The Rapid And Selective Removal Mechanism Of Cr(Ⅵ) On As-designed Amine Polymer Adsorbents From Waters

Chromium and its compounds are widely used in steel,leather,electroplating,metallurgy and other industries,resulting in a large number of chromium-containing wastewater with high toxicity,strong acid and alkali and complex composition,if it were not effectively treated,it will seriously threaten the life of human beings and the ecological balance.Chromium usually exists in the form of Cr（III）and Cr（Ⅵ）in waters,and Cr（Ⅵ）is 100 times more toxic than Cr（III）,so it is urgent to develop efficient removal technology of Cr（Ⅵ）in waters.Adsorption has been considered to be the most effective method to treat chromium-containing wastewater and recover chromium resources.Being affected by complex components in actual wastewater,traditional adsorbents have exposed the following disadvantages:low utilization rate of adsorption sites,easy destruction of structure,and low selectivity for Cr（Ⅵ）.How to improve the utilization rate of specific sites of adsorbents,enhance the chemical tolerance of adsorbents and broaden the p H range of adsorbents is the key technical bottleneck of economic and efficient treatment of chromium-containing wastewater and maximize the recovery of chromium resources.To solve these problems,this paper is based on the strong interaction between amine groups and Cr（Ⅵ）:electrostatic and redox.Quaternary ammonium salt polymers with high exposure adsorption sites were designed and prepared for rapid and selective removal of Cr（Ⅵ）from water and the adsorption mechanism was explored;The proton enhancement mechanism of Cr（Ⅵ）removal driven by reduction of different types of amine polymers under strong acidic conditions was investigated;Bronster-amine polymers with proton buffering function were constructed for Cr（Ⅵ）removal at alkaline p H.Specific research contents and conclusions are as follows:（1）A series of novel quaternary ammonium salt polymers（QAPs）with high density adsorption sites were designed and prepared.QAPs have a high N exposure rate（72.6%～86.1%）.The results showed that the maximum adsorption capacity of Cr（Ⅵ）by QAPs was positively correlated with the N exposure rate（R²=0.922）.When the ratio of functional monomer to crosslinker was 5:1,the N exposure rate of QAPs（5:1（TRIM））was the highest（86.1%）.The maximum adsorption capacity of5:1（TRIM）was 211.8 mg Cr·g^-1,and the initial adsorption rate was 61.49mg·g^-1·min^-1,which are 3.4 times and 4.5 times of Cr（Ⅵ）ion imprinted polymer（Cr（Ⅵ）-IIP）,respectively.In the flow adsorption experiment,5 mg·L^-1Cr（Ⅵ）was completely removed within 5 seconds by 5:1（TRIM）.In addition,5:1（TRIM）has a significant selectivity for Cr（Ⅵ）adsorption,and 100%high purity chromium was obtained in the mixed solution where Cr（Ⅵ）coexists with high concentrations of SO₄^2-,NO₃^-and PO₄^3-.The structure and adsorption performance of 5:1（TRIM）was maintained after ten cycles of adsorption-desorption.5:1（TRIM）showed a stable recycling performance and good Cr（Ⅵ）recovery potential.Density functional theory（DFT）calculation and XPS analysis illustrated the mechanism of 5:1（TRIM）rapid selective adsorption of Cr（Ⅵ）.（2）Three kinds of polyamine adsorbents（APs）with different alkalinity and electron donating ability were prepared:benzylamine polymer（Be-AP）,N,n-dimethyl-1-（4-vinylphenyl）methylamine polymer（Me-AP）and phenylamine polymer（Ph-AP）.The adsorption and reduction process of Cr（Ⅵ）on APs at different p H values was studied.It was found that when p H>2,the alkalinity of APs determined the degree of amine protonation on its surface and dominated the removal rate of Cr（Ⅵ）.Be-AP and Me-AP with higher alkalinity showed higher Cr（Ⅵ）removal capacity.In the strong acid environment（p H≤2）,on the one hand,the high concentration of-NRH⁺on APs surface significantly promoted the electrostatic adsorption of Cr（Ⅵ）by APs,and accelerated the mass transfer between Cr（Ⅵ）and APs.On the other hand,when p H≤2,the reduction of Cr（Ⅵ）is promoted due to its high reduction potential（E≥0.437）.For example,phenylamine polymers（Ph-AP）with high reducing activity can dynamically reduce Cr（Ⅵ）as excess Cr（III）in the solid phase was released into the solution.The reduction/adsorption ratio（α）of Cr（Ⅵ）was greater than 0.70.The Cr（III）produced after reduction was further fixed by amine oxidation products,and the proportion of Cr（III）bound on Ph-AP exceeds 67.6%.Selective experiments showed that APs had good anti-anion interference ability,wherein Ph-AP can maintain a high removal rate of Cr（Ⅵ）in the background of strong acid and high concentration of SO₄^2-and NO₃^-.Finally,the mechanism of proton enhanced removal of Cr（Ⅵ）was revealed through FTIR and XPS spectral analysis and the construction of DFT model.（3）Different series of Br（?）nsted acid-amine polymers have been prepared:4-vinylbenzylamine/acrylic acid polymer（Be-Ac）,4-vinylbenzylamine/itaconic acid polymer（Be-It）,4-vinylbenzylamine/2-trifluoromethacrylic acid polymer（Be-Tf）,4-vinyl phenylamine/2-trifluoromethacrylic acid polymer（Ph-Tf）.The effects of dosage of different series of Br（?）nsted acid monomers on polymer properties were also investigated.The successful synthesis of Br（?）nsted acid-amine polymer was confirmed by FTIR and Zeta potential characterization.Series characterization results showed that the introduction of Br（?）nsted acid monomer increased the specific surface area and pore volume of the amine polymer,and decreased the isoelectric point of the amine polymer.TOC test results showed that the introduction of Br（?）nsted acid monomer increased the stability of the polymer in acid solution.The acid-base test and acid-base titration experiments of the polymer proved that the introduction of Br（?）nsted acid monomer reduced the alkalinity of the polymer and could effectively buffer the p H change caused by the amine protonation.The experimental results of adsorption performance evaluation showed that when the ratio of amine monomer to Br（?）nsted acid monomer was 4:1,each series of Br（?）nsted acid-amine polymers exhibited the best adsorption performance.The maximum adsorption capacity of Ph4Tf1 polymer for Cr（Ⅵ）reached 312.9 mg Cr·g^-1under the condition of strong acid（p H=1）.While the adsorption rate of Be4Tf1 polymer was rapid,and the initial adsorption rate（h₀）was up to 1063.61 mg·g^-1·min^-1at p H=1,and the high removal rate（76.4%～100%）of Cr（Ⅵ）at a wide range of p H（p H=1-10）,which were both due to the proton buffer function of Br（?）nsted acid site.