Study On The Performance And Mechanism Of Cobalt-doped Lanthanum Oxides For The Removal Of Arsenic From Water

Arsenic（As） contamination in the water environment has been an urgent and pending environmental pollution problem that has caused great concern worldwide.Because of its high toxicity and carcinogenicity,long-term exposure to arsenic-contaminated environments,even at low concentrations,poses a significant threat to human health.Arsenate As（Ⅴ）and arsenite As（Ⅲ）are the two most common forms of inorganic arsenic found in natural water environments.Among the many arsenic removal techniques,adsorption is considered as a promising conventional method due to its low cost,simple operation,high efficiency and eco-friendliness.Among the various nanometallic adsorbents that have been developed for the removal of arsenic from polluted water,lanthanum（La）,a rare earth metal,a relatively abundant and inexpensive element,is considered as a promising material for arsenic removal by virtue of its large number of coordination sites and high affinity for arsenic.Based on the above,the present study aims to prepare Co-doped lanthanum oxide composites and investigate its application in the removal of As（Ⅴ）and As（Ⅲ）.The adsorption performance and interaction mechanisms were studied in detail.The specific research contents and conclusions are as follows.（1）Preparation and characterization of Co-doped La（OH）₃ composites（Co-La）.The composites of Co-La have high specific surface area,micron-sized walnut-like morphology and rich maro-mesoporous structure.Such a unique structure can expose more active sites thus enhancing the adsorption performance of arsenic.（2）The performance and mechanism of Co-La for the removal of As（Ⅴ）from water were investigated.The adsorption experiments showed that Co-La exhibited excellent adsorption capacity for As（Ⅴ）with a saturation capacity at 165.29 mg/g and pseudo-first order kinetic rate constant（k₁）to 0.05501 min^-1.The adsorption of As（Ⅴ）by Co-La is a spontaneous,heat-absorbing and entropy-increasing process.Co-La was able to maintain a high adsorption capacity for As（Ⅴ）in a wide p H range（3.54-9.2）.Except for CO₃^2-and Si O₃^2-,which have a large effect on the adsorption of As（Ⅴ）,NO₃^-、Cl^-and SO₄^2-have almost no effect on the adsorption of As（Ⅴ）.Co-La has good adsorption cycle stability performance（removal rates of more than 90%in all three cycles）.XRD,FTIR and XPS analyses showed that the adsorption of Co-La on As（Ⅴ）was mainly attributed to the ligand exchange between the hydroxyl groups on the surface of Co-La and As（Ⅴ）.The results indicate that Co-La is a promising material for the removal of As（Ⅴ）from water,but has a poor adsorption capacity for As（Ⅲ）.（3）To further improve the removal performance of As（Ⅲ）,Co-La was used in conjunction with peroxymonosulfate（PMS）to construct a Co-La activated PMS system（Co-La/PMS）to improve the removal performance of As（Ⅲ）and to investigate its removal mechanism.The results showed that the Co-La/PMS system could remove 99.4%As（Ⅲ）in 10 min when the molar ratio of PMS to As（Ⅲ）was 1.2.The maximum removal of As（Ⅲ）capacity in this system reached up to243.90?mg/g and the pseudo-first order kinetic rate constant（k₁）to 1.0771 min^-1,which was 4.6times higher and 239 times faster than that of Co-La adsorption alone,respectively.The enhancement of the maximum adsorption capacity is attributed to the fact that PMS can drive La（OH）₃ of Co-La to transform into transition states La₂SO₄（OH）₄,thus generating a new exchangeable adsorption site SO₄^2-.Meanwhile,the ultra-fast removal kinetics are attributed to the fact that the Co element and oxygen-defects of Co-La can activate PMS to produce reactive oxygen species transforming As（Ⅲ）into(As（Ⅴ）.The Co-La/PMS system maintains excellent arsenic removal capability over a wide p H range（2.63-10.11）.CO₃^2-and Si O₃^2-inhibit the removal of arsenic by the Co-La/PMS system,while NO₃^-、Cl^-and SO₄^2-and organic matter have almost no significant effect on the removal of arsenic.The Co-La/PMS system was found to be a promising natural water treatment strategy.Meanwhile,the Co-La/PMS system has excellent stability and reusability.In addition,almost complete removal of As（Ⅲ）within 1 min by dynamic filtration unit（more than 97%and the effluent concentration was 6.67μg/L）,which is much lower than the WHO standard of 10μg/L of arsenic in drinking water.The excellent arsenic removal performance of the Co-La/PMS system will make it potential for application in arsenic contamination treatment applications.