Natural leaching

The second reason for acid-digestion is the determination of the total soil elemental content of, e.g. potassium, phosphorus or trace elements. This is seldom done for potassium in normal soil samples, mainly because the total K in soils is of no value as an index to the availability of K to plants, nor is it always of value in tracing the movement or accumulation of applied fertilizer K (Pratt, 1965). The unreactive soil phosphorus is obtained by subtracting the naturally leached reactive phosphorus from the total phosphorus, and a method for determining the latter by extraction with sulphuric acid and potassium persulphate is cited by Turner and FHaygarth (2000). They analysed... 

Cameron, J., 1963. Discussion on natural leaching of uranium ores. Trans. Inst. Min. Metall., 72 507—517. 

Continued growth in the production of aluminum is unlikely to be hampered by a shortage of mineral since the earth s crust consists of about 8% aluminum, chiefly as aluminosilicates. Even though aluminum is the most abundant metallic crustal element, bauxite ores suitable for aluminum recovery only occur in more limited areas where natural leaching processes have concentrated the aluminum-containing minerals. Since the free metal is chemically reactive it is never found in nature in this form. 

The data from field studies is largely confirmed by the performance assessments of proposed nuclear waste r sitories. The intrusion scenario is calculated to have a lower risk than the undisturbed natural leach and migration scenario. The latter qualitatively agrees with the Oklo data and indicates no unacceptable risks result from a carefully chosen and designed geologic repository in which the nuclear wastes are enq>laced with appropriate packaging. 

Groundwater resources provide approximately one half of the nation s water supply as well as a major fraction of water used for self-supplied industry. Since both the consequences of human activities and the natural leaching processes may compromise the quality of water, treatment is frequently necessary prior to use. Preferably, the treatment would be accomplished by engineered treatment plants prior to distribution to the consumer. However, this is not always possible, particularly in cases where consumers are geographically distant from the resource or where distribution system treatment other than at the point of use is too expensive. 

Leaching (1796) v. The process of the extraction of a component from a solid material by treating the material with a solvent that dissolves the component of interest but not the remaining principle material. Natural leaching occurs when rain water dissolves soluble salts from soil. Some components of a paint coating leach from the dried film after application to a substrate when exposed to rain or when immersed in water. 

Many of the early investigations of prey chemical defenses were conducted by placing potential consumers into water containing secondary metabolites that had been extracted from potential prey. These assays lacked ecological realism in that (1) many of these compounds are not naturally leached into seawater (2) if they were, they would disperse rapidly and not be present at significant concentrations and (3) the potential predators would not normally encounter the... 

Diffusion and Mass Transfer During Leaching. Rates of extraction from individual particles are difficult to assess because it is impossible to define the shapes of the pores or channels through which mass transfer (qv) has to take place. However, the nature of the diffusional process in a porous soHd could be illustrated by considering the diffusion of solute through a pore. This is described mathematically by the diffusion equation, the solutions of which indicate that the concentration in the pore would be expected to decrease according to an exponential decay function. 

In the United States the analytical methods approved by most states are ones developed under the auspices of the Association of Official Analytical Chemists (AOAC) (3). Penalties for analytical deviation from guaranteed analyses vary, even from state to state within the United States (4). The legally accepted analytical procedures, in general, detect the solubiUty of nitrogen and potassium in water and the solubiUty of phosphoms in a specified citrate solution. Some very slowly soluble nutrient sources, particularly of nitrogen, are included in some specialty fertilizers such as turf fertilizers. The slow solubihty extends the period of effectiveness and reduces leaching losses. In these cases, the proportion and nature of the specialty source must be detailed on the labeling. 

Acidic suspensions such as those produced by acid leaching often respond to natural products such as guar as well as nonionic polyacrylamides and anionic polyacrylamides containing sulfonic acid groups. 

The phenoxyalkanoic herbicides are acidic in nature and thus subject to some degree of ionization. The extent to which the herbicide ionizes is controlled by the acid dissociation constant (fQ of the herbicide in question and the soil solution pH (238). The leaching potential is significantly influenced by these reactions. 

Fluridone is a weak base with low water solubiUty. Sorption of fluridone increases with decreasing pH (436). Leaching of fluridone was not significant in field study, and the persistence has been determined to be less than 365 days. The degradation of fluridone appears to be microbial in nature, and accelerated breakdown of the herbicide occurs upon repeated appHcations (437). Fluorochloridone is shown to degrade by hydrolysis at pH 7 and 9, but not at lower pH. The half-Hves for this reaction are 190 and 140 days for pH 7 and 9, respectively. Breakdown by photolysis occurs rapidly with a half-hfe of 4.3 days at pH 7 (438). An HA is available for acifluorfen. 

Potassium nitrate, essential in the manufacture of black gun powder, was produced by the Chinese, who had developed gun powder by the tenth century AD. The process involved the leaching of soil in which nitrogen from urine had combined with mineral potassium. By the early 1800s, potassium nitrate had become a strategic military chemical and was stiU produced, primarily in India, by using the ancient Chinese method. The caUche deposits in Chile are the only natural source of potassium nitrate (2). These deposits are not a rich source of potassium nitrate, purifying only to about 14% as K O. 

An extensive pesticide properties database was compiled, which includes six physical properties, ie, solubiUty, half-life, soil sorption, vapor pressure, acid pR and base pR for about 240 compounds (4). Because not all of the properties have been measured for all pesticides, some values had to be estimated. By early 1995, the Agricultural Research Service (ARS) had developed a computerized pesticide property database containing 17 physical properties for 330 pesticide compounds. The primary user of these data has been the USDA s Natural Resources Conservation Service (formerly the Soil Conservation Service) for leaching models to advise farmers on any combination of soil and pesticide properties that could potentially lead to substantial groundwater contamination. 

Hydrometallurigcal Processes. In hydrometaHurgical processes, metal values and by-products are recovered from aqueous solution by chemical or electrolytic processes. Values are solubilized by treating waste, ore, or concentrates. Leaching of copper ores in place by rain or natural streams and the subsequent recovery of copper from mnoff mine water as impure cement copper have been practiced since Roman times. Most hydrometaHurgical treatments have been appHed to ores or overburden in which the copper was present as oxide, mixed oxide—sulfide, or native copper. PyrometaHurgical and hydrometaHurgical processes are compared in Reference 34. 

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