Study On The Adsorption And Solidification Of Cd2+ In Brown Earth By Alternanthera Philoxeroides-bentonite Composite Adsorbent Based On Response Surface Optimization Method

At present,the water and soil environment in China is increasingly polluted by the heavy metal cadmium?Cd?,which has strong toxicity,bioaccumulation and nonbiodegradable,seriously threatening human health,yield and quality of agricultural products.Therefore,the researchers are required to continuously develop and improve methods and technologies for remediation of heavy metal pollution.There have been many studies on the adsorption/solidification of heavy metals by single adsorbents such as slaked lime,activated carbon,and biochar,etc.However,few studies on the combining two or more different adsorption materials into composite adsorption materials by physical or chemical methods.The complementary and synergistic effects on the adsorption performance of materials with different properties in its composition needs further study.In this study,the invasive weed?Alternanthera philoxeroides?was used as organic raw material to prepare biochar under the O₂-limited condition at 350??Bc?,and a new composite adsorbent?BcB?was prepared by the combination of Bc and inorganic adsorbent material bentonite?Be?by physical method.The physicochemical properties of the BcB Bc and Be were characterized by Elemental analysis,Fourier transform infrared spectroscopy,Scanning electron microscopy,and X-ray diffraction.The significant factors that affecting the adsorption effect of Cd²⁺were screened out through the Plackett-Burman experiments.After obtaining the significant factors,the single factor steepest climbing experiment was further used.Combined with response surface optimization methodology?RSM?,the second-order model of the adsorption rate of Cd²⁺was obtained,and the best adsorption parameters of Cd²⁺were determined.The effects of the dosage of BcB,the initial Cd²⁺content in the soil,and the curing time on the solidification efficiency of Cd²⁺in brown soil were investigated through single-factor experiments.Based on the results of the single-factor experiments,the RSM was used to optimize the solidification conditions of Cd²⁺.The main conclusions were as follows:?1?The three adsorbent materials were alkaline due to their pH values all greater than7.Compared with the Bc and Be,the C and H content of the BcB decreased,while the O element,the ratio of O/C and?O+N?/C increased.In addition,the vibrational absorption peak of the inorganic functional group Si-O-Si on the surface of the BcB and the appearance of Ca,Mg elements indicate that the Be might has been loaded onto the surface of Bc.?2?The results of Plackett-Burman experiment showed that the pH,the initial Cd²⁺ concentration and the adsorbent dosage were main significant factors.The results of the RSM experiment with fewer experiments and fewer cycles were close to the traditional single-factor experiment.The optimal adsorption parameters of Cd²⁺were:the pH 6.86;the initial Cd²⁺concentration 49.26 mg·L^-1;and the adsorbent dosage 0.04 g.At this time,the model adsorption rate was 96.26%,and the actual adsorption rate was 95.13%,which was close to each other.The increase of adsorption temperature was beneficial to the adsorption of Cd²⁺.At 298 K and 313 K,the adsorption behavior of Cd²⁺was more in accordance with Freundlich model.When the adsorbent dosage was less than 0.04 g,the adsorption strength of three adsorbents for Cd²⁺was:BcB>Bc>Be.?3?The solidification efficiency of Cd²⁺in brown soil was positively related to the change in the adsorbent dosage,and negatively related to the change of the initial Cd²⁺content.When the adsorbent dosage was less than 3%,the solidification strength of three adsorbents for Cd²⁺was:BcB>Bc>Be;when it was higher than 3%,the strength was:Bc>BcB>Be.The effect of solidification time on the solidification efficiency of Cd²⁺in soil was positive correlation between 8-38 days and negative correlation between 39-48days.The highest solidification efficiency of Cd²⁺was 38.16%at 38 days.The acid extractable content had the lowest value of 23.11%,and the maximum of the reducible,organic,and residue states were 17.53%,3.04%,and 56.99%,respectively.?4?The response surface optimization results of box Behnken were as follows:the linear and quadratic effects of the adsorbent dosage,the initial Cd²⁺content and the solidification time on the response value were very significant.The optimal solution was obtained through the response surface model that the initial Cd²⁺content in the brown soil was 12.88 mg·kg^-1,the compound adsorbent dosage was 5 g,and solidification time was 38days,and the Cd²⁺solidification efficiency reached the highest value of 42.75%,which was close to the actual solidification efficiency value?45.17%?measured by the verification experiment.