Experiments were conducted to evaluate the hydraulic conductivity of geosynthetic clay liners (GCLs) containing granular sodium bentonite that were permeated with coal combustion product (CCP) leachates. Chemical properties of the CCP leachates were selected from a nationwide survey of CCP disposal facilities. Five synthetic leachates were selected from this database to represent a range of CCP disposal facilities: typical CCP leachate (geometric mean of CCP chemistry), strongly divalent cation leachate (aka low-RMD leachate), flue gas desulfurization (FGD) residual leachate, high ionic strength ash leachate, and trona ash leachate. Typical GCLs from two U.S. manufacturers were used. Hydraulic conductivity tests were conducted on non-prehydrated and subgrade hydrated (by compacted soil for 60 days) GCL specimens at effective stresses ranging from 20 to 450 kPa. At 20 kPa, GCLs permeated directly had high hydraulic conductivity ($>{10}^{-6}\mathrm{m}/\mathrm{s}$ ) to trona leachate and moderate to high hydraulic conductivity (${10}^{-10}$ to ${10}^{-7}\mathrm{m}/\mathrm{s}$ ) to the other CCP leachates. Hydraulic conductivity was strongly related to the ionic strength of the leachate and inversely related to the swell index of the bentonite when hydrated in leachate, as demonstrated in past studies on leachates from other waste streams. Increasing the effective stress from 20 to 450 kPa caused the hydraulic conductivity to decrease up to three orders of magnitude. Hydration on a subgrade prior to permeation has only modest impact on the hydraulic conductivity to CCP leachate. Hydration by permeation with deionized (DI) water prior to permeation with trona leachate resulted in hydraulic conductivity up to three orders of magnitude lower than obtained by direct permeation, suggesting that deliberate prehydration strategies may provide chemical resistance to CCP leachates.

Experiments were conducted to evaluate the hydraulic conductivity of geosynthetic clay liners (GCLs) containing bentonite–polymer composites (BPCs) permeated with coal combustion product (CCP) leachates. Eight synthetic CCP leachates were used: the five characteristic CCP leachates from a leachate database and three based on actual leachate chemistries. Hydraulic conductivity tests were conducted on non-prehydrated and prehydrated BPC GCL specimens at an effective stress of 20 kPa. Prehydration was achieved by contact with a subgrade for 60 days or by permeation with deionized (DI) water. Comparisons are made to the hydraulic conductivity and swell index of conventional sodium bentonite (NaB) GCLs permeated with the same leachates. The hydraulic conductivity of BPC GCLs with low polymer loading ($<1.9\%$ ) to the CCP leachates is similar to that of NaB GCLs, with hydraulic conductivity related inversely to swell index and directly to ionic strength. For higher polymer loading ($\ge 1.9\%$ ), the hydraulic conductivity of BPC GCLs appears to be controlled by the polymer hydrogel and is not related directly to the swell index of the BPC or ionic strength of the leachate. Higher hydraulic conductivity of BPC GCLs is associated with greater polymer elution and nonuniformity of the polymer within the GCL. Subgrade hydration had a modest beneficial effect on the hydraulic conductivity of a BPC GCL with low polymer loading ($<1.9\%$ ). Prehydration by permeation with DI water resulted in low hydraulic conductivity ($4.3\times {10}^{-11}\mathrm{m}/\mathrm{s}$ ) for this same GCL with one of the strongest leachates.

This study analyzes the leaching behavior of elements from red mud (bauxite residue) at pH values ranging from 2 to 13. The leaching characteristics of metals and contaminated anions in five red mud samples produced by Bayer and combined processes were analyzed using the batch leaching technique following the US Environmental Protection Agency (USEPA) Method 1313. In addition, the geochemical model of MINTEQ 3.1 was used to identify the leaching mechanisms of metals. The results showed that Ca, Mg, and Ba follow the cationic leaching pattern. Al, As, and Cr show an amphoteric leaching pattern. The leaching of Cl⁻ is unaffected by the pH. The maximum leaching concentration of the proprietary elements occurs under extremely acidic conditions (pH = 2), except for As. The leaching concentration of F⁻ reaches 1.4–27.0 mg/L in natural pH conditions (i.e., no acid or base addition). At the same pH level, the leaching concentrations of Pb, As, Cr, and Cu are generally higher from red mud produced by the combined process than that those of red mud from the Bayer process. The leaching concentration of these elements is not strongly related to the total elemental concentration in the red mud. Geochemical modeling analysis indicates that the leaching of metal elements, including Al, Ca, Fe, Cr, Cu, Pb, Mg, Ba, and Mn, in red mud are controlled by solubility. The leaching of these elements depended on the dissolution/precipitation of their (hydr)oxides, carbonate, or sulfate solids.

A nationwide investigation was carried out to evaluate the geochemical characteristics and environmental impacts of red mud and leachates from the major alumina plants in China. The chemical and mineralogical compositions of red mud were investigated, and major, minor, and trace elements in the leachates were analyzed. The mineral and chemical compositions of red mud vary over refining processes (i.e., Bayer, sintering, and combined methods) and parental bauxites. The main minerals in the red mud are quartz, calcite, dolomite, hematite, hibschite, sodalite, anhydrite, cancrinite, and gibbsite. The major chemical compositions of red mud are Al, Fe, Si, Ca, Ti, and hydroxides. The associated red mud leachate is hyperalkaline (pH > 12), which can be toxic to aquatic life. The concentrations of Al, Cl⁻, F⁻, Na, NO₃²⁻, and SO₄²⁻ in the leachate exceed the recommended groundwater quality standard of China by up to 6637 times. These ions are likely to increase the salinization of the soil and groundwater. The minor elements in red mud leachate include As, B, Ba, Cr, Cu, Fe, Ni, Mn, Mo, Ti, V, and Zn, and the trace elements in red mud leachate include Ag, Be, Cd, Co, Hg, Li, Pb, Sb, Se, Sr, and Tl. Some of these elements have the concentration up to 272 times higher than those of the groundwater quality standard and are toxic to the environment and human health. Therefore, scientific guidance is needed for red mud management, especially for the design of the containment system of the facilities.

Tests were conducted on eight soils to determine how coal combustion product (CCP) leachates may affect the hydraulic conductivity of compacted soil liners (CSLs) used for CCP disposal facilities. The soils represent a broad range of particle-size distributions, Atterberg limits, and mineralogy, and meet minimum compositional recommendations for CSLs. Hydraulic conductivity tests were conducted with five characteristic CCP leachates from the Electric Power Research Institute (EPRI) database of CCP leachates. The testing confirmed that seven of the soils are suitable for a CSL. Five of the seven suitable soils have hydraulic $\text{conductivity}<1\times {10}^{-9}\mathrm{m}/\mathrm{s}$ when permeated with any of the CCP leachates at 28 kPa effective stress (disposal facility with first lift of CCP placed), as do all but one soil when the effective stress is 450 kPa. Larger increases in hydraulic conductivity are associated with soils having lower hydraulic conductivity to deionized water and significant montmorillonite content. Soils exhibiting the smallest increases in hydraulic conductivity have little to no montmorillonite. Hydraulic conductivity to CCP leachate is not related systematically to any of the primary index properties, indicating that mineralogy is a better indicator of sensitivity to CCP leachates than index properties. Increasing the effective stress from 28 to 450 kPa ($\approx 2$ to 30 m CCP depth) results in an average reduction hydraulic conductivity of $10\times$ . The average reduction is $2\times$ at 100 kPa and $5\times$ at 250 kPa.