Mechanism Of Eco-friendly And Biomimetic Biocides Against Microbiologically Influenced Corrosion Under Biofilms

Microbiologically influenced corrosion(MIC)is a common phenomenon in industries,in which sulfate reducing bacteria(SRB)induced corrosion is one of the most serious and widely distributed.SRB threaten the security of metal substrates through their metabolic activities and they are difficult to treat using physical and chemical means.In industries,various biocides are employed to mitigate SRB-MIC,but biofilms can protect the embedded sessile cells,which hinders a biocide's penetration and leads to bacterial resistance.Large biocide concentrations used in biofilm treatment leads to environmental problems and increased costs.Hence,new environment–friendly,highly–effective biocides are desired to control MIC under biofilms.This topic is meaningful in theory and engineering practice.This work covered several case studies.1.To minimize the number and dosage of chemicals in corrosion mitigation,it is desirable to develop a bifunctional chemical possessing excellent anti–bacterial as well as superior corrosion inhibition properties.A small-molecule inhibitor was used to kill SRB cells by penetrating into biofilms.The inactivated biofilms can hinder further corrosion and thus reduce the need for biocides.Anti–bacterial and anti–corrosion properties of sodium pyrithione(SPT)were studied in simulated oilfield produced water.In the presence of 30 mg/L SPT,a 4–log reduction and a 5–log reduction were observed for planktonic and sessile SRB cell counts,compared to those without SPT.At a high concentration of 80 mg/L SPT,both planktonic and sessile cells became undetectable.SRB sessile cells,biofilms and corrosion products on carbon steel decreased with higher SPT concentrations with corrosion inhibition efficiencies reaching up to 82 % with 80 mg/L SPT based on weight loss data.Due to the formation of an ion gradient in cells,transmembrane driving force caused cells to collapse,leading to cell death.Moreover,SPT molecules could form a layer to replace water molecules on the surface of carbon steel,and offer electrons to the unoccupied d–orbitals of iron to form a coordinate bond.SPT molecules can inhibit corrosion by both physisorption and chemisorption on carbon steel surface.2.Inspired by nature,cationic antimicrobial peptides,which are potent against a broad spectrum of microorganisms were synthesized and applied in SRB corrosion mitigation.This method avoids discharge of toxic and microbial resistance inducing chemical biocides,which provides a new environment–friendly method in SRB corrosion mitigation.Cationic antimicrobial peptides were synthesized from a combinatorial library of antimicrobial peptides in this work.A cationic antimicrobial peptide(KWRKWWKRFK)was chosen and systematically assessed for its bactericidal properties against Desulfovibrio vulgaris(a common SRB species).The cell counts decreased by 2–log in planktonic and 1–log in sessile cell counts in the presence of 80 mg/L peptide,and became undetectable at 160mg/L of the peptide.Confocal laser scanning microscope(CLSM)imaging data were in agreement with cell count data trends,showing a good inhibition effect by the peptide.Morphological images showed a reduction of the amounts of corrosion products and biofilm biomass with the increase of the peptide concentration.With 160 mg/L,corrosion rate dropped to 0.056 mm/a,corresponding to a corrosion efficiency of 88 %.Weight loss and pitting data indicated that it can mitigate both uniform and localized corrosion.The good bactericidal efficacy of the cationic antimicrobial peptide is attributed to the distribution of a special charge and its amino acid sequence.The morphology of SRB cells suggested that the bactericidal action of the peptide damaged outer cell membranes that led to cytoplasmic leaks,thus it mitigated the SRB biofilm and its corrosion on X80 carbon steel.3.To decrease the cost and increase the efficiency of peptides,loop-like structure peptide sequence was constructed instead of linear antimicrobial peptides.A patented biomimetic cyclic peptide codenamed “Peptide A”(CSVPYDYNWYSNWC)was chosed as an enhancer of THPS(tetrakis hydroxymethyl phosphonium sulfate)biocide.It was observed that anti-bacterial and anti-corroison effects of 20 mg/L(w/w)THPS alone were insufficient.However,the SRB sessile cell count,but not planktonic cell count,decreased considerably with the addition of 10 ?g/L and 100 ?g/L Peptide A to THPS,which indicated that it enhanced biofilm removal by dispersing sessile cells,but not by killing cells.Enhancement efficiencies(further reductions in corrosion rate)reached to 83 % for 100 ?g/L Peptide A,indicating that Peptide A was a good biocide enhancer for THPS.The anticorrosion outcome of 20 mg/L THPS + 10 ?g/L Peptide A combination was close to that for50 mg/L THPS.Thus,the addition of Peptide A(?g/L level)can be an effective way to reduce THPS dosage.4.As an alternate approach based on the peptides above,Bacilus subtilis which could produce antibiotics,was chosen to incubate with D.vulgaris.B.subtilis was chosen to incubate with SRB because the similar growth conditions like temperature,p H and carbon source.The SRB cells decreased by 4–log in planktonic and 5–log in sessile cell counts in the presence of B.subtilis,which were mainly attributed to its metabolic products including antibiotics.Those antibiotics inhibited the growth of the SRB effectively.Corrosion inhibition efficiency in the presence of mixed culture bacteria reached up to 94 % based on weight loss.Electrochemical results showed that the corrosion inhibition outcome and the electrochemical behavior were closed to those with B.subtilis alone,further proving that B.subtilis decreased X80 carbon steel corrosion by competitive growth and metabolic antibiotics.