Adsorption Of Cr(Ⅵ) By Schwertmannite Synthesized At Constant P H And Its Phase Stability

Cr（VI）is a kind of heavy metal element with strong migration,high toxicity and great harm to the ecological environment and human health,which has attracted wide attention.Adsorption is a simple,effective and low-cost method to treat Cr（VI）polluted environment,and the selection of appropriate adsorbent is the key to efficient pollution control.Schwertamnnite（Sch）is a kind of iron hydroxysulfate mineral that exists naturally in Fe and SO₄^2-rich environment.Its unique pore structure and abundant surface active groups(such as-OH and SO₄^2-)make it have strong affinity for oxyanions such as Cr（VI）and can fix them firmly in structure through ion exchange and coordination-complex reactions.Therefore,as a kind of green adsorption material with good application prospects,schwertmannite has been widely concerned.However,schwertamnnite also has the following disadvantages:（1）In the synthesis system of schwertmannite,the production and utilization rate of raw materials are low in one-time synthesis,which often require multiple cycle synthesis and takes a long time,resulting in high cost;（2）Acid pickling is often used to remove rust on the plating substrate in the production process of factories,therefore most of the Cr（VI）containing wastewater is acidic.However,the adsorption capacity of schwertmannite to Cr（VI）is low in acid environment,so its adsorption performance needs to be further optimized.In addition,the metastable structure of schwertmannite makes it easy to transform to other mineral phase with better crystallinity.Although it has the prospect for Cr（VI）soil amendment,the complex water environment may catalyze the phase transformation in practical application.It has been reported that OH^-can accelerate the crystallization process of iron-based minerals,products with optimized structure and better adsorption performance can be obtained.Therefore,in this paper,we synthesized schwertmannites by chemical method.In this process,OH^-was added to maintain the constant pH of the systems in order to accelerate the mineralization processes,schwertamnnites with different pH control conditions were synthesized.A series of means were used to characterize and analyze the structure of these schwertmannites,and their adsorption properties for Cr（VI）were also compared.Furthermore,the mechanism of the improvement of Cr（VI）adsorption performance was analyzed.Finally,the phase transformation behavior and mechanism of these schwertmannites under different water conditions（simulated soil environment）were investigated.This study provides a theoretical basis for remediation technologies for Cr（VI）contaminated environment.The main results are as follows:（1）The characteristics of schwertmannites（morphology,crystallinity and surface properties）prepared under different pH control conditions were analyzed.The results showed that the yield of the systems with constant pH were more than 3 times higher than that without pH control,in which the precipitation rate of Fe³⁺and SO₄^2-could reach 100%and 21%,respectively.The crystallinity of schwertmannites formed from constant pH control systems decreased with the increasing pH.The morphology of schwertmannite（Sch）obtained from system without constant pH control was sphere with burr structure and particle size of 700m,and its specific surface area was 18.9m²/g;When the constant pH of the system was at 2.5and 2.7,the corresponding schwertamnnites（Sch-2.5 and Sch-2.7）were nanospheres with particle size of 150 nm,and the specific surface areas were 63.3 and 65.1 m²/g,respectively;When the pH of the system increased to 2.9,the morphology of obtained schwertmannite（Sch-2.9）was flaky block,and its specific surface area was only 1.6 m²/g.This was because the precursor size of schwertmannits were controlled by the concentration of OH^-and SO₄^2-,resulting in significant differences in morphology and specific surface area.In these schwertmannites,the surface-OH group abundance of Sch-2.7 is the highest.（2）The adsorption behavior of Cr（VI）by Sch-2.7 and Sch under acid conditions,the factors affecting the Cr（VI）adsorption process（pH,temperature and coexisting ions）and the mechanism of the improvement adsorption performance of Sch-2.7 were studied.The model fitting showed that the adsorption for Cr（VI）by Sch-2.7 and Sch were monolayer chemisorption.The maximum adsorption capacity of Cr（VI）was 57.14（pH=3）,61.67（pH=4）and 66.65 mg/g（pH=5）under the acid condition of Sch-2.7,while the maximum adsorption capacity of Cr（VI）corresponding to Sch was only 20.35,24.77 and 27.17 mg/g.With the increasement of temperature,the maximum Cr（VI）adsorption capacity of Sch-2.7and Sch increased,and the optimal Cr（VI）adsorption pH of them was 6.The Cr（VI）adsorption of Sch-2.7 was not affected by coexisting cations such as K⁺,Cd²⁺and Cu²⁺,but the adsorption process was significantly inhibited by coexisting anions such as PO₄^3-.The main reasons for the improvement of adsorption performance of Sch-2.7 were as follow:（1）More Fe-OH active sites were exposed at high specific surface area;（2）Compared with Sch,the structural SO₄^2-of Sch-2.7 was easier to exchange with Cr（VI）.（3）The effect of water content on the phase transition behavior of Sch-2.7 and Sch were investigated.The mineral phases of them were stable under dry treatment.Under wet and flooded conditions,Sch-2.7 changed obviously after 30h and 96h,while Sch remained stable without obvious change.Under moist treatment,the surface phase of Sch-2.7 transformed into a hydrophobic mixture of ferricopiapite and Fe（OH）₃.The co-catalysis of O₂ and water were the main reason for the phase transition.Therefore,the formation of hydrophobic products gradually prevented water from entering the interior of Sch-2.7 and inhibiting further transition process.Under flooding treatment,Sch-2.7 was transformed to goethite,and the exchange of OH^-and SO₄^2-was the main reason for the formation of goethite.After adsorption of Cr（VI）of Sch-2.7,the above phase transformation behavior did not occur,indicating that Cr（VI）can effectively stabilize the mineral structure.Therefore,compared with Sch prepared by common chemical method,the Cr（VI）adsorption performance of Sch-2.7 prepared by pH control was significantly improved and with strong anti-interference ability.Although the stability of Sch-2.7 was significantly affected by water content,the adsorption of Cr（VI）further inhibited this process.In conclusion,this study proposed a strategy to optimize the adsorption performance of schwertmannite and constructed an efficient Cr（VI）adsorbent,which provided a theoretical basis for the subsequent practical application of environmental remediation.