Journal of Hazardous, Toxic, and Radioactive Waste

Phosphogypsum (PG) is a waste product of phosphate fertilizer and phosphoric acid industries. Vast quantities of the waste product are generally left stockpiled on the ground or discharged into rivers or ponds, and pose serious environmental concerns. Several countries are facing problems with the handling, storage, and utilization of PG. This article is a comprehensive review of the physicochemical, microstructural, geotechnical, radiation, and leaching characteristics of raw PG. The performance of PG with other admixtures (cement, fly ash, lime, ground granulated blast furnace slag) and soils (clays, loess, sands) has also been reviewed. In addition, the factors, such as pH, calcination, forms of PG, and curing methods, that affect the geotechnical, durability, and leaching characteristics of PG mixtures are also discussed. Based on the review, it has been observed that raw PG has high sulfate content, high water absorption capacity, and high variability in strength, leaching, and radioactivity properties; thus, using raw PG alone as a backfill or embankment material is not suitable. However, the geotechnical, durability, and leaching properties of PG may be enhanced by using it in combination with other admixtures. The excessive usage of PG should also be undertaken with caution, as the higher amount of PG may lead to a decrease in the strength and the formation of cracks. There are studies that have reported the optimum content of PG; however, the range is quite varied and can differ for soil types. Therefore, it is recommended to investigate the optimum content of PG before the start of any road or ground improvement project. The compaction characteristics of PG-admixed soils are highly contrasting and hence need more comprehensive studies. The strength characteristics and resilient modulus are also highly variable and hence need more exhaustive research for actual usage of PG in road construction. Other critical issues that require attention have been highlighted, and associated research gaps that could be explored in future research works have been identified.

In this study, the possible utilization of mixed construction and demolition waste (CDW) is evaluated for constructing geosynthetic-encased stone columns. A detailed geotechnical characterization is presented, including investigation of the physical, mechanical, and hydraulic properties of the CDW. Further, the performance of a group of 36 geosynthetic-encased stone columns (GECs) constructed using CDW (CDW-GECs) is investigated by 3D numerical modeling. Both the load-bearing ability and the settlement characteristics of the CDW-GECs showed similar performance to that of GECs made of natural crushed stone aggregates (CSA-GECs). Vertiсаl stresses imparted to the CDW-GEC were 3 times the tоtаl vertiсаl stresses imparted tо the surrоunding sоil, resulting in a stress concentration ratio of 3. This is due to the better stiffness оf the CDW-GECs compared with the surrounding soil and soil arching effect. The CDW-GECs helped in dissipating excess hydrostatic pressure developed in the clay by acting as a vertical drain. Around 65% of the total settlement happened during construction itself. Maximum hoop tension in geosynthetic encasement of the CDW-GECs was developed at a depth of 2.5 times the diameter of the column and was 15 kN/m. The maximum value of hoop tension decreases as the distance of the stone column from the center of the embankment increases. Tension mobilized in the basal geogrid was in agreement with the differential settlement pattern with a maximum value near to the center of the embankment, which is found to be 23 kN/m. The present study indicates that CDW can be considered as a sustainable and valuable resource for the construction of geosynthetic-encased stone columns.

In bauxite residue (BR), the abatement and rebound of pH when ameliorated with different amendments have been reported as prime concerns. In addition, to the best of the authors’ knowledge, no research has focused on the characterization of BR during the decrease and rebound in pH, which happens over an extended period. This characterization is essential to convert posttreated residue into green construction materials and to affirm that it does not pose a threat to the environment. This study aimed to investigate two important points. The evaluation of the efficacy of commonly and widely employed conventional additives and mineral acids while mitigating the pH of the residue and its characterization included sedimentation and leaching with a change in pH. The uniqueness of this study lay in the exploration of the latter point and pH rebound for ≤180 days. Cement, gypsum (G), fly ash (FA), and ground granulated blast furnace slag (GGBS) as conventional additives, and nitric (HNO₃) and hydrochloric acids (HCl) as mineral acids were selected to ameliorate BR. A significant pH rebound with time occurred from 6.59 to 9.51 and 7.54 to 9.78 when treated with 1M HCl and HNO₃, which indicated the influence of the curing period and ameliorant on the alkalinity. Conventional additives, except for G, and their combinations proved ineffective when mitigating and maintaining pH within acceptable limits (i.e., 8.5). Settling analysis revealed accelerated particle settling at pH 4.5–7.0, which indicated implications for the safe disposal, washing, or slurry thickening of BR. The extensive leaching studies for potentially toxic elements (PTEs), which were further endorsed with a field case study, demonstrated that the amended residue was environmentally safe to be used as a construction material.

Nanoscale zerovalent iron (nZVI) was synthesized and employed for treating chromium(VI) containing aqueous solution through adsorption using a series of microcosm experiments. The experiments were then performed to study the effects of a light nonaqueous phase liquid, toluene, on Cr(VI) removal by nZVI. Results showed that a maximum removal efficiency of almost 90% was achieved within 300 min of the treatment process, while it was suppressed by approximately 25% in toluene’s copresence. The efficiency was found to be higher (>80%) at pH 3–7 and reduced to 30.3% at pH 9. No significant change in the removal was observed due to a change in pH under toluene’s copresence (57%–65% removal for all pH ranges). The removal efficiency also reduced from 62.7% to 37.1% and 82.1% to 28.3% with an increase in Cr(VI) concentration (25–200 mg/L), with and without coexisting toluene, respectively. The surface morphology and structure of nZVI pre- and posttreatment with Cr(VI) and toluene were analyzed through scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The results confirmed the presence of Cr(VI) and toluene on the nZVI surface. The adsorption of Cr(VI) by nZVI was best described by pseudo-second-order kinetics (R² = 0.97 in both cases) and the Langmuir isotherm equilibrium model (R² = 0.95, 0.81). The maximum adsorption capacity (Q_max) for Cr(VI) without and with toluene was 64.52 and 58.47 mg/g of nZVI particles, respectively. Findings from this study provide crucial information about cocontamination and its noticeable effects on remediation of the contaminants of concern.

The current study deals with the interaction of toxic metals with soils by conducting a series of leaching tests on four kaolinitic soils procured from locations with varying pollution levels, such as a pristine location, vicinity of steel mill works, battery works unit, and an abandoned municipal waste dump site. The procured soils were spiked with 3,000 mg/kg of toxic metals, lead (Pb), and nickel (Ni) to understand the contaminant transport. An innovative amendment, such as synthesized nano-calcium silicate (NCS), was also mixed with the soils for toxic metal immobilization. Leaching tests. such as the toxicity characteristic leaching procedure (TCLP), extended TCLP, and caged TCLP, were conducted on soil samples. These tests were successful in predicting the interactive behavior of the composite mix containing soil, NCS, and toxic metals, comprehensively. The vulnerability of soils based on TCLP can be expressed in the order of Soil C > Soil D > Soil B > Soil A. These results were concurrent with extended TCLP, wherein the response of these composites at various pH levels was presented. Caged TCLP quantified the leaching rate for different mixtures, and it was found that the leaching rate for virgin soils reduced from 1.3 to 0.075 cm/yr after the NCS amendment. The addition of NCS resulted in the agglomeration and aggregation of soil particles. This increased the density and reduced the void spaces of the soil–nano mix, thereby encapsulating the toxic metals permanently within its interstices. The highest attenuation was observed for Pb²⁺ compared with Ni²⁺ for all the mixtures considered. Thus, leaching tests can be considered a prospective short-term technique for monitoring polluted soil media compared with tedious and time-dependent column studies.

Abstract

The present work evaluates and compares the performance of clay soil stabilized with red mud, bioenzyme, and red mud–bioenzyme as a subgrade material in pavement construction. Finite-element simulations have been performed using PLAXIS-2D to evaluate pavement parameters, such as surface deflection, compressive strain at the top of the subgrade layer, and tensile strain at the bottom of the asphaltic layer. Based on strain values, the rutting life and fatigue life of stabilized subgrade pavement have been evaluated. The effect of various axle loads (40 to 280 kN) and climatic conditions (20°C to 40°C) on pavement parameters have been studied. The results show that the compressive strain and tensile strain in pavement increase with an increase in axle loads. The fatigue life of soil–red mud, soil–bioenzyme, and soil–red mud–bioenzyme subgrade pavements are improved by 4 times, 6.5 times, and 13.4 times, respectively, and rutting life is improved by 9.3 times, 20.5 times, and 38.9 times, respectively, as compared with clay soil subgrade pavement. The strain and surface deflection values of stabilized subgrade pavements are found to be 30% to 55.4% and 15% to 42.7% lower, respectively, than clay soil subgrade pavement. Based on analysis, optimum axle load and design life of stabilized subgrade pavements have been discussed.

Practical Applications

The present study describes the applicability of clay stabilized with red mud, bioenzyme, and red mud–bioenzyme as a subgrade material in pavement. The use of red mud, bioenzyme, and red mud–bioenzyme increase the strength of the weak clay soil subgrade layer. The increased strength will offer additional resistance to the pavement to withstand the load generated by vehicular movement. The use of red mud will also reduce the land-associated problem faced by aluminum industries for aluminum effluent safe disposal. The service life of stabilized subgrade pavement is 4 times to 13 times higher than the service life of clay subgrade pavement. The surface deflection values of stabilized subgrade pavements are found to be 15% to 42.7% lower than clay soil subgrade pavement. This improves the riding comfort for the passengers. The study reveals that the movement of overweight vehicle reduces the service life of pavement. The rate of deterioration of pavement life is faster under higher temperature. The study also suggests the probable axle load that can be allowed on the stabilized subgrade pavement.

The efficacy of lime treatment to improve sulfatic soils has previously been explored. The formation of ettringite (E) and thaumasite due to the reaction between lime–clay–sulfate in the presence of moisture leads to the distress of lime-treated sulfatic soils. Further, sulfate contamination in the field that is due to industrial chemical discharges, groundwater contamination, accidental spillage of chemicals, acid rain, acid mine drainage, or any other sources occurs frequently. The alteration in soil behavior that has been subjected to prolonged submergence or soaking under sulfate contamination significantly affects the strength, stiffness, and durability of lime-treated soils. Therefore, the behavior of lime-treated soil that has been submerged or soaked under sulfate contaminants at different times has been addressed in this study. This study explored the compaction characteristics [maximum dry density (MDD) and optimum water content (OWC)], physicochemical [pH and electrical conductivity (EC)] and strength behavior [unconfined compressive strength (UCS)] of a lime-treated soil that was submerged under gypsum and sodium sulfate (Na₂SO₄) solutions with varying sulfate concentrations (i.e., 0–30,000 ppm) for different periods. The strength of the lime-treated soil that was subjected to sulfate contamination (i.e., designated as submergence) was compared with a lime-treated sulfatic soil (i.e., designated as nonsubmergence). The submergence of the lime-treated soil that was subjected to sulfate contamination resulted in a reduction in strength compared with the same under the nonsubmergence conditions. The strength of the lime-treated soil that was subjected to Na₂SO₄ contamination reduced with the increase in submergence ≤28 days; however, a marginal achievement in strength was witnessed with gypsum contamination, particularly at a higher concentration of 16,000 ppm. Therefore, the prolonged submergence of the treated soil under sulfate contamination ≤28 days led to a reduction in the strength of the lime-treated expansive soil. In addition, variations in the strength of the lime-treated soil that was subjected to submergence conditions depended on several factors, such as submergence period, type of sulfate, and concentration. The mechanisms of strength variations in the lime-treated soil for submergence and nonsubmergence conditions were determined by performing microanalyses and physicochemical examinations. The formation and growth of E and cementitious compounds were controlled by the availability of water, sulfate ions (SO₄^2–), and the duration of submergence, which led to alterations in the soil matrix and variations in the strength behavior of lime-treated soil.

In this study, the energy, economic, and environmental (3E) analysis on two waste-to-energy (WtE) technologies, for example, anaerobic digestion (AD) and incineration, for the treatment of municipal solid waste in Surat (Gujarat, India) was conducted. For energy criteria, the potential for energy recovery from municipal solid wastes (MSW) through AD and incineration was considered. The economic analysis that was carried out considered preprocessing, transportation, land, capital investment, and operational and maintenance costs that were incurred. The revenue that was generated by selling the electricity and compost was considered along with the recycling value of the paper waste. For environmental analysis, the global warming and acidification potentials (APs) were considered from the emissions that were produced during the treatment, transport, and disposal along with the displaced emissions. The results indicated that the average daily solid waste generation from Surat was 0.33 kg/person and the average total solid waste generation was 2080 t/day. Organic waste was the largest fraction (35.6%) followed by inert waste (13.5%), construction and demolition waste (12.4%), and recyclable waste, such as glass, rubber, and metal (11.2%). AD had less energy generation potential (9.89 MW) compared with incineration (16.99 MW). From an economic perspective, incineration had a higher profit (INR 1.48 million/day) than AD (INR 1.38 million/day). The environmental analysis showed that incineration had greater potential to reduce global warming and acidification compared with AD. The 3E analysis revealed that incineration was the superior technology choice.

Sustainable water management is the critical issue in dealing with circular water economy and water scarcity, for further overcoming drought in the future. Industrial cooling tower systems consume about 70% of total fresh water use, especially in semiconductor industries of Taiwan. The development of high energy efficiency technology for cooling water blowdown reclamation can minimize the water and carbon footprint, which is beneficial to the environment and the economy. In this study, resin wafer electro-deionization (RW-EDI), utilizing immobilized conventional ion exchange resin in porous material to enhance ion mobility, was used to demonstrate the desalination of cooling water blowdown from an incineration plant in north Taiwan. With a feed total dissolved solids (TDS) concentration of 900 mg/L, RW-EDI exhibits an excellent performance; more than 99.75% of salts can be removed. The energy consumption was 0.26–0.49 kW · h/m³, with productivity increases from 9.1–15.9 L/(h m²) of the desalination process under various applied voltages. For economic evaluation of the RW-EDI process, the levelized cost of water was from 0.31 to 0.43 USD/m³, where the material costs were far greater than the operational energy cost. Briefly, it can be concluded from the results that the RW-EDI process is a crucial component in the portfolio of water supply options by providing impaired-water reclamation.