The factors and optimal conditions for DTCS preparation were determined by using single factor experiment,Plackett-Burman experiment,the steepest climbing experiment and central composite design method(CCD)in the response surface methodology.The characterization of CS and DTCS were done by using elemental analysis,SEM,XPS,FTIR,specific surface area and pore size distribution techniques.The suitable conditions for Cd(Ⅱ)removal with DTCS were studied by static adsorption experiments.The main results of experiments are as follows:(1)The feasible preparation conditions of DTCS obtained by single factor experiment were:10 mesh(2.0 mm)of stalk particle size,45 min of soaking time,1:1:2 of CS/CS2/NaOH mass ratio,25℃of pre-reaction temperature,30 min of pre-reaction time,40℃of main reaction temperature,and 1 h of main reaction time.(2)Stalk particle size,reactant ratio,pre-reaction temperature and main reaction temperature were selected by Plackett-Burman experiment as key factors for preparation of DTCS.(3)The center points of the optimal response area of DTCS preparation conditions were:20 mesh(2.0 mm)of stalk particle size,45 min of soaking time,1:2:3 of CS/CS2/NaOH mass ratio,25℃of pre-reaction temperature,30 min of pre-reaction time,45℃of main reaction temperature,and 90 min of main reaction time,which were determined by the steepest climbing experiment.(4)The optimum preparation conditions of DTCS optimized by CCD were:10 mesh(2.0mm)of stalk particle size,45 min of soaking time,1:1.62:3 of CS/CS2/NaOH mass ratio,20℃of pre-reaction temperature,30 min of pre-reaction time,40℃of main reaction temperature,and 90 min of main reaction time.(5)Various techniques like elemental analysis,XPS and FTIR confirmed that dithiocarboxyl groups was introduced successfully onto the molecular structure of corn stalk.(6)The highest Cd(Ⅱ)removal efficiency by DTCS at Cd(Ⅱ)concentrations of 5 mg/L,15 mg/L,25mg/L,50 mg/L and 100 mg/L were 93.72%,100%,100%,99.89%and 98.74%,respectively,which were obtained under conditions of adsorption temperature of 50℃,oscillation speed of 150 r/min and pH value of 3.0～6.0.Otherwise,Cd(Ⅱ)removal efficiency for 50 mg/L cadmium-containing water samples with DTCS decreased slightly after storing for 10 days at room temperature.The highest Cd(Ⅱ)removal efficiency with waste filtrate at Cd(Ⅱ)concentrations of 5 mg/L,15 mg/L,25 mg/L and 50 mg/L reached 74.58%92.60%,97.30%and 97.94%,respectively.(7)The adsorption kinetics conformed to the pseudo-second-order adsorption kinetic model(R~2 greater than 0.99),which suggested that the adsorption process was mainly based on chemical adsorption.Langmuir isotherm adsorption model(R~2 in the range of0.8946～0.9984),Freundlich isotherm adsorption model(R~2 in the range of 0.9319～0.9898),Dubinin-Radushkevich isotherm adsorption model(R~2 in the range of 0.9385～0.9664)could well describe adsorption process,which revealed that both chemical adsorption and physical adsorption happened during the adsorption procedure.The maximum Cd(Ⅱ)adsorption capacity with DTCS obtained from isothermal adsorption experiment was 55.19 mg/g.In addition,the thermodynamic parameters indicated exothermic behavior and chaotic process of the adsorption.The novel heavy metal adsorbent DTCS with the low cost of preparation and the high efficiency for heavy metal ions implies that it has potential utilization value in the treatment of heavy metal wastewater.