Study On The Adsorption Performance And Mechanism Of Cd(?) On Aluminium Hydroxide-modified Attapulgite Prepared By Different Aluminum Sources

With the development of industrial technology,heavy metal pollution incidents are increasing frequently in water bodies,and cadmium pollution incidents are more prominent especially.Cadmium is highly toxic and could cause harm to humans,animals and plants at lower concentrations.Therefore,exploring cost-effective treatment technologys for cadmium pollution is a hot spot in the field of environmental science.Attapulgite is a kind of clay mineral with abundant reserves in our country.Attapulgite is cheap,has a large specific surface area and strong adsorption performance,have been widely used for the removal of heavy metals and other pollutants in water bodies.However,the raw attapulgite has relatively weak adsorption performance for cadmium in water.Therefore,some modification methods are used to treat the attapulgite to improve its adsorption performance for cadmium in aqueous solution,which has certain research value.In this paper,the inorganic aluminum salts Al₂(SO₄)₃·18H₂O and Al Cl₃·6H₂O and the organic aluminum salt Al(OH)(CH₃COO)₂·2H₂O were used to prepare aluminum hydroxide-modified attapulgite through co-precipitation method.Then the aluminum hydroxide-modified attapulgite that prepared by the inorganic aluminum salts were calcined at 600°C to get the calcined aluminum hydroxide-modified attapulgite.The modified materials above were used as adsorbents,with Cd(?)as the target pollutant.The adsorption experiments were studied under different p H values of the solution,different initial concentrations of Cd(?),different reaction time,different reaction temperature and different doses,to research the influence of performance for Cd(?)by adsorbents.Combined with scanning electron microscope(SEM),energy spectrum(EDS),transmission electron microscope(TEM),infrared spectroscopy(FTIR),X-ray diffractometer(XRD),Zeta potential and other analytical testing methods,the microscopic morphology,structure,crystal form and surface groups of modified attapulgite were investigated.Study the difference of Cd(?)adsorption performance on modified attapulgite that prepared by different aluminum salts in-depth,and discuss the adsorption mechanism.1.Using Al₂(SO₄)₃·18H₂O and Al Cl₃·6H₂O as aluminum source prepared aluminum hydroxide-modified attapulgite(ATP-SO₄^2-and ATP-Cl-⁾,to research the adsorption performance of Cd(?),and compared with raw ATP.The results showed that:(1)The adsorption capacities of ATP,ATP-SO₄^2-and ATP-Cl^-on Cd(?)increased with the increasing of solution p H,initial Cd(?)concentration and adsorption time.The adsorption capacities was ATP-Cl^->ATP>ATP-SO₄^2-.ATP-Cl^-was more fit with Freundlich isotherm model,ATP and ATP-SO₄^2-were more fit with Langmuir isotherm model.The adsorption equilibrium times of the three adsorbents was 12 h,and all conformed to the pseudo-second-order model.(2)The Gibbs free energy changes(?G°)of the three adsorbents were all negative,indicating that the adsorption reaction was spontaneous and belongs to the physical adsorption type.The enthalpy changes(?H°)were-20.78,-49.49 and-31.74 k J·mol^-1,respectively,which indicating that the adsorption reactions of the three adsorbents were all exothermic.The entropy changes(?S°)were-45.38,-80.82 and-147.91 J·mol^-1·K^-1,respectively,indicating that the adsorption reactions of the three adsorbents proceed in an orderly direction.(3)The characterization methods showed that the aluminum hydroxide successfully coated onto surfaces of ATP-SO₄^2-and ATP-Cl^-.However,the dispersion of rod crystals in ATP-Cl^-were better than that of ATP-SO₄^2-,and the surface roughness of ATP-Cl^-was increased.The bond positions and groups of ATP-SO₄^2-and ATP-Cl^-had a certain change than ATP,but no changes in crystal form.The negative surface charge after modification was less than before unmodified,and the negative surface charge of ATP-Cl^-was still more than that of ATP-SO₄^2-.2.The raw ATP and the above-prepared ATP-SO₄^2-and ATP-Cl^-were calcined at a temperature of 600?to prepare C-ATP,C-ATP-SO₄^2-and C-ATP-Cl^-,respectively.Research the adsorption performance of these three adsorbents of Cd(?).The results showed that:(1)The adsorption capacities of Cd(?)by C-ATP,C-ATP-SO₄^2-and C-ATP-Cl^-increased with the increasing of solution p H,initial Cd(?)concentration and reaction time.The adsorption capacities was C-ATP>C-ATP-Cl^->C-ATP-SO₄^2-,which all fit to the Langmuir isotherm model.The adsorption equilibrium times of the three adsorbents was 2h,and all conformed to the pseudo-second-order model.(2)The Gibbs free energy changes(?G°)of the three adsorbents were all negative,indicating that the adsorption reaction was spontaneous and belongs to the physical adsorption type.The enthalpy changes(?H°)were-38.61,35.69 and 13.82 k J·mol^-1,respectively,indicating that the adsorption process of C-ATP was exothermic,while the adsorption processes of C-ATP-SO₄^2-and C-ATP-Cl^-were endothermic.The entropy changes(?S°)were-89.94,144.36 and 66.33 J·mol^-1·K^-1,respectively,indicating that the adsorption reaction of C-ATP proceeded in an orderly direction,while C-ATP-SO₄^2-and C-ATP-Cl^-was going towards disorder.(3)The characterization methods showed that the rod crystals of the three adsorbents all have sintering and fracture phenomena,and the agglomeration of C-ATP-SO₄^2-rod crystals was more obvious.After calcination,the bond positions and groups significantly changed than before,and the aluminum hydroxide transformed into?-Al₂O₃ phase under high temperature.There was no obvious change in the crystal structure.The negative charges on the surface of the three adsorbents were reduced compared with those before the modification,however,the amount of negative charges were close to the same as the p H increased.3.Using basic aluminum acetate as aluminum source to prepare aluminum hydroxide-modified attapulgite(ATP-AC),and to research the adsorption performance of Cd(?).The results showed that:(1)The adsorption capacity of ATP-AC on Cd(?)increased with the increasing of solution p H,initial Cd(?)concentration and reaction time,which is better fit with the Freundlich isotherm model.The maximum adsorption capacities at 293 K,303 K and 313K were 5.50,10.35,12.95 mg·g^-1,respectively.The equilibrium time was 6 h,which conformed to the pseudo-second-order model.(2)The Gibbs free energy change(?G°)of ATP-AC was negative in the temperature range of 293 to 313 K,indicating that the adsorption reaction proceeded spontaneously,and the temperature rise was more conducive to spontaneous progress.The enthalpy change(?H°)was 53.22 k J·mol^-1,indicating that the adsorption reaction of the adsorbent was exothermic.The entropy changes(?S°)was 204 J·mol^-1·K^-1,indicating that the adsorption reaction of the adsorbent was proceeding in an orderly direction.(3)The characterization methods showed that the aluminum hydroxide was successfully coated onto surface of ATP-AC.The outline of the aluminum hydroxide floc was more obvious.There were certain changes in the groups and bond positions after modification,but the crystal structure did not changed significantly.The amount of negative surface charge were reduced compared with that before the modification.Overall,the aluminum hydroxide-modified attapulgite prepared from different inorganic aluminum salts affects the adsorption effect due to the different anions.The adsorption performance of Cd(?)was ATP-Cl^->ATP>ATP-SO₄^2-.After calcination of ATP,ATP-SO₄^2-and ATP-Cl^-at 600?,the adsorption performance of Cd(?)was significantly improved.And the adsorption performance was C-ATP>C-ATP-Cl^->C-ATP-SO₄^2-.The modified attapulgite(ATP-AC)prepared by organic aluminum salt had better adsorption performance than ATP,ATP-SO₄^2-and ATP-Cl^-.All these provide theoretical guidance for the application of attapulgite modified by aluminum hydroxide in water treatment.