Organic sulfur precipitator

Figure 1. Precipitation of metals form EDTA waste with lime and organic sulfur precipitant at a temperature of 150°F (65°C) and a pH of 10.8. 

The decomposition of dithionite in aqueous solution is accelerated by thiosulfate, polysulfide, and acids. The addition of mineral acid to a dithionite solution produces first a red color which turns yellow on standing subsequentiy, sulfur precipitates and evolution of sulfur dioxide takes place (346). Sodium dithionite is stabilized by sodium polyphosphate, sodium carbonate, and sodium salts of organic acids (347). 

Some products are precipitated from the fermentation broth. The insoluble calcium salts of some organic acids precipitate and are col-lec ted, and adding sulfuric acid regenerates the acid while forming gypsum (calcium sulfate) that constitutes a disposal problem. An early process for recovering the antibiotic cycloserine added silver nitrate to the fermentation broth to precipitate an insoluble silver salt. This process was soon obsolete because of poor economics and because the silver salt, when diy, exploded easily. 

If the injected acid itself contains iron (III), a precipitation of the asphaltic products can occur when it comes in contact with certain crude oils. This leads to practically irreversible damage of the zone treated. The amount of precipitate generally increases with the strength and concentration of the acid. Certain organic sulfur compounds, such as ammonium thioglycolate, mercaptoethanol, cysteamine, thioglycerol, cysteine, and thiolactic acid [581], can reduce the iron (HI). 

Sulfur occurs mainly in proteins that typically display a C/S ratio of about 50. The processes responsible for the direct primary production of organically bound sulfur are the direct assimilation of sulfate by living plants and microbiological assimila-tory processes in which organic sulfur compounds are synthesized. Land plants use sulfate available from precipitation, marine phytoplankton use ocean water sulfate. 

In the freshwater peat swamp, bacterial reduction of organic sulfur in plant tissues may be an important process in the formation of pyrite (93). Altschuler et al. (93) proposed that in the Everglades peat, pyrite precipitates directly by the reaction of HS or organic sulfide (produced by reduction of oxysulfur compounds in dissimilatory respiration) with ferrous iron in the degrading tissues. Pyrite formation in low-sulfur coal may be accounted for by this process. 

In these clastic sediments the dominant form of sulfur is pyritic, while organic sulfur is usually present only in trace amounts. For this reason, much work on sulfur in these sediments focuses on pyrite formation and its crystallization has been studied in detail by Berner (IT), Sweeney and Kaplan (12). Rickard (13). Rickard (14) and others. Under saline and hypersaline conditions precipitation of monosulfides may be the initial step. Sulfur is then added to these precipitates, converting them to pyrite. Laboratory studies indicate that if griegite is present in the original precipitate, sulfurization may produce framboidal aggregates (12). Conversion may depend on chemical factors such as H2S concentrations (9). In contrast, in conditions that are undersaturated with respect to monosulfides, but supersaturated with respect to pyrite, pyrite may form directly and rapidly from... 

As the treatment temperature was raised, the quantity of sulfur removed by the first step increased greatly while coal recovery declined. The decline in recovery was gradual up to 250 C and then more precipitous beyond. The overall reduction in total sulfur content for both steps increased slightly and the reduction in apparent organic sulfur content somewhat more as the temperature of the first step was raised. The overall reduction in ash content for both steps also rose but then reached a plateau at 250°C. 

In the pedosphere, the main outputs of sulfur are represented by river runoff and biogenic H2S, and inputs by dead organic matter, precipitation, dry deposition and fertiliser application. Friend (1973) assumes that the pedosphere is in dynamic equilibrium and retains a constant sulfur concentration. This is partially supported by experiments cited by Eriksson (1963) where, under SO2 exposure, soils rich in sulfate evolved H2S while those poor in sulfate gained sulfate. Removal of sulfur from the pedosphere by river run-off was estimated by Friend (1973) to be 89 Tg S y" based on differences between total run-off and the volcanic and rock weathering inputs to rivers. Friend also estimated that biogenic H2S released from the pedosphere was in the order of 58 Tg S y, assuming that sulfur inputs and removals from the pedosphere are balanced. The plant-soil cycle probably represents a net transfer of atmospheric sulfur (uptake by live plants) to the pedosphere (plant decay) although some H2S is released back to the atmosphere in the latter process. 

The degree to which a pollutant is taken up, which also determines its potential toxicity, is determined by its bioavailability. Bioavailability refers to the ability of a chemical to move from the environment into a living organism. Bioavailability depends upon the ionization state and speciation of a chemical. Because certain organic compounds and clays can strongly bind various hydrocarbon chemicals and metals, the amount of organic carbon and clay in the soil, sediment, and water determines the bioavailability of these compounds. Bioavailability of metals is also dependent on the amount of sulfur precipitates of other metals in soils and sediments. 

Some simple organic species. Precipitated sulfur, as obtained during analysis when HCl is added to a polysulfide extract of metallic sulfides, is soluble in benzene or low-boiling petroleum ether. This has value when looking for traces of As or Sb sulfides. 

The highest concentration of hydrogen sulphide in biogas is noted in the early stages of waste decomposition. The decrease in the concentration of H S is most likely caused by the precipitation of the sulphides in the reaction with heavy metals (such as Cu and Fe) or their oxides, which are present in the deposited material. Sulphides as water insoluble compounds remain in the mass of waste (Parker et al. 2002). The organic sulfur compounds in the greatest concentrations in landfill gas are dimethyl sulphide (DMS), carbon disulphide, methyl mercaptan, dimethyl disulphide (DMDS) at the concentrations of 0.007-180 mg m" 0.09-61.6 mg m" 0.084-17.94 mg m 0.0124-0.942 mg m" respectively (Kim et al. 2005, Shin et al. 2002). 

Citric Acid Separation. Citric acid [77-92-9] and other organic acids can be recovered from fermentation broths usiag the UOP Sorbex technology (90—92). The conventional means of recovering citric acid is by a lime and sulfuric acid process ia which the citric acid is first precipitated as a calcium salt and then reacidulated with sulfuric acid. However, this process generates significant by-products and thus can become iaefficient. 

Anhydrous aluminum triduotide, A1F., is a white crystalline soHd. Physical properties are Hsted ia Table 2. Aluminum duotide is spatingly soluble ia water (0.4%) and iasoluble ia dilute mineral acids as well as organic acids at ambient temperatures, but when heated with concentrated sulfuric acid, HF is hberated, and with strong alkah solutions, aluminates are formed. A1F. is slowly attacked by fused alkahes with the formation of soluble metal duotides and aluminate. A series of double salts with the duotides of many metals and with ammonium ion can be made by precipitation or by soHd-state reactions. 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

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