Chromium Removal Performance And Mechanism With Dithiocarboxylated Wheat Straw As Heavy Metal Adsorbent

The development of industrialization will produce a large number of production wastewater,and the wastewater containing chromium mainly comes from electroplating,pharmaceutical and tanning industries.The chemical forms of chromium are Cr（VI）and Cr（III）,of which Cr（VI）is more toxic than Cr（III）,so the treatment of chromium-containing wastewater mainly focuses on the treatment of Cr（VI）,which exists in the form of oxygenated anions such as Cr₂O₇^2-,HCr O₄^-,Cr O₄^2-,et al.,and has certain oxidation capacity.There are many ways to treat Cr（VI）-containing wastewater,and adsorption method has the advantages of simple operation and wide source of adsorbent,which can be used to treat chromium-containing wastewater.At present,some waste crop straws can be used as a natural adsorbent for the treatment of heavy metal wastewater,but they has the problems of low adsorption capacity and poor adsorption performance,so crop straw will be modified to improve its adsorption performance.In this paper,a noval heavy metal adsorbent,called dithiocarboxylated wheat straw（DTWS）,was prepared by introducing a sulfur-containing group（dithiocarboxylate group）on waste wheat straw（WS）as a raw material through chemical modification.The adsorption performance and adsorption mechanism of DTWS for Cr（VI）and Cr（III）removal were investigated by single-factor experiments,various adsorption models and characterization methods.The adsorption conditions of DTWS for Cr（VI）removal were optimized by response surface experiments.Furthermore,the effects of various coexisting inorganic cations,inorganic anions,organic substances and turbidity on Cr（VI）removal by DTWS were studied by adding coexisting substances to chromium-containing water samples under different concentration conditions.A certain amount of waste filtrate was generated in the preparation process of DTWS,and the effect of the filtrate as flocculant on the removal of Cr（VI）was studied through flocculation experiments.The main research results are as follows:（1）In the process of Cr（VI）adsorption by DTWS,the dosage of DTWS,adsorption vessel,oscillation rate,solution p H,initial concentration of Cr（VI）,adsorption temperature and adsorption time could affect the removal effect of Cr（VI）.The conditions of Cr（VI）removal by DTWS were initially determined by the single-factor method as the oscillation rate of 200 r·min^-1,the adsorption temperature of 30℃and the adsorption time of 2 h.Under the adsorption condition,the highest removal rate was 99.12%when DTWS adsorbed Cr（VI）containing water samples with initial concentration of 25 mg·L^-1 and p H 6.0.The adsorption of Cr（VI）by DTWS was more consistent with the quasi-secondary kinetic equation and Langmuir model,and the whole adsorption process of DTWS for Cr（VI）was a spontaneous exothermic process.The adsorption mechanism of DTWS for Cr（VI）was mainly electrostatic adsorption,physical adsorption and chemisorption including oxidation-reduction reaction and coordination reaction,in which the dithiocarboxyl group introduced in wheat straw played a great role.（2）In the process of Cr（III）adsorption by DTWS,the dosage of DTWS,solution p H and initial concentration of Cr（III）could affect the removal of Cr（III）,and the solution p H had a greater effect on the Cr（III）adsorption by DTWS.The best removal of Cr（III）by DTWS was achieved at the solution p H of 4,and the highest Cr（III）removal rate was 98.28%at the dosage of DTWS was 5 g·L^-1.When the solution p H was above 5,Cr（OH）₃ precipitation was generated.The adsorption isothermal model results indicated that the adsorption of Cr（III）by DTWS was more in line with the Freundlich model,which was a multi-layer adsorption of non-homogeneous material.（3）Through Plackett-Burman experiments,it was found that the solution p H,oscillation rate and adsorption time had significant effects on Cr（VI）adsorption by DTWS in the process of Cr（VI）adsorption by DTWS,and these three reaction conditions had interactive effects on the removal of Cr（VI）.The centroid values of the influencing factors were determined by using the steepest climbing experiment.The optimum adsorption conditions for the removal of Cr（VI）by DTWS were further optimized by CCD experiments in the response surface method as the oscillation rate of 70 r·min^-1,the p H value of 3,the reaction time of 145 min,and the reaction temperature of 50℃.The kinetic experiments for the adsorption of Cr（VI）by DTWS were conducted under the optimized conditions,and the results showed that the optimized conditions did not change the potential of Cr（VI）removal by DTWS.The results showed that the optimized conditions did not change the potential mechanism and rate-controlling step of DTWS for the removal of Cr（VI）,but strengthened the influence of the adsorption process by liquid film diffusion.（4）When some coexisting substances existed the solution containing Cr（VI）,the removal of Cr（VI）and total Cr by DTWS would be affected accordingly.Most inorganic cations could inhibit the removal of Cr（VI）and total Cr due to the competition with Cr for adsorption,but Fe³⁺would promote the removal of Cr（VI）by DTWS due to the hydrolysis.The inorganic anions had a certain inhibitory effect on the removal of Cr（VI）and total Cr by DTWS,among which IO₃^-and Br O₃^-had a strong inhibitory effect on the removal of Cr（VI）and total Cr due to their strong oxidation effect.（5）The filtrate generated in the preparation process of DTWS as flocculant to treat wastewater containing Cr（VI）was influenced by the filtrate dosage,solution p H,initial concentration of Cr（VI）and coexisting substances.The influence of solution p H was greater,and the best removal of Cr（VI）was determined by the filtrate at p H 2.When the filtrate dosage was 4 m L,the removal rate of Cr（VI）could reach 96.66%.However,Cr（VI）would show a trend of decrease with the increase of p H value.When some coexisting substances existed in the solution containing Cr（VI）,inorganic cations and organic substances showed inhibition,but inorganic anions had a certain promotion effect,and the coexisting turbidity would inhibit the Cr（VI）removal at low turbidity and promote the Cr（VI）removal at high turbidity.DTWS shows good adsorption performance for treating chromium-containing wastewater,and has good removal effect on Cr（VI）and Cr（III）for various acid and alkaline wastewater.The Cr removal is less affected by conventional inorganic cations,inorganic anions and common organic substances in chromium-containing wastewater from electroplating.At the same time,the filtrate produced during the preparation of DTWS also has a good ability to remove Cr（VI）.Conclusionanlly,the whole treatment process of chromium-containing wastewater by DTWS reflects the concept of treating waste with waste and green environment.