Acid deposition recovery from

Acid deposition is an on-going, long-term environmental problem with a greater environmental impact than previously projected. Since many ecosystems are now more sensitive to the input of additional acids, their recovery from the adverse affects of acid rain will most likely be delayed. In Acid in the Environment, vjq provide the perspectives of various authors with respect to the lessons learned and future prospects associated with the issue of acid deposition. We use an interdisciplinary approach that combines a discussion of important ecological issues associated with acid deposition with an analysis of domestic and international policies to control the emission of pollutants that cause acid rain. In this context. Acid in the Environ-... 

Cobalt recovery from acidic metal ion solutions is impeded by the evolution of hydrogen. However, if the pH of the solution is gradually increased, it is found that at pH > 4 this deposition is possible. 

Singh, R., Khwaja, A., Gupta, B., and Tandon, S. (1999) Extraction and separation of nickel(II) using bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex 301) and its recovery from spent catalyst and electroplating bath residue. Solv. Extraction Ion Exch., 17 (2), 367-390. Malinowska, B., Rakib, M., and Durand, G. (2001) Ammonia recycling and cadmium confinement in chemical bath deposition of CdS thin layers. Prog. Photovoltaics, 9 (5), 389—404. 

Quantifying the sources and rates of input of base cation nutrients (calcium, magnesium, potassium, and sodium) to forest ecosystems is an important goal in forest biogeochemistry, particularly when seeking to understand the recovery from environmental disturbances such as acid rain and forest clear-cutting. The earliest study to use isotopes as an indicator of atmospheric inputs to soils was by Dymond et al. (1974), who used strontium isotope measurements of micas in Hawaiian soils to determine that a significant proportion of the potassium input to Hawaiian soils was from deposition of dust transported... 

Dahlgren R. A., McAvoy D. C., and Driscoll C. T. (1990) Acidification and recovery of a spodisol Bs horizon from acidic deposition. Environ. Sci. Technol. 24, 531-537. 

A value of current efficiency below 100% usually indicates that by-products are formed. Another possibility is that reverse reaction happens, for example, dissolution of deposited metal. In metal recovery from acidic solutions, the hydrogen evolution reaction is a common side reaction that lowers the current efficiency. The current efficiency can also vary with the current density. Current efficiency is a widely used measure of tankhouse proficiency in producing metal, but it does not provide a direct measure of metal quality. 

Leaching and electrolysis processes can be used for metal recovery from waste electrical and electronic equipment. Metals such as Ag, Au, Cu, Pb, Pd, Sn, are dissolved from shredded electronic scrap in an acidic aqueous chloride electrolyte by oxidizing them with aqueous dissolved chlorine species. In the electrochemical reactor, chlorine is generated at the anode for use as the oxidant in the leach reactor and the dissolved metals are deposited from the leach solution at the cathode. The very low concentrations of the precious metal ions require the use of porous electrodes with high specific surface areas and high mass transport rates to achieve economically adequate reactor productivities and space-time yields [72]. 

Mixtures of acetaldehyde and acetic acid may be obtained121 by passing acetylene (2 to 3 volumes) and air (10 volumes) mixed with a large excess of steam over the zinc, copper, nickel, or cadmium salts of vanadic, molybdic, or chromic acids deposited upon a suitable base, such as pumice, at temperatures ranging from 300° to 400° C. For example, yields of 75 to 80 per cent acetaldehyde along with 5 per cent acetic acid have been obtained by using basic zinc vanadate at 380° C. The aldehyde is separated by fractional condensation in a column and the condensed fraction which is poor in aldehyde is utilized to furnish steam for the catalytic treatment of more acetylene. The fractions rich in acetaldehyde serve for the direct recovery of the aldehyde or may be oxidized immediately to acetic acid by passage over a suitable catalyst. In this way, the process may also be applied directly to the preparation of acetic acid from acetylene. 

As a result of a major reduction in the emission of acidifying S and N compounds since the 1970s in Europe (see Section 7.4.5), acid deposition on boreal lakes and their catchments has also strongly declined. Recently, Stendera and Johnson (2008), assessing the recovery of boreal lake ecosystems from acidification, used different indicators (water chemistry, biota), different trophic levels (primary producers and consumers) and different habitats within the lakes (pelagic, benthic) to compare the response of both acidified and reference lakes to the reduction in acid deposition. 

Delay in recovery from acid deposition and threats to stream organisms from increased frequency and magnitude of high-discharge, low-pH events in upland regions of Northern Europe. 

Acidic deposition results in the accumulation of sulfur and nitrogen in forest soils. As sulfate is released from the soil in response to decreases in emissions and atmospheric deposition of sulfur, it is transported to adjacent streams and lakes (Likens et al. 2000). The recovery of surface waters in response to emission controls has therefore been delayed and will not be complete until the sulfate left by a long legacy of acidic deposition is released from the soil. 

Recovery from acidic deposition involves decreases in emissions resulting from regulatory controls, which in turn lead to reductions in acidic deposi-... 

Table 3.3. Indicators of chemical recovery from acidic deposition...

To date, emissions targets set in the United States and Europe have been met or exceeded. There are widespread decreases in surface water concentrations of sulfate and some waters are showing increases in ANC. Nevertheless, data suggest that these targets may not be sufficient to achieve the full recovery of sensitive ecosystems. In order to evaluate the extent to which historic and future emissions reductions will facilitate ecosystem recovery from acidic deposition, it is necessary to use acidification models to project the future relationship between emissions, deposition, and chemical recovery of acid-sensitive forest watersheds (see Box 3.5). 

Long-Term Changes in Boreal Lake and Stream Chemistry Recovery From Acid Deposition and the Role of Climate... 

There are several consistent themes that run through this volume that indicate what we still need to do. One of the highest priorities is to find a more effective way to reduce major sources of acid deposition other than sulfur dioxide. This means reducing emissions of nitrous oxides and ammonia that are derived from dispersed sources (vehicles and farming activities, respectively) and so are more difficult to control. Another theme is that we need to understand and monitor the ecological effects of acid deposition in a wide range of ecosystems, and determine if there are ways to speed their recovery from decades of acid deposition. 

Electrodeposition is the preferred method for preparing a thin, smooth, uniform, and stable source for counting, as discussed in Section 3.7. Empirically developed procedures are applied to the heavy elements in aqueous solution because they cannot be reduced to the metal the source is deposited at the cathode in what is believed to be hydroxy forms after partial reduction. Deposition is in acid solution, under various conditions that yield near-quantitative recovery from the solution. Test results specify the volume, reagent content, applied volts and amperes, and optimum time period (see Section 6.4.1). The cathode can be a platinum disk, but a polished stainless steel disk is satisfactory. 

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