Removal of Cd2+ from water by Friedel's salt (FS: 3CaO x A12O3 x CaCI2 x 10H2O): sorption characteristics and mechanisms

A review of chloride ions removal from high chloride industrial wastewater: Sources, hazards, and mechanisms

To reduce metal pipe corrosion, improve product quality, and meet zero liquid discharge (ZLD) criteria, managing chloride ion concentrations in industrial wastewaters from metallurgical and chemical sectors has become increasingly important. This review provides detailed information on the sources, concentration levels, and deleterious effects of chloride ions in representative industrial wastewaters, and also summarizes and discusses various chloride ion removal techniques, including precipitation, ion exchange, physical separation, and advanced oxidation (AOPs). Among these, AOPs are particularly promising due to their ability to couple with other technologies and the diversity of their auxiliary technologies. The development of dechlorination electrode materials by electro-adsorption (CDI) can be inspired by the electrode materials used in chloride ion battery (CIB). This review also provides insights into exploring the effective combination of multiple chloride removal mechanisms, as well as the development of environmentally friendly composite materials. This review provides a theoretical basis and development direction for the effective treatment and secondary utilization of chlorine-containing industrial wastewater in the future.

Synthesis of layered double hydroxides from municipal solid waste incineration fly ash for heavy metal adsorption

The process of urbanization has resulted in a continuous growth of the production of municipal solid waste, consequently leading to the increase of municipal solid waste incineration fly ash (MSWI FA) over time. This has prompted the need for effective disposal and value-added utilization strategies for MSWI FA. In this study, a hydrothermal method was employed to synthesize CaAl layered double hydroxides (LDHs) using MSWI FA as the raw material. The main objective was to investigate how different synthesis parameters affect the crystallinity of the layered bimetallic hydroxides. Subsequently, the synthesized LDHs were characterized using various techniques such as BET, SEM, XRD, FT-IR, and XPS. The results revealed the presence of calcium and aluminum cations in the interlayer region of the synthesized material, with chloride ions, sulfate ions, and acetate ions being the predominant anions. Moreover, the formation of LDHs presents an effective approach for the self-purification of leachates derived from MSWI FA. The LDHs exhibited excellent adsorption capacity for Cd2+ and Cu2+ in wastewater, with maximum values of 730 mg·g-1 and 446 mg·g-1, respectively. The adsorption mechanisms involved isomorphous substitution, complexation, as well as the precipitation of hydroxides or interlayer anions. This method presents a novel approach for effectively utilizing MSWI FA to produce environmentally friendly value-added adsorbents.

Solidification/stabilization of highly toxic arsenic-alkali residue by MSWI fly ash-based cementitious material containing Friedel's salt: Efficiency and mechanism

Arsenic-alkali residue (AAR) and MSWI fly ash (MFA) are hazardous wastes, which still lack effective treatment methods. In this study, a novel solidification/stabilization (S/S) method for AAR with MFA-based cementitious material (MFA-CM) containing Friedel's salt was proposed. The efficiency and mechanism of S/S was mainly focused. Abundant Friedel's salt as well as a few C-S-H gel and ettringite (AFt) were found as hydration products of MFA-CM. 12% of AAR was well solidified/stabilized by MFA-CM, accompanied by As leaching concentration reducing from 10,687 mg/L to less than 5 mg/L. In order to investigate S/S mechanism of As, removal mechanism of As during co-precipitation synthesis of Friedel's salt was studied. During co-precipitation process, As was successively removed by formation of calcium arsenate precipitates, formation of As-Friedel's salt (replacement of Cl^- by AsO₄^3-), and adsorption of Friedel's salt. The S/S mechanism of As by MFA-CM was found to be similar to the removal mechanism of As during co-precipitation. With the prolonging of curing time, As was mainly solidified/stabilized by formation of calcium arsenate precipitates and As-Friedel's salt, and adsorption of Friedel's salt. Thus, this study provides a novel harmless treatment method for highly toxic arsenic-containing wastes by "treating the wastes with wastes".

Reactivity and Heavy Metal Removal Capacity of Calcium Alginate Beads Loaded with Ca–Al Layered Double Hydroxides

Layered double hydroxides (LDHs) present multiple applications due to their versatility and reactivity. Thus, Ca–Al LDHs with Friedel’s salt structure (HC) have been proposed as heavy metal scavengers due to their buffering capacity at basic pHs. Nevertheless, the control of the reactivity of LDHs such as HC is necessary to optimize their applications. Here, the reactivity of an HC prepared by a coprecipitation method was modified by its inclusion in calcium alginate (CaAlg) beads prepared by ionic gelation. The obtained beads (CaAlg/HC) showed good dispersion of the HC particles in the alginate matrix and were used to test the acid base reactivity and heavy metal uptake capacity compared with pure CaAlg beads and HC powder separately. The pH buffering capacity of CaAlg beads was enriched by the inclusion of HC that, in turn, was modulated in its reactivity. Thus, the HC dissolution times changed from mere seconds for the powder to tens of minutes when enclosed in the beads in a kinetic profile determined by the diffusive step. On the other hand, Cu2+ uptake capacity of CaAlg/HC beads combined the Cu(OH)2 precipitation capacity of HC with the complexation capacity of alginate, reaching good affinity and capacity for the obtained beads. Nevertheless, the precipitation of the hydroxide was produced outside the bead, which would induce the addition of an additional separation step to produce an acceptable Cu2+ elimination.

Cotreatment of MSWI Fly Ash and Granulated Lead Smelting Slag Using a Geopolymer System

Municipal solid waste incineration fly ash (MSWI FA) and granulated lead smelting slag (GLSS) are toxic industrial wastes. In the present study, granulated lead smelting slag (GLSS) was pretreated as a geopolymer precursor through the high-energy ball milling activation process, which could be used as a geopolymeric solidification/stabilization (S/S) reagent for MSWI FA. The S/S process has been estimated through the physical properties and heavy metals leachability of the S/S matrices. The results show that the compressive strength of the geopolymer matrix reaches 15.32 MPa after curing for 28 days under the best parameters, and the physical properties meet the requirement of MU10 grade fly ash brick. In addition, the toxicity characteristic leaching procedure (TCLP) test results show that arsenic and heavy metals are immobilized effectively in the geopolymer matrix, and their concentrations in the leachate are far below the US EPA TCLP limits. The hydration products of the geopolymer binder are characterized by X-ray diffraction and Fourier transform infrared methods. The results show that the geopolymer gel and Friedel's salt are the main hydration products. The S/S mechanism of the arsenic and heavy metals in the geopolymer matrix mainly involves physical encapsulation of the geopolymer gel, geopolymer adsorption and ion exchange of Friedel's salt.

Crystalline/Amorphous Blend Identification from Cobalt Adsorption by Layered Double Hydroxides

In this study, the adsorption behavior of CaAl-Cl layered double hydroxide (CaAl-Cl-LDH) with a controlled pH value (pH = 6) on Co(II) ions ([Co] = 8 mM) is investigated. The comprehensively accepted mechanism of cobalt adsorption on LDH is considered to be co-precipitation, and the final adsorbed products are normally crystalline Co-LDH. One unanticipated finding is that crystalline/amorphous blends are found in the X-ray diffraction (XRD) pattern of Co-adsorbed LDH. To shed light on the adsorption products and the mechanisms in the adsorption process of Co(II) in an aqueous solution by CaAl-Cl-LDH, a series of testing methods including Fourier-transform infrared spectroscopy (FT-IR), Scanning electron microscope (SEM), High-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma (ICP) are applied to clarify the interaction between cobalt and CaAl-Cl-LDH. According to the comprehensive analysis, the formation of the crystalline/amorphous blends corresponds to two adsorption mechanisms. The crystalline phases are identified as Co₆Al₂CO₃(OH)₁₆·4H₂O, which is attributed to the co-precipitation process occurring in the interaction between Co(II) and CaAl-Cl-LDH. The formation of the amorphous phases is due to surface complexation on amorphous Al(OH)₃ hydrolyzed from CaAl-Cl-LDH.

Experimental and theoretical approaches for Cd(II) biosorption from aqueous solution using Oryza sativa biomass

Biomass of Oryza sativa (OS) was tested for the removal of Cd(II) ions from synthetic and real wastewater samples. Batch experiments were conducted to investigate the effects of operating parameters on Cd(II) biosorption. Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive x-ray spectroscopy were used to examine the surface characteristics of the Cd(II)-loaded biomass. The maximum removal efficiency of Cd(II) was 89.4% at optimum pH 6.0, biosorbent dose 10.0 g L(-1), initial Cd(II) 50 mg L(-1), and biosorbent particle size 0.5 mm. The applicability of Langmuir and Freundlich isotherms to the sorbent system implied the existence of both monolayer and heterogeneous surface conditions. Kinetic studies revealed that the adsorption process of Cd(II) followed the pseudo-second-order model (r2: 0.99). On the theoretical side, an adaptive neuro-fuzzy inference system (ANFIS) was applied to select the operating parameter that mostly influences the Cd(II) biosorption process. Results from ANFIS indicated that pH was the most influential parameter affecting Cd(II) removal efficiency, indicating that the biomass of OS was strongly pH sensitive. Finally, the biomass was confirmed to adsorb Cd(II) from real wastewater samples with removal efficiency close to 100%. However, feasibility studies of such systems on a large-scale application remain to be investigated.

Succinate-bonded pullulan: An efficient and reusable super-sorbent for cadmium-uptake from spiked high-hardness groundwater

Chemically modified pullulan was evaluated for its sorption efficiency and selectivity to remove cadmium (Cd) from spiked high-hardness groundwater (GW). Pullulan esterified with succinic anhydride using dimethylaminopyridine showed a fairly high degree of substitution value as confirmed by (1)H NMR spectroscopy. Pullulan succinate (Pull-Suc) was converted into the sodium salt (Pull-Suc-Na). The effect of contact time (5-200min) and pH (2-8) on Cd-uptake by the sorbent (Pull-Suc-Na) was investigated. The sorbent showed more than 90% Cd-removal in first 15min from distilled water (DW) and GW solution, respectively. Comparison of Pull-Suc-Na with other polysaccharidal sorbents suggested its high efficiency (DW 476.2mg/g and GW 454.5mg/g) and selectivity for the removal of Cd by an ion exchange mechanism, which is further supported by the negative Gibbs free energy values calculated from Langmuir isotherms. A Langmuir isotherm kinetic model provided the best fit for the sorption of Cd using Pull-Suc-Na. The sorbent showed a negligible decrease in Cd-uptake over three regeneration cycles. The thermal stability testing of the sorbents indicated that Pull-Suc-Na (sorbent) is more stable than Pull-Suc.