| With the rapid economic growth and industrialization process acceleration in China,the increasing level of heavy metals that are discharged into environment as industrial wastes,represent a serious threat to human health,living resources,and ecological systems.Heavy metals are known as poisonous chemical elements.They do not degrade into harmless end products in the metabolism and accumulate in the food chain.Thereby,it is necessary for us to find a rational method to control the pollution.Traditional technologies for heavy metal treatments include chemical precipitation,electrolysis,membrane separation and ion exchange,and are often costly or ineffective and rather complicated,which result in less technically appealing in actual use.The methods of converting agricultural residues into ion adsorbents to removal low concentration of heavy metal in the wastewater have the advantages such as the most abundant renewable organic resource on the earth,low cost and easily being applied and spreaded.Cassava is widely cultivated in Guangxi and lots of residual Cassava stalks are produced every year,which is normally discarded.Using the cassava stalks as the meterial for heavy adsorbents is therefore of great value for environmental protection and for utilization of the waste resources.In this study,cassava stalk was modified with ethylenediaminetetraacetic dianhydride(EDTAD).The preparation conditions of modified adsorbents were optimized by single factor experiment.The resulting product was used to adsorb heavy metal ions such as Pb2+,Zn2+,Cd2+,and Cu2+ ions from aqueous single metal ion solutions.It provides a theoretical and practical basis for the utilization of agricultural waste resources and the control of heavy metal pollution and resource recycling.The research content and conclusions are as follows:1.Preparation conditions optimization and characterization of the modified adsorbentOptimized reaction parameters such as temperature,reaction time,sample weight and the extent of solvent was studied.Experimental results showed that optimal synthesis conditions were as follows:While 500mg mercerized cassava stalk was modified,the temperature was kept at 70℃,the reaction time was 32h,weight ratio of cassava stalk to EDTAD was 1:4,and the volume of DMF as catalyst and solvent was 30mL.the maxmium weight-growth rate of modified cassava stalk was 15.51%.SEM images indicated that a lot of round particles appeared on the surface of modified cassava stalk.The surface of the modified cassava stalk became more irregular.A lot of grooves and cracks appeared on the surface,indicating specific surface area was increased.Infrared spectroscopy showed that the stretching vibration peak at 1744 cm-1 represent C=O bond in natural cassava stalk,but it is disappeared from the IR spectroscopy after alkalifying.The absorption peak appearred again in the modified cassava stalk indicated that EDTA dianhydride was successfully connectted on the surface of cassava stalk.XRD graph illustrated that crystalline region intensity was decreased in modified cassava stalk,modifier molecule can easily access into the internal cellulose and increased chemical reaction activity after alkalization treatment.Elemental analysis indicated that the nitrogen content is increased after cassava stralk was modified with EDTA dianhydride2.Research on heavy metal ions adsorption on modified cassava stalk①The effect of the amount of adsorbent the experiment was carried out when the initial concentration of heavy metals was 200mg/L,reaction temperature 30℃,reaction time 1 h and the pH value of the solution is not adjusted.60 mg modified cassava was added in the solution and the maximum adsorption rate was 94.39%for Pb2+ adsorption.while 200mg,120mg,210mg of modified cassava stalk was added in the Zn2+ Cd2+ Cu2+solution,the maximum adsorption rate was 96.52%,94.33%,95.76%,respectively.With the increased of modified cassava stalk dosage,the adsorption rates were on the rise.②Effect of pH on the adsorption capacity of adsorbents The experiment was carried out when the initial concentration of heavy metal ions was 200mg/L,reaction temperature 30℃,reaction time 1 h.The pH value was adjusted from 1-8.Experimental results showed that while pH lower than 2,the adsorption removal rate was limited.The optimal pH value for Pb2+adsorption was 5 and maximum adsorption removal rate was 95.67%.The maximum adsorption removal rate for Zn2+was 98.92%at pH=6,96.37%for Cd 2+ at pH=5,97.84%for Cu2+ at pH=6,respectively.③Effect of oscillation rate on the adsorption capacity of adsorbents ion The experiment was carried out when the initial concentration of heavy metal ions was 200mg/L,reaction temperature 30℃,reaction time 1 h and at optimal pH value.The oscillation rate was adjusted.Experimental results showed that the optimal oscillation rate was 150rpm for Pb2+,Zn2+ and Cd2+and maximum adsorption rates were 96.73%,98.32%,96.39%,respectively.But optimal oscillation rate for Cu2+ was 200rpm and maximum adsorption rate is 96.09%.④The effect of contact time on the adsorption capacity of adsorbents The experiment was carried out when the initial concentration was 200mg/L,reaction temperature was 30 ℃ and at optimal pH value.Thecontact time was adjusted.Experimental results showed that the adsorption process was divided into three stages..the adsorption capacity was rapidly rising from 0 to 10min for Pb2+ and Cd2+;The adsorption capacity increased slowly from 10 to 40 min.The adsorption capacity maintain contant after 40 min.The adsorption characteristics for Zn2+ and Cu2+ removel is similar to the former description.The rapidly rising stage was from 0 to 5 min,while the second stage was from 5 to 40 min.After 40 min past,the adsorption capacity maintain contant.⑤Kinetic model fitting Three kinetic models were applied for the kinetic curve fitting,the results showed that the second-order kinetic equation was appropriate to desribe the adsorption precess.The correlation coefficients were above 0.99.⑥Adsorption isothermal model study The experiment was carried out with contact time of 40 min,at 20,25,30,35 ℃.The adding amount of modified cassava stalk for the treatments of Pb2+,Zn2+,Cd2+,Cu2+ removal were 125,140,80,210mg,respectively.And the corresponding pH values for these treatments were adjusted to 5,6,5,6,respectively.The initial concentration of the heavy metals(50,100,150,200,300,400 mg/L for Pb2+;160,180,200,240,280,320,360 mg/L for Zn2+;50,100,150,200,250,300 mg/L for Cd2+;10,20,35,97,180,310,420,480 mg/L for Cu2+)was adjusted at different temprature(25,35,45,55℃ for Pb2+;20,30,40℃ for Zn2+;35,40,45,50℃ for Cd2+;30,40,50℃ for Cu2+).Experimental results showed that adsorption amount followed a downward trend while the temperature gone up.The adsorption of four heavy metal ions can be described with Langmuir isotherm model.The maximum adsorption capacity for Pb2+ was 196.34,192.2,188.6,181.7mg/g at 25,35,45,55 ℃ ℃,respectively;While the maximum adsorption capacity for Zn2+ 82.64,81.97,80.64 mg/g at 20,30,40 ℃,respectively;the maximum adsorption capacity of Cd2+ 181.82,175.44,167.29,151.52 mg/g at 35,40,45,50℃,respectively;the maximum adsorption capacity of Cu2+was 56.73,54.05,52.91 mg/g at 30,40,50℃,respectively.⑦ Regeneration experiment 0.5mol/L HCl、HNO3、citric acid、EDTA.2Na、NaCl and NaHCO3 were used as regenerator for the saturated adsorbent.The experimental results showed that 94.23%、94.49%、94.23%、97.63%、39.82%and 24.57%regeneration rates for Zn2+were attached after three times sorption and desorption cycle.The regeneration rates were 98.26%、98.21%and 97.63%while 0.5mol/L ethylenediaminetetraacetic disodium was used as regenetator after each cycle.The adsorption capacity was 55.48、54.95 and 53.75 mg/g.The adsorption capacity was down by 1.57%,4.2%and 5.94%,respectively. |
PDF Full Text Request