Effect Of Humic Acid On Corrosion Characteristics Of Microbacterium＿oxydans On Cast Iron Pipes Of Water Supply

The drinking water can reach the hygienic standard of drinking water after treatement.However,the secondary pollution is easy to occur in the process of water distribution through water supply network system.Metal pipes are most widely used in water supply network.The pipes can react with all kinds of chemical substances carried in drinking water and cause corrosion of the inner wall of pipes.The harmful effects of corrosion include the breakage of metal pipeline,perforation and pipe bursts.Affecting water distribution capacity and increasing the energy consumption.Causing the growth and reproduction of microorganisms on the inner wall of the pipeline.Corrosion will also cause secondary pollution of drinking water,leading to the increase of turbidity and the phenomenon of"red water".Iron-oxidizing bacteria are the main reason of microbiologically influenced corrosion of iron pipes.They can form biofilms on the inner wall of metal pipes and interact with metal to promote corrosion.Humic acid,as a natural organic substance in water,can not only provide nutrients for bacterial growth,but also react with metals to promote the corrosion of metal pipes.This study took microbiological influenced corrosion as the breakthrough point,combined with the natural organic matter existing in water,and studied the growth characteristics of an iron-oxidizing bacteria selected from the water supply pipeline under different conditions,such as p H,temperature,dissolved oxygen,carbon source,nitrogen source and different molecular weight humic acids.Moreover,the effects of humic acids with different molecular weight on the corrosion characteristics of iron-oxidizing bacteria were also investigated.The main conclusions are as follows:（1）After separation and purification,a pure strain of iron-oxidizing bacteria was successfully screened from the scale of cast iron pipes,and the strain was named ZT-1.The strain was round,smooth and short rod-shaped.It was identified as Microbacterium＿Oxydans.（2）The strain can be applied to a wide range of p H,and can grow well in the p H range of 5～9.In the temperature range of 20～40℃,the strain grew well,and the growth rate of the strain was accelerated with the increase of temperature.When the rotation speed was 200 r/min,the growth of the strain was the best.The growth under different nitrogen sources is NO₃^--N>NH₄⁺-N>NO₂^--N,NO₃^--N and NH₄⁺-N could better promote the growth of the strain.The growth of the stain under different carbon source conditions showed that sodium succinate>ammonium ferric citrate>fulvic acid>humic acid,and macromolecular organic carbon was more difficult to be absorbed and utilized by bacteria.After grading the humic acid,the strain had the best growth condition under the condition of<1k Da HA,and the strain had the worst growth condition under the condition of>100k Da HA.（3）In the corrosion experiment,the dissolved oxygen of tap water+ZT-1,<1 k Da HA+ZT-1,10-30 k Da HA+ZT-1 and>100 k Da HA+ZT-1 decreased by 3.23 mg/L,3.53mg/L,3.7 mg/L and 3.63 mg/L,respectively,which indicated that dissolved oxygen was consumed in the corrosion process.The p H value increased by 0.11,0.13,0.16 and 0.14,respectively,indicating that OH^-was formed during the reaction.The average turbidity of the effluent were 26.93NTU,28.57NTU,32.09NTU and 43.66NTU in 0～30 days,and 17NTU,14.69NTU,10.19NTU and 24.05NTU in 30～60 days,respectively.The turbidity gradually decreased as the reaction progressed.（4）Under the four reaction conditions of tap water+ZT-1,<1 k Da Ha+ZT-1,10-30 k Da Ha+ZT-1 and>100 k Da Ha+ZT-1,the corrosion rates of cast iron coupons after 20 days were 0.166 mm/a,0.183 mm/a,0.207 mm/a and 0.23 mm/a,respectively.During the period of 0～30 days,the average total iron concentration of effluent under the four groups were 2.62 mg/L,3.17 mg/L,3.77 mg/L and 7.21 mg/L,respectively.The average Fe²⁺concentration were 0.141 mg/L,0.184 mg/L,0.184 mg/L and 0.394 mg/L,respectively,which indicated that the addition of humic acid promoted the corrosion,and with the increase of the molecular weight of the addition of humic acid,the corrosion rate of cast iron surface accelerated.（5）After 60 days,the corrosion rates of cast iron coupons under the four conditions were 0.127 mm/a,0.115 mm/a,0.117 mm/a and 0.109 mm/a,respectively.The concentrations of total iron in the effluent were basically maintained at about 1.33 mg/L,1.18 mg/L,1.05 mg/L and 1.63 mg/L.（6）After the reaction,a protective film was formed on the surface of the cast iron,and with the increase of the molecular weight of the added humic acid,the adhesion effect on the surface was more obvious.The corrosion products produced were mainly iron oxides,such as?-Fe OOH（goethite）,β-Fe OOH（akaganeite）,γ-Fe OOH（lepidocrocite）,Fe₂O₃（hematite）,Fe₃O₄（magnetite）and so on,finally mainly with more stable?-Fe OOH,Fe₂O₃,Fe₃O₄and other relatively stable iron oxides.