Investigation on stabilization/solidification characteristics of lead-contaminated soil using innovative composite model of cement and soda residue

Abstract

The cement solidi cation/stabilization method of heavy metal contaminated soils has been promoted in engineering practice and applied on a large scale for site remediation, but it still reveals some scienti c problems in the current complex and variable global extreme climate. To solve these problems and explore cement-based soil remediation technology, this study used the waste soda residue produced in large quantities in the "ammonia-soda process" as a composite additive, and established an innovative composite model of cement and soda residue by adding different ratios, which was applied to the remediation experiments of lead-contaminated soil. The innovative composite model solidi cation/stabilization of cement and soda residue for uncon ned compressive strength and toxic leaching properties under different soil environmental conditions were investigated. Moreover, curing and leaching mechanisms are discussed, and future industrial practice was evaluated. The results showed that the addition of soda residue improved the early (20 days) uncon ned compressive strength (UCS) of the composite curing agent for lead-contaminated soil by an average of 23.1% Mpa.When the percentage of soda residue composite was 40%, the UCS strength was 0.96 Mpa, which reached the maximum. The concentration of Pb 2+ in the leachate of the cement-soda residue composite curing agent was greatly reduced (average 3.28 times) compared with that of a single cement in the same situation, with an average leached Pb 2+ concentration of 1.87 mg•L -1 . This indicates that the addition of alkali residue improved the curing effect. The curing mechanism was divided into four steps, mainly a complex physicochemical reaction between the cement-soda residue composite and soil particles. The leaching mechanism of cement-soda residue to aqueous solution is mainly the consumption of acid ions by alkaline substances. This study will provide scienti c data to support potential lead-containing soil in site remediation technologies and future large-scale engineering applications. Highlights 1. The cement and soda residue were compounded in a direct homogeneous mixture.2. The UCS, TCL, curing and leaching mechanisms were researched and discussed.3. The doping ratios of S b /S d =20% and S s /S b =40% have the best curing effect. 4. cement-soda residue composite reduces the leaching of Pb 2+ by 3.28 times, i.e. 1.87 mg•L -1 . 5. The curing mechanism was divided into four steps, mainly a complex physicochemical reaction.

Research status of soda residue in the field of environmental pollution control

“…Hydration refers to the primary reaction that occurs when alkali residue is introduced into the soil and persists throughout the soil remediation process. 68 Complex chemical reactions take place between the soil and alkali residue components, such as CaCO 3 , Ca(OH) 2 , SiO 2 , and Al 2 O 3 , generating several hydration products, 69 as shown in eqn ( 23)- (28). The formation of various hydration products contributes to soil compaction, improves the leaching characteristics, and enhances the adsorption and encapsulation of the hydration products.…”

“…Hydration refers to the primary reaction that occurs when alkali residue is introduced into the soil and persists throughout the soil remediation process. 68 Complex chemical reactions take place between the soil and alkali residue components, such as CaCO 3 , Ca(OH) 2 , SiO 2 , and Al 2 O 3 , generating several hydration products, 69 as shown in eqn (23)–(28). The formation of various hydration products contributes to soil compaction, improves the leaching characteristics, and enhances the adsorption and encapsulation of the hydration products.CaCO 3 + Ca(OH) 2 → CaCO 3 ·Ca(OH) 2 CaCO 3 + 2CaO + SiO 2 + ( n + 1)H 2 O → CaSiO 3 ·CaCO 3 ·Ca(OH) 2 · n H 2 OSiO 2 + Ca(OH) 2 + n H 2 O → CaO·SiO 2 ·( n + 1)H 2 OAl 2 O 3 + Ca(OH) 2 + n H 2 O → CaO·Al 2 O 3 ·( n + 1)H 2 OCa(OH) 2 + SiO 2 + n H 2 O → CaO·SiO 2 ·( n + 1)H 2 O2Ca(OH) 2 + CaCO 3 + SiO 2 + n H 2 O → CaSiO 3 ·CaCO 3 ·Ca(OH) 2 ·( n + 1)H 2 O…”