Phosphorus-Iron-Sulfur Interactions

FIGURE 9.57 Interactions of sulfate on the reduction of Fe(III) and release of Fe(II) and phosphate. SRB, sulfate-reducing bacteria FeRB, iron-reducing bacteria. (Modified from Roden and Edmonds, 1997.) 

In sulfate-dominated wetlands, production of sulfide (through biological reduction of sulfate) and formation of ferrous sulfides may preclude phosphorus retention by ferrous iron in regulating phosphorus bioavailability (Caraco et al., 1991). In iron- and calcium-dominated systems, Moore and Reddy (1994) observed that iron oxides likely control the behavior of inorganic phosphorus under aerobic conditions, whereas calcium phosphate mineral precipitation governs the solubility under anaerobic conditions. This difference is in part due to a decrease in pH under aerobic conditions as a result of oxidation of ferrous iron compounds, whereas an increase in pH occurs under anaerobic conditions as a result of reduction of ferric iron compounds. The juxtaposition of aerobic and anaerobic interfaces promotes oxidation-reduction of iron and its regulation of phosphorus solubility. 

FIGURE 9.58 Relationship between sequential reduction select electron acceptors and soluble phosphorus release in a wetland soil. (Modified from D Angelo and Reddy, 1994.) 

FIGURE 9.59 Maximum amount of dissolved phosphate released per unit of carbon mineralized during anaerobic decomposition. (Adapted from Roden and Edmonds, 1997.) 

FIGURE 9.60 A schematic showing phosphorus cycling in periphyton mats of a wetland. 

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X-ray Photoelectron Spectroscopy (XPS) tests were conducted on surfaces lubricated with a sulfur-containing extreme pressure additive, dibenzyl sulfide (Baldwin, 1976 Bird and Galvin, 1976). The films can arise from the use of additives that contain sulfur, phosphorus, chlorine, bromine, or boron and the differences in reactivity are affected by the formation of protective layers. Triboinduced electrons are said to activate the formation of iron halides, iron phosphates and iron sulfides (Dorison and Ludema, 1985 Grunberg, 1966 Kajdas, 2001 McFadden et al., 1998 ). When a chemical reaction takes place, e.g., oxygen interacts with aluminum to form aluminum oxide, a large oxygen peak is seen at approximately 500 eV in the Auger electron spectra (Benndorf et al., 1977 Nakayama et al., 1995). 

The concept of the interaction film in lubricant additive action is based on the postulate that material in the rubbing surface and the additive both contribute to the formation of the film by extensive chemical reaction with each other. The additives contain characteristic elements l. g. sulfur, chlorine, phosphorus) in characteristic chemical structures. The material in the rubbing surface is usually metallic iron is the metal most frequently encountered in technological practice. 

Figure 10-20. Phosphorus (I) and sulfur (II) in an interaction film on hardened cast iron under dynamic conditions. (a) Effect of load and running time 1% zinc dialkyldithiophosphate in SAE 10 base oil. (b) Effect of change from compounded oil to base oil. Treated parts furnished with surface coating of manganese phosphate. Data by Loesser, Wiquist and Twiss [70].

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