Study On Degradation Of 2,4-dichlorophenol In Groundwater By Modified Fenton System Based On Calcium Peroxide

Recent years, with the fast and continuous development of economy, contamination of groundwater has been more and more serious. Remediation of groundwater contaminated by the persistent organic pollutants（POPs） represented as chlorophenols, due to their bioaccumulative, highly toxic and refractory has been a difficult and hot subject. In-situ chemical oxidation technologys, such as Fenton-like technology, potassium permanganate oxidation technology and persulfate oxidation technology are favored by scholars. Among them, Fenton- like technology which involves hydrogen peroxide（H₂O₂） and iron generated from aquifer materials is known as a cost-effective and environmentally friendly technology. However, the instability of H₂O₂ in subsurface severely affects the remediation efficiency. Large amount of injected H₂O₂ directly decompose into water and oxygen, without participating into Fenton- like reaction to generate hydroxyl radicals, resulting in lower utilization of H₂O₂. So in this study, a modified Fenton based on calcium peroxide（CaO₂） was proposed to remediate groundwater contaminated by 2, 4-dichlorophenol（2,4-DCP） on the base of properties of CaO₂ for slow release of H₂O₂.This paper firstly studied the properties of CaO₂ for slow release of H₂O₂ and O2 and the influencing factors. Secondly, the degradation performance and mechanism of 2,4-DCP by modified Fenton system based on CaO₂ was revealed. The intermediate products in the degradation were detected and the degradation pathway was proposed. Then the nano CaO₂ capped by polyethylene glycol（PEG） was composed and used for combining iron to degradate 2,4-DCP. The degradation performance and the influencing factors were studied. Finally, the migration characteristics of PEG-capped nano CaO₂ in saturated porous medium were investigated. The results were as follows:（1） CaO₂ can dissolved in H2 O slowly and release H₂O₂ and O2 at a slow rate. O2 and H₂O₂ were released directly from CaO₂ dissolution and H₂O₂ was not an essential intermediate in the conversion of CaO₂ to O2. The two release paths formed a parallel reaction system, with a competitive relationship between the two paths.（2） The release of H₂O₂ and O2 followed a pseudo-zero-order kinetics pattern and a pseudo- first-order kinetics pattern, respectively. Increasing temperature and decreasing p H could accelerate both O2 and H₂O₂ release rate. However, increasing temperature could decrease the H₂O₂ yield and increase the O2 yield, which were similar to the impact of increasing p H.（3） H₂O₂ generated from CaO₂ can combine iron to form Fenton-like system and can remove 2,4-DCP with a high removal efficiency. The removal efficiency with 340 reaction time reached 95%. In this system, disproportionation of H₂O₂ was reduced since not all the H₂O₂ is available at once as with liquid H₂O₂.（4） According to our study, both HO? and O2-? presented in modified fenton system based on CaO₂ and ?OH is the dominant reactive oxygen specie responsible for 2,4-DCP degradation in the system. The main degradation intermidiates were 2-chlorohydroquinone and 6-dichlororesorcinol. The TOC removal efficiency can reach 12.6% and the organic matters left in the solution were low- molecular-weight organic acids.（5） The PEG-capped nano CaO₂ was prepared from Ca（OH）2 and H₂O₂. The mean diameter nano CaO₂ was test as 10²0nm. Covering PEG made nano CaO₂ separated from outside and can be long preserved. When nano CaO₂ combined iron, modified Fenton system was formed and showed a strong oxidation potential.（6） The PEG-capped nano CaO₂ particle shows a very excellent migration property in the saturated porous media because of the small particle size, small molecular weight and the hydrophilicity of cladding material.