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22 sulfate ions into solution, which is precipitated as barium sulfate when heated to near boiling and after addition of barium chloride (30). The precipitate is filtered, weighed, and dried to quantify sulfates in solution. Sulfides present in soils are not determined by the test, as HCl digestion releases sulfur as H2S gas. Acid dissolution solublizes most sulfate components in soil and therefore yields the maximum possible sulfate level in soil. This method is an overestimation of the sulfate problem as the pH in lime-treated soils is high (alkaline) and some of the sulfate species mentioned above may not be soluble under those conditions. Again, the test for acid soluble sulfates may be considered a conservative approach for screening purposes. But this method does not differentiate among different sulfur species, which is important in deciding minerals that can provide sulfate ions for ettringite formation. Total Reduced Sulfur Reduced sulfur is formed when sulfate-reducing bacteria converts SO42- to S2- in anoxic and reducing environments. Aerobic bacterial reduction of organic components in soil creates this reducing environment resulting in formation of reduced sulfur. Reduced sulfur is mostly finely crystalline and impure in nature. H2S evolved reacts with Fe2+ in solution forming a metastable iron sulfide, which is transformed into pyrite during diagenesis. Currently, no standard procedure is available for quantification of total reduced sulfur in soils (30). As reduced sulfur is closely associated with pyrites in soil, indirect methods can be used to determine its concentration. The difference between total iron determined by nitric acid digestion and nonpyritic iron from hydrochloric acid digestion gives the concentration of reduced sulfur in soils. Indirect methods may not be a very accurate all the time, as nitric acid digestion may not completely dissolve all the pyrites, leading to underestimation of reduced sulfur, whereas the oxidation of organic matter can release iron resulting in overestimation of pyrites. If mono- sulfides are present in soils, acid soluble sulfate content may be overestimated as the H2S gas released is oxidized by atmospheric oxygen present in dilute hydrochloric acid solution. Pyrite concentration will also be overestimated in presence of organic sulfur, elemental sulfur, acid insoluble sulfur, and iron mono-sulfides. Total Sulfur Total sulfur in soils is determined by digestion of samples using an oxidizing acid system. A mixture of nitric acid and hydrochloric acid is used as the digestive system (30). Quantification of total sulfur can be done based on gravimetric analysis of precipitated barium sulfate in solution. METHODS FOR SULFATE QUANTIFICATION IN SOILS A wide range of test methods are currently used to extract and quantify the amount of sulfates in soils. These methodologies use different sulfate measurement techniques, e.g., chromatography, ICP, gravimetry, colorimetry, etc., and different sample preparation techniques. However, most of the test methods are based on determining water soluble sulfates in the soil. The extraction techniques are often derived from water chemistry analysis and are modified for application in soils. Since the sulfate availability in treated soils is dependent on dissolution and movement of sulfate ions in natural water, an extraction process using water as the solvent is acceptable. Again, the accuracy of sulfate extraction depends on the type and the solubility properties of sulfate minerals present in soils. Therefore, use of as high an extraction ratio of soil
23 to water as possible in an attempt to solubilize all available sulfate ions is recommended. Although the different test methods follow accepted principles, the procedures and sequences of testing are different, which can significantly influence the end result of the tests (32, 36). A comparison of AASHTO sulfate testing methods to other available test methods is given in Table 2. The following sections outline some of the key steps in different sulfate quantification techniques that are currently in practice: Texas Department of Transportation, Colorado Department of Transportation, and AASHTO T 290. TxDOT MethodâTex-620-J Tex-620-J is a gravimetric method based on precipitation of BaSO4 by adding a barium chloride solution as a reagent to the soil-water mixture. Thirty (30) grams of soil passing the 425 μm sieve are mixed with 300 mL of deionized water, giving a dissolution ratio of 1 part soil to 10 parts water for analysis. The mix is then brought to near the boiling point and kept there for 24 hours. The sample is stirred intermittently to disintegrate the sulfate minerals within the soil matrix. At the end of the heating period the sample is filtered through a No. 42 Whatman filter paper and the filtrate collected for further analysis. Ten (10) mL of concentrated HCl is added to 80 mL of the filtered solution (46). This prevents the precipitation of barium carbonate and barium phosphate in solution. Barium chloride is then added and the solution is heated to near 100°C for 10 minutes. The precipitate is then filtered using a 2.5 μm filter paper to collect the precipitated barium sulfate, which is washed and weighed. The results are compared with the weight of natural soil to determine the percent of sulfates in solution. U.S. Army and Air Force Method Gravimetric Method The U.S. Army and Air Force method-TM 5-822-14/AFJMAN 32-1019 (47) uses a gravimetric technique to determine the concentration of sulfates in solution. Ten (10) grams of soil is mixed with 300 mL of demineralized water, giving a dilution ratio of 1:30. Fifteen (15) mL of HCL is then added to the mix, which is heated for 1.5 hours. The solution is filtered using Whatman No. 40 filter paper using hot water to facilitate filtration of the solute. MgCl2 is added to 100 mL of the above filtrate until precipitation ends, at which time the solution is filtered with Whatman No. 42 filter paper, again using hot water. One hundred (100) mL of the resulting solution is collected and heated to near the boiling point, and barium chloride is added slowly until there is no further precipitation. Boiling of the solution is continued for 5 minutes, and the solution is left to stand overnight in a warm place. The solution is then filtered using Whatman No. 42 filter paper, and the filtrate is washed with hot water until chlorides are removed. The filter paper is dried and ignited and the residue is weighed to determine the sulfate content. Turbidimetric Method Point five (0.5) N ammonium acetate solution with pH 4.2 (from adding dilute HCl) is added to a 10 gram representative sample of air dried soil to form a solution ratio of 1:5 by weight (47). The mixture is boiled for 5 minutes and filtered through Whatman No. 40 filter paper until a clear filtrate is obtained. Ten (10) mL of extracted solution is diluted to 40 mL with distilled water. Two tenths (0.2) of a gram of barium chloride is added, and the mixture is diluted to 50 mL. The solution is stirred for 1 minute and the turbidity measured using a
24 spectrophotometer at 30 second intervals for 4 minutes. The maximum reading is considered as the turbidity and the value is compared with a standard curve to determine the sulfate ion concentration. University of Texas Arlington (UTA) Method The University of Texas Arlington method formulated by Petry (38) is also based on gravimetric analysis and uses a dilution ratio of 1:10. Ten (10) grams of soil is dissolved in 100 mL of distilled water, and the solution is shaken for 30 minutes to disintegrate sulfate salts in soil matrix. The mix is then centrifuged at 4,500 rpm for 15 minutes to obtain a clear extract. If the filtrate is not clear, centrifuging is repeated at a higher speed for longer durations until a clear extract is obtained. After centrifugation the solution is filtered through Whatman No. 541 filter paper and diluted to 200 mL with distilled water. The pH of the solution is then adjusted to between 5 and 7 using concentrated HCl, and the filtrate is heated to near the boiling point. Warm BaCl2 is added to the solution until no precipitate is obtained. The precipitate is then digested at 80°-90°C for 12 hours and filtered through a 0.45 μm filter membrane. The precipitate is dried and weighed to determine the sulfate content in soil. The modified University of Texas Arlington method is similar to the regular UTA method, but with minor changes to improve the efficiency of the test procedure for use in fine- grained soils. The pore size of the filter membrane is reduced to 0.1 μm to improve the efficiency of removal of suspended fine clay particles, as they tend to interfere with precipitated sulfate compounds. This can give a higher sulfate reading during gravimetric analysis (32). Since the average grain size, for fine clays like montmorillonite, is smaller than normal clay particles a longer settling time will be needed for these clays. Hence a centrifugation speed of 14,000 rpm for duration of 30 minutes is used to remove the suspended particles from soil samples. Ion Chromatography Ion chromatography (IC) is a good technique by which to measure sulfate concentrations at lower concentrations (36). Sulfate measurement using ion chromatography involves dissolving sulfate compounds and then introducing small quantities of the aliquot into the IC system. The sample is passed through ion exchange columns using inert compounds like polyetheretherketone. The different ions are attracted to the resins in the column and released at different times by the conductivity detectors. The conductivity of the solution is compared with conductivities of standard solutions to quantify the concentration of ions. Harris et al. (36) in their comparison of various available sulfate test procedures emphasized the capability of ion chromatography in measuring low sulfate concentrations. The retention time of ions decreases with increase in concentration, and hence for higher concentrations, the dissolution ratio must be increased. TxDOT Colorimetric Method Tex 145-E (45) determines the sulfate content of soils based on colorimetric techniques. The technique measures the cloudiness of a liquid and correlates that to concentration. Ten (10) g of an air dried soil sample passing the 425 μm sieve is added to 200 mL distilled water in a high-density polyethylene bottle. The solution, having an initial dilution ratio of 1:20, is shaken vigorously for 1 minute to disintegrate the sulfate salts and then left idle for 12 hours. After 12 hours the sample is filtered and 10 mL of the filtrate is collected in a glass vial and used to
25 calibrate the colorimeter for the initial sulfate level in the solution. A barium chloride tablet is then added to the vial and dissolved completely to precipitate barium sulfate, which appears as turbidity in the solution. The colorimeter is used to measure the turbidity of the solution, which, in turn, provides the sulfate ion concentration in the solution. The dilution ratio must be increased if the sulfate content is above the measuring limit of the colorimeter. CDOT Colorimetric Method Like the TxDOT method discussed earlier, the Colorado Department of Transportation (CDOT) method (CP-L 2103) is also based on the principle of colorimetry. The procedures are similar with only minimal differences between the two. One of the differences is the dilution ratio for extracting soluble ions. The TxDOT method (45) uses a soil to water dilution of 1:20, whereas CDOT (48) recommends a 1:10 dilution at a higher temperature. Since the solubility of sulfates is linearly dependent on the dilution ratio, the TxDOT method will identify a larger value for soluble sulfates than the CDOT method if the sulfate content of the soil is greater than the quantity a 1:10 ratio can solubilize. However, 1:10 dilution ratio is capable of extracting sulfates higher than the threshold levels considered problematic in soils and hence use of this lower dilution is justified on this basis (4). Another difference between the two methods is that the CDOT method recommends the soil water mixture to be left idle for 16 hours at a temperature close to 140°F to dissolve the soluble sulfates, whereas the TxDOT recommends only 12 hour dissolution at room temperature. This difference in procedures is insignificant if gypsum is the major sulfate source in soil, as the temperature dependency of gypsum solubility is insignificant when compared to other calcium sulfate forms like anhydrite and hemi-hydrate at water temperature below boiling conditions (49, 50). Solubility of anhydrite generally decreases with increases in temperature but increases with time (49). Hence using a higher temperature for longer duration might favor the dissolution of anhydrites if any are present. TxDOT Conductivity Method Measurement of conductivity of a solution extracted from soil reflects the presence of soluble salts including sulfate concentrations in soil. Tex 146-E (40) describes a method of conductivity testing where 5 g of air dried soil passing the 425 μm sieve is placed in solution and the conductivity of the solution is measured. In the first step, 100 mL of distilled water is added to a high-density polyethylene bottle, and the initial conductivity of the water is recorded. The soil sample is placed in the bottle, and the sample is shaken vigorously for 1 minute and the conductivity is measured immediately after that. The sample is kept idle for 12 hours, after which it is shaken vigorously for 1 minute and the conductivity measured. The conductivity of distilled water is subtracted from the two readings in order to determine the conductivity of the soil solution. An initial conductivity reading above 238 µS or a difference of 50 µS between the initial and final readings indicates the presence of excessive soluble ions in solution. If this is the case, the soil should be tested using colorimetric techniques to identify the magnitude of soluble sulfates. Inductively Coupled Plasma Atomic Emission Spectroscopy Inductive coupled argon plasma (ICP) atomic emission spectroscopy is an effective method by which to determine the level of sulfate ions in solution. Soil samples are digested or solubilized using appropriate sample preparation methods prior to analysis. Soil samples are made to dissociate in an argon plasma stream producing element-specific spectral lines by the
