Study On Migration And Transformation Mechanisms Of Chromium Between Scale/Water Phase In Drinking Water Distribution System

The presence of residual disinfectant in the drinking water distribution system increases the exposure risk of Cr（Ⅵ）in the main water phase,while the reducing substances such as Fe（II）in scale phase make it possible to the detoxification and enrichment of Cr（Ⅵ）.It is of great significance for controlling Cr（Ⅵ）release in drinking water and reducing the potential health risk of human body to clarify Cr（Ⅵ）migration and REDOX behavior between scale and water in the drinking water distribution system.This study focused on the ternary system of corrosion scales,chromium and water,the exposure risk of Cr（Ⅵ）caused by oxidation of residual disinfectant in the drinking water distribution system was the key point.The migration and transformation behavior of chromium in scale/water phase was analyzed from the perspectives of the corrosion scale composition,the valence state and existing form of chromium,and the water quality conditions.The influence of water quality environment on chromium release under Na Cl O disinfection system was investigated to provide basis for Cr（Ⅵ）control in actual drinking water distribution system operation.The main research results are as follows:（1）The steel corrosion scales showed obvious layered structure.The surface layer,shell-like layer and porous core layer had different morphology,composition,crystal structure and surface characteristics.α-Fe OOH andγ-Fe OOH were the main components of the surface layer.Fe（II）was the most abundant in the shell-like layer.The presence of a large amount of Fe₃O₄ made it more stable and difficult to be destroyed.The composition of the porous core layer was more complex,and theα-Fe OOH and Fe（II）contents were between the other two layers.（2）Surface complexation and REDOX were the main mechanisms found for Cr（Ⅵ）enrichment.In the surface layer and porous core layer,Cr（Ⅵ）enrichment was mainly in the form of surface complexation,while REDOX was the main way of Cr（Ⅵ）enrichment in the shell-like layer.The enrichment of Cr（Ⅵ）in each layer of corrosion scales was the highest in the surface layer,lower in the porous core layer and the lowest in the shell-like layer.Surface precipitation was the main mechanism of Cr（Ⅲ）enrichment in corrosion scales,and there was no significant difference between different layers.（3）Under the action of disinfectant,Cr（Ⅲ）in water phase was gradually transformed into Cr（Ⅵ）and enriched in corrosion scales.Although the enrichment amount of total chromium did not change much,the proportion of Cr（Ⅵ）enrichment increased and the release risk is reduced.The results of dynamic release experiment showed that disinfectant could promote the release of total chromium and the transformation of Cr（Ⅲ）in pre-enriched Cr（Ⅲ）corrosion scales,and the promoting effect could be ranked as Na Cl O≈Cl O₂>NH₂Cl.In the actual operation of the drinking water distribution system,in order to control the release of Cr（Ⅵ）,the combined chlorine disinfectant should be given priority.When using free chlorine or chlorine dioxide for disinfection,the dosage of disinfectants should be strictly controlled to prevent the disinfectants from oxidizing non-toxic Cr（Ⅲ）released from corrosion scales into highly toxic Cr（Ⅵ）in the water phase,so as to minimize the potential health risks.（4）Due to the unstable enrichment form of Cr（Ⅲ）,the chromium release rate in the pre-enriched Cr（Ⅲ）scale was faster than that of the pre-enriched Cr（Ⅵ）group and the pre-enriched Cr（Ⅵ）+Cr（Ⅲ）group,and the release amount was the largest.Na Cl O consumption wss also the largest.The influence of solution p H on chromium release was different in non-disintection system and disinfection system.Under the disinfection system,higher p H and more HCO₃^-inhibited chromium release in pre-enriched Cr（Ⅵ）scale group,but promoted the release of chromium in pre-enriched Cr（Ⅲ）scale group.Alkaline condition promotion to the conversion of Cr（Ⅲ）to Cr（Ⅵ）,and the re-enrichment of Cr（Ⅵ）under acidic condition could explain this phenomenon.Due to electron shuttle catalysis,Br^-promoted Cr（Ⅲ）preconcentration scale significantly.Ca²⁺and Mg²⁺inhibited the release of chromium in each group.Due to the precipitability of Cr（Ⅲ）at neutral p H and the oxidation of disinfectant,the content of Cr（Ⅲ）in finished water might be low,and most of the chromium in the water phase is enriched in the form of Cr（Ⅵ）in corrosion scales.Therefore,during the operation of the actual drinking water distribution system,maintaining a moderately alkaline p H,slightly higher alkalinity,slightly higher hardness and low bromine concentration is beneficial to inhibit Cr（Ⅵ）release risk.The specific control range of the corresponding indicators should be determined according to the actual drinking water source,corrosion degree of pipe network and operation status.