Study on the stability, transport behavior and OH− release properties of colloidal Mg(OH)2

Ammonia removal and simultaneous immobilization of manganese and magnesium from electrolytic manganese residue by a low-temperature CaO roasting process

A large amount of open-dumped electrolytic manganese residue (EMR) has posed a severe threat to the ecosystem and public health due to the leaching of ammonia (NH4+) and manganese (Mn). In this study, CaO addition coupled with low-temperature roasting was applied for the treatment of EMR. The effects of roasting temperature, roasting time, CaO-EMR mass ratio and solid-liquid ratio were investigated. The most cost-effective and practically viable condition was explored through response surface methodology. At a CaO: EMR ratio of 1:16.7, after roasting at 187 °C for 60 min, the leaching concentrations of NH4+ and Mn dropped to 10.18 mg/L and 1.05 mg/L, respectively, below their discharge standards. In addition, the magnesium hazard (MH) of EMR, which was often neglected, was studied. After treatment, the MH of the EMR leachate was reduced from 60 to 37. Mechanism analysis reveals that roasting can promote NH4+ to escape as NH3 and convert dihydrate gypsum to hemihydrate gypsum. Mn2+ and Mg2+ were mainly solidified as MnO2 and Mg(OH)2, respectively. This study proposes an efficient and low-cost approach for the treatment of EMR and provides valuable information for its practical application.

Colloidal magnesium hydroxide Nanoflake: One-Step Surfactant-Assisted preparation and Paper-Based relics protection with Long-Term Anti-Acidification and Flame-Retardancy

Enhancing the interfacial dispersion and suspension stability is crucial for magnesium hydroxide (Mg(OH)₂) nanomaterials in the long-term deacidification of paper-based cultural relics. However, because of the low specific surface area and the poor solvent compatibility of as-prepared large-sized Mg(OH)₂, it often tends to agglomerate and settle down during the usage and storage, that is harmful for paper protection due to its unevenly deacidification and nonuniformly distribution on paper cellulose. Herein, we propose a feasible preparation of colloidal Mg(OH)₂ ultrathin nanoflakes with high dispersion stability via a simple one-step surfactant-assisted strategy. The surfactant acts as both a structure-direct agent to confine the growth of Mg(OH)₂ with rich active sites and a surface modifier to enhance its solvent adaptability and dispersion stability, avoiding the common fussy procedure with additional steric stabilizer. Owing to the evenly interaction with free acid species therein and the uniformly distribution on the paper fiber as alkaline reserve, the as-obtained Mg(OH)₂ presents the superior paper protection performance characterized by its safer pH of 7.29 for the original aged paper (pH = 5.03) and the excellent long-term anti-acidification effect with competitive pH of 5.47 after accelerated-aging at 105 °C for 5 months. Furthermore, Mg(OH)₂ nanoflakes with surfactant-modified structure also endue them as an improved flame retardant for multifunctional paper protection. The protection with Mg(OH)₂ has little effect on the paper surface properties and cellulose crystallinity, in line with the principle of least intervention. This work will put forward a feasible way toward colloidal Mg(OH)₂ nanoflakes with excellent paper protection performance, shedding light on the development of emerging protection materials for paper-based cultural relics.

Viscosity modification enhanced the migration and distribution of colloidal Mg(OH)2 in aquifers contaminated by heavy metals

Mg(OH)₂ is extensively considered as an potential material for groundwater remediation because its injection could provide a long-term pH buffering system. In this study, colloidal Mg(OH)₂ was regarded as an alternative reagent for the in-situ remediation of heavy metal polluted groundwater. However, experiments demonstrated that the transport performance of colloidal Mg(OH)₂ in groundwater was depressed by the contamination of heavy metals and its stabilization performance for heavy metals was deteriorated. To solve these difficulties, the transport properties of colloidal Mg(OH)₂ was enhanced by viscosity modification by adding xanthan gum (XG). Column tests were conducted to investigate the transport performance of colloidal Mg(OH)₂ with and without viscosity modification, and to evaluate its stabilization effect for Pb and Cd polluted aquifer. Experimental results indicate that although the injection pressure increased during the migration of colloidal Mg(OH)₂, the increased viscosity effectively could decrease the intensity of Brownian motion of Mg(OH)₂ particles and reduce the collision efficiency between colloidal particles and aquifer media. Thus, deposition of Mg(OH)₂ particles on aquifer media significantly reduced after viscosity modification, and its migration performance in groundwater was effectively enhanced. In contrast, the distribution of colloidal Mg(OH)₂ was more uniform after viscosity modification, and immobilization of heavy metals in contaminated aquifer was noticeably improved, furthermore the exchangeable fraction of Pb and Cd is significantly reduced.

Transport and release of electron donors and alkalinity during reductive dechlorination by combined emulsified vegetable oil and colloidal Mg(OH)2: Laboratory sand column and microcosm tests

Emulsified vegetable oil combined with colloidal Mg(OH)₂ (EVO-CM) can slowly release electron donors and OH^- into groundwater and is therefore regarded as a promising amendment for enhanced in situ treatment of chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE). However, its migration ability in different porous media and the simultaneous release of electron donor and pH buffer during enhanced reductive dechlorination (ERD) have never been evaluated in detail. In this study, EVO-CM with uniform drop size and desirable stability was prepared. Laboratory-scale column tests were conducted to investigate the transport and spatial distribution of the EVO-CM in different porous media. Batch microcosm experiments were carried out to study the dechlorination efficiency under different EVO:Mg(OH)₂ ratios. Experimental results indicate that prepared EVO-CM emulsions can be transported effectively through different porous media with grain size ranging from 0.1-1.0 mm. The emulsified vegetable oil (EVO) and colloidal Mg(OH)₂ showed synchronous movement through the porous media column systems, providing both electron donor and alkalinity at the same location. The retention degree of EVO-CM is greater in the finer grained media and decreases with increasing distance from the inlet. The injection of EVO-CM lead to a significant mobilization of TCE in the column. The reductive dechlorination of TCE in the microcosms was remarkably enhanced in the presence of EVO-CM. The coaddition of sufficient colloidal Mg(OH)₂ effectively limits the deleterious pH decline caused by acid release. The microcosm achieves an optimum dechlorination efficiency when the EVO:Mg(OH)₂ ratio is 1:1.