Magnesium Oxide to Remove Metals in Water
Conventional practice for treating mine drainage and other water sources contaminated with heavy metals is lime precipitation and settling of the hydrous oxides. This produces a voluminous toxic sludge that results in another type of disposal problem. In addition, settling of the precipitated hydrous oxides often does not produce sufficiently pure water to meet statutory limits for discharge, so the decant has to be polished by filtration through sand or other granular media. Metal hydroxides are difficult to filter because of their small size and their low resistance to the hydraulic shear forces encountered in conventional granular filters. Flocculation with organic polyelectrolytes is often necessary to achieve efficient filtration.
Magnesium oxide (MgO) is similar to lime (CaO) and can be used analogously to precipitate heavy metals. Although MgO is less soluble than lime at high pH, in acidic to slightly alkaline water it provides more neutralization capacity per unit weight than lime, owing to its lower molecular weight. When a stoichiometric excess of MgO is used to precipitate heavy metals from wastewater, the resulting sludge is up to 4 times more compact than that produced by liming. This was attributed to the unique positive electrokinetic charge of the MgO surface at neutral and slightly basic pH, in contrast to the negative surface charge of most metal hydroxides. The resulting electrostatic attractive force causes the fresh heavy metal precipitate to cement, or adhere to, the MgO surface and expel water molecules from the spaces between the precipitated particles, giving a denser solid.
Forms of Magnesia
MgO is often obtained from seawater or other brines that are rich in MgCl2. Lime is added to the brine to produce Mg(OH)2 precipitate and CaCl2 brine. The precipitate is dehydrated by calcining to produce MgO. Depending on the temperature and duration of the calcination, a multitude of MgO products can be obtained with different properties and reactivities. By calcining at lower temperatures (<700° C), much of the original porosity associated with the crystal structure of Mg(OH)2 is retained and a very active magnesia is produced. By calcining at higher temperatures (“dead burning”), the MgO is fused, the porosity is lost, and a tough, relatively inert, crystalline material is produced.
Periclase is a natural magnesium oxide mineral sometimes found in marble. It easily alters to brucite or hydrous magnesia, MgO·H2O, which is one of the many sources of dead-burned magnesite. Granular, dead-burned MgO has been demonstrated to be an efficient deep-bed filter medium for removing particulates flocculated with aluminum salts. In previous research, pure MgO powder was used as a precipitant to remove heavy metals. One objective of this research was to find whether granular periclase can also precipitate heavy metals, thereby achieving both precipitation and filtration steps in the same bed. It was found that the granular filter material has sufficient activity to raise the pH of unbuffered water several units. At typical flow rates in deep-bed filters the effluent can have a pH of 10 or greater when the influent pH is about 7. This increase in pH is enough to cause heavy metals to precipitate as oxides or hydroxide as they pass through the filter.
These precipitates were observed to be quite strongly attached to the MgO granules. One objective of this study was to determine if the presence of dissolved heavy metals in the influent would impair the ability of the MgO to filter suspended solids. Early results indicated that significant metals removal occurred in the filter, so the second objective was to measure the metals removal capacity of the MgO and to test the feasibility of removing both suspended solids and dissolved metals in the same bed. The third objective was to determine the feasibility of recovering these metals by eluting them from the loaded MgO bed with either chelating agents or acids.
Electrical Phenomena at Interfaces
The surface charge on metal oxides or hydroxides in water is pH dependent and is described by Lippmann’s equation:
where σ is the surface charge density, i.e., charge per unit surface area, γ is the interfacial tension, and E is the interfacial potential. The derivation of equation 1 is given in the appendix.
The electrocapillary curves in figure 1 show the variation in surface charge with respect to pH (or interfacial potential) for a silicate mineral such as amphibole asbestos and for MgO. The maximum of each curve corresponds to a surface charge density of zero, and the pH resulting in zero net surface charge is called the zero point of charge (ZPC). The surface charge of a metal oxide is therefore dependent on the pH and moves from positive to negative with increasing pH, becoming zero at some intermediate pH. A method was developed
to remove asbestos fibers from water, at pH intermediate to the ZPC of MgO and asbestos, based on the electrokinetic attractive forces between the unlike surface charge on asbestos and MgO.
The ZPC values vary from oxide to oxide and correspond to the pH of minimum solubility of each metal oxide. In general, oxides with cations in higher oxidation states are more acidic and will have a lower ZPC. The ZPC is mainly dependent on the ratio of charge (Z) to radius (R) of the cation in the pure solid. In a simple electrostatic model the ZPC decreases with increasing ionic potential, Z/R, and corrections for crystal field effects (coordination number) are made to refine the model. The ZPC’s of various metal oxides were compiled by Parks and are shown in table 1. Depending on the measurement method employed and the…
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