Efficiency modeling of solidification/stabilization of multi-metal contaminated industrial soil using cement and additives

Cement-based solidification of nuclear waste: Mechanisms, formulations and regulatory considerations

This review paper provides a comprehensive analysis of cement-based solidification and immobilisation of nuclear waste. It covers various aspects including mechanisms, formulations, testing and regulatory considerations. The paper begins by emphasizing the importance of nuclear waste management and the associated challenges. It explores the mechanisms and principles in cement-based solidification, with a particular focus on the interaction between cement and nuclear waste components. Different formulation considerations are discussed, encompassing factors such as cement types, the role of additives and modifiers. The review paper also examines testing and characterisation methods used to assess the physical, chemical and mechanical properties of solidified waste forms. Then the paper addresses the regulatory considerations and compliance requirements for cement-based solidification. The paper concludes by critically elaborating on the current challenges, emerging trends and future research needs in the field. Overall, this review paper offers a comprehensive overview of cement-based solidification, providing valuable insights for researchers, practitioners and regulatory bodies involved in nuclear waste management.

Does Current Knowledge Give a Variety of Possibilities for the Stabilization/Solidification of Soil Contaminated with Heavy Metals?-A Review

Stabilization/solidification of contaminated soil is a process that allows simultaneous strengthening of the soil structure, disposal of contamination and recycling of industrial waste, implemented as substitutes for Portland cement or additives to improve the properties of the final product obtained. Extremely intensive development of studies pertaining to the S/S process prompted the authors to systematize the binders used and the corresponding methods of binding the contamination, and to perform an analysis of the effectiveness expressed in geomechanical properties and leachability. The study pays close attention to the types of additives and binders of waste origin, as well as the ecological and economic benefits of their use. The methods of preparing and caring for the specimens were reviewed, in addition to the methods of testing the effectiveness of the S/S process, including the influence of aging factors on long-term properties. The results of the analyses carried out are presented in the form of diagrams and charts, facilitating individual evaluation of the various solutions for the stabilization/solidification of soils contaminated with heavy metals.

Rapid solidification of Portland cement/polyacrylamide hydrogel (PC/PAM) composites for diverse wastewater treatments

Cementitious solidification is an effective but time-consuming method for waste disposal, and the incorporation of polyacrylamide hydrogel in Portland cement paste is a simple way to enhance the time-efficiency of cementitious solidification. In this study, a series of Portland cement/polyacrylamide hydrogel (PC/PAM) composites suitable for the wastewater treatment were prepared by a one-pot method and their time-dependent reaction processes, mechanical properties and microstructures were tested. Based on the gelation time method, PC/PAM composites showed great solidification efficiency when treating simulated radioactive liquids, organic dye waste and solutions with strong alkalinity and acidity. At temperatures ranging from 5 °C to 40 °C, it took only a few minutes for these composites to solidify wastes. Also, PC/PAM composites containing wastes had a compressive strength that is more than 2 MPa after reacting for 3 days and were suitable for landfill or secondary treatments. The rapid gelation and sufficient strength development demonstrated that PC/PAM composites have great potential for application in solidifying multi-component wastes, especially in some emergency circumstances.

Novel and stable PC/PAM composites were designed and could serve as a matrix for rapid solidification of various wastewaters, greatly facilitating the applicability of the cementitious solidification method in emergency circumstances.

Solidification/stabilization of Pb2+ and Zn2+ in the sludge incineration residue-based magnesium potassium phosphate cement: Physical and chemical mechanisms and competition between coexisting ions

In order to exhaustively investigate the physical and chemical mechanisms of heavy metal immobilization in sludge incineration residue (SIR)-based magnesium potassium phosphate cement (MKPC), this work investigated the influence of Pb²⁺ and Zn²⁺ on the compressive strength and microstructure of SIR-based MKPC, and the efficiency of Pb and Zn immobilization. Taking the difference of Ksp (solubility product) of different heavy metal compounds as the entry point, the physical and chemical mechanisms of Pb and Zn immobilization, and the competitive mechanism between coexisting ions, were comprehensively analyzed. It was discovered that Pb²⁺ is in the form Pb₃(PO₄)₂, and Zn²⁺ is immobilized in the form Zn₂(OH)PO₄ [Zn₃(PO₄)₂ is preferentially formed, when the pH > 7, Zn₃(PO₄)₂ is converted to Zn₂(OH)PO₄]. The low solubility of heavy metal phosphates is the main reason that Pb²⁺ and Zn²⁺ are well immobilized. The preferential formation of Pb₃(PO₄)₂ (K_sp = 8 × 10^-43) and Zn₃(PO₄)₂ (K_sp = 9.0 × 10^-33) reduced the amount of MgKPO₄·6H₂O (K_sp = 2.4 × 10^-11), resulting in a decrease in compressive strength. Besides, coexisting Pb²⁺ and Zn²⁺ has a competitive effect: Pb²⁺ will weaken the immobilization efficiency of Zn²⁺. The new exploration of these mechanisms provide a theoretical basis for rationally adjusting the Magnesia/Phosphate ratio to enhance the compressive strength and improve the efficiency of heavy metals immobilization.

Stabilization/solidification characteristics of organic clay contaminated by lead when using cement

Research about cement treated soil has examined various characteristics of strengthened and stabilized soil, but has mainly focused on either the unconfined compressive strength or potentially toxic element (PTE) stabilizing results respectively in response to cement dosing. This study investigates how factors including cement concentration, lead concentration, humic/fulvic acid content and curing age affect these two geotechnical and environmental characteristics. A laboratory study was conducted to measure unconfined compressive strength, and lead leaching under several test conditions. Knowing that humic acid and fulvic acid can weaken cementation in cement treated soil but can stabilize PTEs such as lead by different chemical reactions, it was found that the acids generally reduce lead stabilization in cement treated soil. In addition, the stabilized strength reaches a peak at a specific lead content in soil. Finally, scanning electron microscopy was used to observe more detailed changes and mechanisms.

Solidification/stabilization of toxic metals in calcium aluminate cement matrices

The ability of calcium aluminate cement (CAC) to encapsulate toxic metals (Pb, Zn and Cu) was assessed under two curing conditions. Changes in the consistency and in the setting time were found upon the addition of the nitrates of the target metals. Both Pb and Cu caused a delay in CAC hydration, while Zn accelerated the stiffening of the mortar. Compressive strengths of the metal-doped mortars, when initially cured at 60 °C/100% RH, were comparable with that of the free-metal mortar. Three different pore size distribution patterns were identified and related to the compounds identified by XRD and SEM. Sorbent capacities of CAC for the toxic metals were excellent: a total uptake was achieved for up to 3 wt.% loading of the three metals. In this way, CAC mortars were perfectly able to encapsulate the toxic metals, allowing the use of CAC for waste management as proved by the leaching tests.

Equilibrium leaching of toxic elements from cement stabilized soil

The toxicity characteristics leaching procedure (TCLP) is commonly used to assess the efficiency of solidification/stabilization (S/S) of pollutants in wastes, despite recent objections to this method. In this study, formulations of 7, 10, 15 and 20% (w/w) of calcium aluminate cement (CAC) and sulfate resistant Portland cement (SRC) were used for S/S of soil from brownfield contaminated with 43,149, 10,115, 7631, 6130, 90, 82 mg kg(-1) of Zn, Pb, Cu, As, Cd and Ni, respectively. CAC produced S/S soil monoliths of higher mechanical strength (up to 7.65 N mm(-2)). Mass-transfer analysis indicated surface wash-off as a mechanism of toxic elements release, and equilibrium leaching as a crucial parameter of S/S efficiency assessment. In the expected range of field soil pH after S/S (pH 7-9), the TCLP gave markedly different results than the multi-point pH equilibrium leaching method (using nine targeted pH values): up to 2953-, 94-, 483-, 1.3-, 27- and 1.5-times more Zn, Pb, Cu, As, Cd and Ni, respectively, was determined in the TCLP leachate. S/S with CAC reduced leachability of toxic elements more effectively than SRC. Our results indicate that, under given field conditions, the TCLP significantly underrates the efficiency of S/S of contaminated soil with cementitious binders.

Long-term leaching behavior of phenol in cement/activated-carbon solidified/stabilized hazardous waste

The long-term leaching behavior of phenol in solidified/stabilized (S/S) hazardous wastes cured for 28 d with different amounts of activated carbon (AC) was investigated using synthetic inorganic acid (H(2)SO(4):HNO(3) = 2:1, pH = 3.2), acetic acid buffer (HAc/NaAc, pH = 4.93), and deionized water as leachants to simulate the leaching of phenol in three exposure scenarios: acid-precipitation, co-disposal, and neutral-precipitation. Phenol immobilization was enhanced by AC adsorption and impaired by the growth of micropores with increasing amount of AC; thus the optimal added amount of AC to be to added S/S wastes was 2%. The leaching behavior of phenol in co-disposal scenario was unpredictable due to inadequate ionization of HAc in the HAc-NaAc buffer solution. The findings indicated that S/S products should be disposed of in hazardous waste landfills rather than municipal solid waste landfills.

Encapsulation, solid-phases identification and leaching of toxic metals in cement systems modified by natural biodegradable polymers

Cement mortars loaded with Cr, Pb and Zn were modified by polymeric admixtures [chitosans with low (LMWCH), medium (MMWCH) and high (HMWCH) molecular weight and hydroxypropylchitosan (HPCH)]. The influence of the simultaneous presence of the heavy metal and the polymeric additive on the fresh properties (consistency, water retention and setting time) and on the compressive strength of the mortars was assessed. Leaching patterns as well as properties of the cement mortars were related to the heavy metals-bearing solid phases. Chitosan admixtures lessened the effect of the addition of Cr and Pb on the setting time. In all instances, chitosans improved the compressive strength of the Zn-bearing mortars yielding values as high as 15 N mm(-2). A newly reported Zn phase, dietrichite (ZnAl(2)(SO(4))(4)·22H(2)O) was identified under the presence of LMWCH: it was responsible for an improvement by 24% in Zn retention. Lead-bearing silicates, such as plumalsite (Pb(4)Al(2)(SiO(3))(7)), were also identified by XRD confirming that Pb was mainly retained as a part of the silicate network after Ca ion exchange. Also, the presence of polymer induced the appearance and stabilization of some Pb(IV) species. Finally, diverse chromate species were identified and related to the larger leaching values of Cr(VI).