Biofilms metals

Keywords Community ecotoxicology, Fluvial biofilm. Metal pollution. Nutrients,... 

Templeton AS, Trainor TP, Traina SJ, Spormann AM, Brown GE Jr (2001) Pb(II) distribution at biofilm-metal oxide interfaces. Proc Nat Acad Sci USA 98 11897-11902 Templeton AS, Trainor TP, Spormann AM, Brown GE Jr (2002b) Selenium speciation and partitioning within B. cepacia biofilms formed on metal oxide surfaces. Geochim Cosmochim Acta (submitted)... 

Microbes can adhere to metal surfaces forming a biofilm, consisting of a community of microorganisms, leading to corrosion [4,5], When the acidic products of bacterial action are trapped at the biofilm-metal interface, their impact on corrosion is intensihed [6],... 

Microorganisms tend to attach themselves to solid surfaces, colonize, proliferate, and form biofilms which may in turn produce an environment at the biofilm/metal interface radically different from the bulk environment in terms of pH, dissolved oxygen, organic and inorganic species. Since the biofilm tends to create nonuniform surface conditions, localized attack might start at some points on the surface leading to localized corrosion, usually in the form of pitting [22]. 

Table 13 shows some of the developmental products that have EPA appHcations pending and may be available in the near future. Sea Nine is a variation on the very successflil isothiazolone chemistry. It is claimed to be an improvement over metallic actives used for antifouling paint and wood preservation (46,47). Decylthioethylamine and its water-soluble hydrochloride are claimed to be especially effective at controlling biofilm in cooling water appHcations (48—50). The hydroxymethylpyra2ole shown is also suggested to have properties that are well suited to the protection of aqueous products or emulsions (51,52). 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

Preconditioning for Particulates Heavy particulate loading of the inlet gas with dust, grease, oils, or other aerosols can be very dam-aging to the pore structure of the filter bed, resulting in an eventual pressure-drop increase. Oils and heavy metals that are deposited on the filter bed can be poisonous to the microorganisms that live within the biofilm. Particulate APC equipment such as fabric filters and venturi scrubbers are generally adequate for this level of particulate removal. 

Biocorrosion of stainless steel is caused by exopolymer-producing bacteria. It can be shown that Fe is accumulated in the biofilm [2.62]. The effect of bacteria on the corrosion behavior of the Mo metal surface has also been investigated by XPS [2.63]. These last two investigations indicate a new field of research in which XPS can be employed successfully. XPS has also been used to study the corrosion of glasses [2.64], of polymer coatings on steel [2.65], of tooth-filling materials [2.66], and to investigate the role of surface hydroxyls of oxide films on metal [2.67] or other passive films. 

NOTE Compare this with similar problems in CW systems—those of easily and accurately (and at low-cost) determining levels of microbiological contamination. In most CW systems, apart from a general maintenance quality indicator, the levels of bulk water planktonic organisms tend to have little relevance to sessile organism-biofilm reactions occurring at the metal-water interface. 

Metal Ecotoxicology in Fluvial Biofilms Potential Influence of Water Scarcity... 

Biofilms and Metals A Community Level Ecotoxicology Approach. 44... 

Among the physical factors, current velocity has a special significance for benthic biofilms because it can modulate the diffusion of metals through the biofilm and their effects [18, 40]. pH and organic complexation are particularly significant for metal bioavailability [42]. Therefore, metal toxicity will also depend on the influence that environmental variability has on its bioavailability. 

Table 1 Summary of metal concentrations (in )ig of total metal concentration) causing toxicity on fluvial biofilms (in terms of effective concentrations EC50) after acute exposure (of few hours of exposure) and chronic exposure (of several weeks of exposure)...

Table 2 Fate and effects of metals in a stream receiving a point-source of metals (upper part of the table) or diffuse input via urban runoff (lower part of the table). Summary of the expected influence of four different hydrological situations base-flow in a rainy period a flood after a rainy period low-flow after a long period of low rainfall (water scarcity) and a flood produced after this drought. Metal concentration (M) metal retention efficiency (measured on the basis of the nutrient spiraling concept) exposure (dose and duration) bioaccumulation (in fluvial biofilms) and metal sensitivity (of biofihns)...

Highlights the worst scenario in terms of metal pollution effects on fluvial biofilms... 

Liehr SK, Chen H-J, Lin S-H (1994) Metals removal by algal biofilms. Water Sci Technol 11 59... 

SRB, a diverse group of anaerobic bacteria isolated from a variety of environments, use sulfate in the absence of oxygen as the terminal electron acceptor in respiration. During biofilm formation, if the aerobic respiration rate within a biofilm is greater than the oxygen diffusion rate, the metal/biofilm interface can become anaerobic and provide a niche for sulfide production by SRB. The critical thickness of the biofilm required to produce anaerobie conditions depends on the availability of oxygen and the rate of respiration. The corrosion rate of iron and copper alloys in the presence of hydrogen sulfide is accelerated by the formation of iron sulfide minerals that stimulate the cathodic reaction. 

A prepassivated platinum electrode and an electrode of the metal of interest have been used to follow the development of a biofilm to determine its effects on the corrosion behavior of structural materials. The time dependence of the open circuit potential of several stainless steels... 

Microsensors have been used to develop profiles in mixed species biofilms. Figure 10 shows concentration profiles of sulfide, oxygen, and pH in a biofilm accumulated on the surface of a mild steel corrosion coupon. The concentration of sulfide is highest near the metal surface, where iron sulfide forms quickly and covers the steel surface if both ferrous and sulfide ions are available. At low ferrous ion concentrations, adherent and temporarily protective films of iron sulfides are formed on the steel surface, with a consequent reduction in corrosion rate. High rates of SRB-induced corrosion of mild steel are maintained only in high concentrations of ferrous ion. 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

When biofilms are formed on metallic surfaces, they can seriously corrode performance oil production facilities, chemical processing plants, paper mills, ships, and water distribution networks. Microbiologically influenced corrosion (MIC) represents the most serious form of that degradation. 

Side stream sampling devices can be used to collect biofilm and corrosion samples. The biofilm, inorganic passive layers, and metal attacked samples can be characterized with scanning electron microscopy and energy dispersive... 

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