The Mechanism And Application Of Microbial Extracellular Polymeric Substances On The Corrosion And Scale Inhibition Effect

With the development of industry,the global problem of water pollution and the shortage of water resources are becoming more and more serious.Industrial water accounts for more than 80%of urban water use,which circulating cooling water is the largest part of industrial water.Circulating cooling water system faces some problems such as corrosion,scale and microbial growth during operation,which could cause equipment damage,safety accidents and water pollution.Therefore,the problems faced by circulating cooling water systems are solved,which could go a long way to alleviate the current water crisis.The addition of inhibitors with corrosion inhibition,scale inhibition and sterilization are the best way for circulating cooling water systems.The commonly traditional inhibitors,including phosphorus,inorganic slats and organic substances,could cause secondary pollution and are toxic and costly.Therefore,the commonly traditional inhibitors are gradually abandoned and the "green" inhibitors with the advantages of non-toxic and biodegradable begin to receive widespread attention,for example,polyaspartic acid and polyepoxysuccinic acid.However,the chemosynthetic process of "green"inhibitors is not "green",which requires a lot of energy and cause pollution.Therefore,the"green" synthesis of inhibitors is an urgent issue to be solved.Extracellular polymeric substances(EPS)are a mixture of polymers secreted by microorganisms,which are considered as a novel,efficient and "green"inhibitors in circulating cooling water systems due to the advantages of non-toxicity,no secondary pollution,biosorption and biodegradability.Due to the various compositions of EPS of different microorganisms,the mechanisms need to be further investigated.The main conclusions are as follows:(1)Poly glutamic acid(y-PGA),which is kind of EPS secreted by part of Bacillus spp.,is an analogue of polyaspartic acid,and the monomers of which contains one more CH2.γ-PGA could be directly produced by Bacillus spp.solving the problem of the synthesis process of polyaspartic acid not "green".Therefore,in this work,we explored the y-PGA of production and identification of Bacillus spp.,molecular weight determination,and the corrosion and scale inhibition performance and mechanisms.The results showed that,Bacillus megaterium OP-1 and B.licheniformis ATC 11946 were the glutamate-independent strains.The molecular weight of y-PGA produced by B.megaterium OP-1 and B.licheniformis ATC 11946 were 15 and 70 kDa,respectively.In addition,the products of four Bacillus spp.had the scale inhibition ability.The molecular weight was the key factor that affected the scale inhibition performance,which the lower the molecular weight,the more effective the scale inhibition.The mechanisms of yPGA were due to its negatively charged functional groups including-OH,-COOH,and-NH3,which combined with Ca2+ to prevent the formation and growth of CaCO3.(2)Low molecular weight y-PGA’ was the excellent scale inhibitors,however,the performance of corrosion inhibition needed to be improved.In circulating cooling water systems,due to single-component inhibitors had some problems such as low efficiency,high dosage and costly,the combination of different functional inhibitors to enhance performances,reduce costs and dosage was widely used.Therefore,in this work,y-PGA,imidazoline and BDTAC were compounded according to the orthogonal experimental design to test the performance of corrosion and scale inhibition.Then,the zinc salts were added to enhance the performance of corrosion inhibition.The results showed that the optimum concentrations of γPGA,imidazoline,BDTAC and zinc salts were 30 mg/L,40 mg/L,100 mg/L,4 mg/L respectively.Also,the performance of scale inhibition of y-PGA had no changes after the addition of imidazoline,BDTAC and zinc salts and BDTAC had the ability of corrosion inhibition.The zinc salts had the synergies with y-PGA and imidazoline to improve the performances of compound inhibitors.(3)In this work,Gram-negative(Pseudomonas putida)and Gram-positive(B.megaterium)bacteria were selected to explore the corrosion and scale performance and mechanisms of soluble EPS(s-EPS).Then,the corrosion and scale inhibition mechanisms of two kind of sEPS were compared to demonstrate the differences between them.The results showed that,firstly,the corrosion and scale inhibition performances of s-EPS of P.putida(s-EPS-P.putida)were obviously superior to those of s-EPS of B.megaterium(s-EPS-B.megaterium).Secondly,humic substances and carbohydrates of s-EPS-P.putida played the key roles in the scale and corrosion inhibition,respectively.The protein-like substances played the major roles for s-EPSB.megaterium.Therefore,the bonding ability of humic substances was better than that of protein-like substances making s-EPS-P.putida superior in scale inhibition.Carbohydrates chelation with Fe3+ could exhibit the highest stability constants,then,forming a dense protective film to protect A3 carbon steel from corrosive ions.Finally,the scale inhibition mechanisms of s-EPS were mainly that the fun ctional groups chelated to the active sites to prevent the formation and growth of CaCO3 crystals.The corrosion inhibition mechanisms were that s-EPS formed a dense protective film by integrating with metal ions on a carbon steel surface to prevent the corrosion of A3 carbon steel.