Sulfate reducers

Arsenic is another element with different bioavailabiUty in its different redox states. Arsenic is not known to be an essential nutrient for eukaryotes, but arsenate (As(V)) and arsenite (As(III)) are toxic, with the latter being rather more so, at least to mammals. Nevertheless, some microorganisms grow at the expense of reducing arsenate to arsenite (81), while others are able to reduce these species to more reduced forms. In this case it is known that the element can be immobilized as an insoluble polymetallic sulfide by sulfate reducing bacteria, presumably adventitiously due to the production of hydrogen sulfide (82). Indeed many contaminant metal and metalloid ions can be immobilized as metal sulfides by sulfate reducing bacteria. 

Although the process requires the addition of a phosphate donor, such as glycerol-2-phosphate, it may be a valuable tool for cleaning water contaminated with radionuchdes. An alternative mode of uranium precipitation is driven by sulfate-reducing bacteria such as Desulfovibrio desulfuricans which reduce U(VI) to insoluble U(IV). When combined with bicarbonate extraction of contaminated soil, this may provide an effective treatment for removing uranium from contaminated soil (85). 

Water Groundwater can be treated in anaerobic bioreactors that encourage the growth of sulfate reducing bacteria, where the metals are reduced to insoluble sulfides, and concentrated in the sludge. For example, such a system is in use to decontaminate a zinc smelter site in the Netherlands (95). 

C. Further warming to 65°C forms white iron sulfate monohydrate [17375-41 -6], FeSO H2O, which is stable to 300°C. Strong beating results in decomposition with loss of sulfur dioxide. Solutions of iron(II) sulfate reduce nitrate and nitrite to nitric oxide, whereupon the highly colored [Fe(H20) (N0)] ion is formed. This reaction is the basis of the brown ring text for the quaUtative deterrnination of nitrate or nitrite. 

Various patents (22—24) have been issued claiming the use of tetrakis(hydroxymethyl)phosphonium sulfate in, for example, water treating, pharmaceuticals (qv), and in the oil industry where this compound shows exceptional activity toward the sulfate-reducing bacteria that are a primary cause of hydrogen sulfide formation and consequent problems associated with souring and corrosion (25). 

Lithium Hypochlorite. High purity, anhydrous lithium hypochlorite [13840-33-0] LiOCl, is a white, lightweight, dusty, hygroscopic, and corrosive powder. The monohydrate is free-flowing, nondusty, and of reasonable density. The presence of diluents such as salt, sodium, and potassium sulfates reduces dustiness, increases bulk density, reduces reactivity, and improves storage stabiUty. The commercial product is marketed in this form. 

In the first case (22), almost stoichiometric amounts of sulfuric acid or chlorosulfonic acid are used. The amine sulfate or the amine chlorosulfate is, first, formed and heated to about 180 or 130°C, respectively, to rearrange the salt. The introduction of the sulfonic acid group occurs only in the ortho position, and an almost quantitative amount of l-aminoanthraquinone-2-sulfonic acid is obtained. On the other hand, the use of oleum (23) requires a large excess of SO to complete the reaction, and inevitably produces over-sulfonated compound such as l-amino-anthraquinone-2,4-disulfonic acid. Addition of sodium sulfate reduces the byproduct to a certain extent. Improved processes have been proposed to make the isolation of the intermediate (19) uimecessary (24,25). 

The manner in which many of these bacteria cany on their chemical processes is qmte comphcated and in some cases not fuUy understood. The role of sulfate-reducing bacteria (anaerobic) in promoting corrosion has been extensively investigated. The sulfates in shghtly acid to alkaline (pH 6 to 9) soils are reduced by these bacteria to form calcium sulfide and hydrogen sulfide. When these compounds come in contact with underground iron pipes, conversion of the iron to iron sulfide occurs. As these bacieria thrive under these conditions, they will continue to promote this reaction until failure of the pipe occurs. 

Sulfate reducers. The best-known form of microbiologically influenced corrosion involves sulfate-reducing bacteria.- Without question, sulfate reducers cause most localized industrial cooling water corrosion associated with bacteria. Desulfovibrio, Desulfomonas, and Desulfotomacu-lum are three genera of sulfate-reducing bacteria. 

These bacteria are anaerobic. They may survive but not actively grow when exposed to aerobic conditions. They occur in most natural waters including fresh, brackish, and sea water. Most soils and sediments contain sulfate reducers. Sulfate or sulfite must be present for active growth. The bacteria may tolerate temperatures as high as about 176°F (80°C) and a pH from about 5 to 9. 

Thiobacillus thiooxidans is an aerobic organism that oxidizes various sulfur-containing compounds to form sulfuric acid. These bacteria are sometimes found near the tops of tubercles (see Chap. 3, Tubercu-lation ). There is a symbiotic relationship between Thiobacillus and sulfate reducers Thiobacillus oxidizes sulfide to sulfate, whereas the sulfate reducers convert sulfide to sulfate. It is unclear to what extent Thiobacillus directly influences corrosion processes inside tubercles. It is more likely that they indirectly increase corrosion by accelerating sulfate-reducer activity deep in the tubercles. 

Virtually all metallurgies can be attacked by corrosive bacteria. Cases of titanium corrosion are, however, rare. Copper alloys are not immune to bacterial attack however, corrosion morphologies on copper alloys are not well defined. Tubercles on carbon steel and common cast irons sometimes contain sulfate-reducing and acid-producing bacteria. Potentially corrosive anaerobic bacteria are often present beneath... 

Sulfate reducers. Active sulfate reducers are found in anaerobic environments. These environments may be highly localized, such as inside a tubercle or beneath a spotty deposit. A thin, fairly regular biofilm is difficult to perceive in such microenvironments. 

Recently, tests have been developed that do not require culturing of sulfate reducers. These tests are based on detecting certain compounds produced by the sulfate reducers and have applicability (in some cases) even if the producing organisms have recently died. Laboratory studies have shown adequate agreement between such tests and live culture analyses when viable organisms are present. 

A typical microbiological analysis in a troubled carbon-steel service water system is given in Table 6.2. Table 6.3 shows a similar analysis for a cupronickel utility main condenser that showed no significant corrosion associated with sulfate reducers. When biological counts of sulfate reducers in solid materials scraped from corroded surfaces are more than about 10, significant attack is possible. Counts above 10 are common only in severely attacked systems. 

Planktonic counts (in water samples) are usually unreliable as an indicator of active corrosion. The presence of any sulfate reducers in the water, however, indicates much higher concentrations of these organisms on surfaces somewhere in the system. 

Corrosion morphologies. Sulfate-reducing bacteria frequently cause intense localized attack (Figs. 6.2 through 6.7). Discrete hemispherical depressions form on most alloys, including stainless steels, aluminum. Carpenter 20, and carbon steels. Few cases occur on titanium. Copper alloy attack is not well defined. 

TABLE 6.3 Typical Microbiological Analysis at Outlet A Main Condenser Suffering No Significant Corrosion by Sulfate-Reducing Bacteria ... 

Figure 6.2 Severely pitted aluminum heat exchanger tube. Pits were caused hy sulfate-reducing bacteria beneath a slime layer. The edge of the slime layer is just visible as a ragged border between the light-colored aluminum and the darker, uncoated metal below.

Pit interiors are characteristically smooth and distinctly hemispherical, but become rougher on less-noble alloys. Pits tend to cluster together, overlapping to form irregularly dimpled surfaces. Frequently, a lightly etched aureole surrounds the pit clusters. These etched areas are often produced by shallow corrosion beneath deposit and slime masses that covered the sulfate reducers in service (Figs. 6.3 and 6.4A and B). 

Figure 6.3 Pitting on the waterside surface of a Carpenter 20 heat exchanger tube caused by sulfate reducers.

Figure 6.5 Many small hemispherical pits on a 304 stainless steel heat exchanger tube end. The heat exchanger was removed from service and stored vertically for an extended period. Deposit accumulated at the lower tube ends where sulfate reducers flourished.

Figure 6.6 Clustered sulfate-reducer pits on a carbon steel tank bottom.

Figure 6.7 A small-diameter carbon steel service water pipe perforated by sulfate-reducer corrosion.

Corrosion products and deposits. All sulfate reducers produce metal sulfides as corrosion products. Sulfide usually lines pits or is entrapped in material just above the pit surface. When freshly corroded surfaces are exposed to hydrochloric acid, the rotten-egg odor of hydrogen sulfide is easily detected. Rapid, spontaneous decomposition of metal sulfides occurs after sample removal, as water vapor in the air adsorbs onto metal surfaces and reacts with the metal sulfide. The metal sulfides are slowly converted to hydrogen sulfide gas, eventually removing all traces of sulfide (Fig. 6.11). Therefore, only freshly corroded surfaces contain appreciable sulfide. More sensitive spot tests using sodium azide are often successful at detecting metal sulfides at very low concentrations on surfaces. 

Figure 6.9 Irregular deposit and corrosion-product mounds containing concentrations of sulfate-reducing bacteria on the internal surface of a 316 stainless steel transfer line carrying a starch-clay mixture used to coat paper material. Attack only occurred along incompletely closed weld seams, with many perforations. Note the heat tint, partially obscured by the deposit mounds, along the circumferential weld.

Figure 6.10 A perforated carbon steel pipe at a weld-backing ring. The gaping pit was caused by sulfate-reducing bacteria (see Case History 6.1).

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

Clostridia frequently are found where sulfate-reducing bacteria are present, often in high numbers inside tubercles. A typical microbiological analysis of tubercular material removed from a troubled service water system main is given in Table 6.4. Clostridia counts above 10 /g of material are high enough to cause concern. When acid producers... 

Sliming is often more severe on inlet tube sheets than on outlets. Bacterial counts are usually quite high, exceeding tens of millions in most slime layers (Table 6.5). Sulfate-reducer counts are also usually high. In waters taken from such systems, bacteria counts are often several orders of magnitude lower. 

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