Wastewater metal removal

Hard-burned magnesias may be used in a variety of appHcations such as ceramics (qv), animal feed supplements, acid neutralization, wastewater treatment, leather (qv) tanning, magnesium phosphate cements, magnesium compound manufacturing, fertilizer, or as a raw material for fused magnesia. A patented process has introduced this material as a cation adsorbent for metals removal in wastewater treatment (132). 

Chemical Oxidation. Chemical oxidation can be appHed ia iadustrial wastewater pretreatment for reduction of toxicity, to oxidize metal complexes to enhance heavy metals removal from wastewaters, or as a posttreatment for toxicity reduction or priority pollutant removal. 

Chemical precipitation can remove 95 percent of the suspended solids, up to 50 percent of the soluble organics and the bulk of the heavy metals in a wastewater. Removal of soluble organics is a function of the coagulant chemical, with iron salts yielding best results and lime the poorest. Metal removal is primarily a function of pH and the ionic state of the metal. Guidance is available from solubihty product data. 

Clay - The use of elay based floeculating agent(s) in eonjunetion with a strong metal preeipitator has proven sueeessful in many wastewater treatment applieations where the objeetives are aimed at metals removal. Clay based floeeulants eleans the wastewater and in some eases replaees multistage conventional treatment system and saves the traditional operational difficulties of treatment with several chemicals such as metal hydroxide precipitation, coagulant, floeeulants and other methods. Commercial clay-based floeeulants usually consist of bentonite and other... 

Biosorption is a process that utilizes biological materials as adsorbents [Volesky, 1994], and this method has been studied by several researchers as an alternative technique to conventional methods for heavy metal removal from wastewater. 

Granular bed filters are used in ten coil coating plants to remove residual solids from the clarifier effluent, and are considered to be tertiary or advanced wastewater treatment. Chemicals may be added upstream to enhance the solids removal. Pressure filtration is also used in this industry to reduce the solids concentration in clarifier effluent and to remove excess water from the clarifier sludge. Figure 7.4 shows a granular bed filter and Table 7.13 presents the heavy metal removal data of a lime clarification and filtration system. 

Option 1 system consists of hydroxide precipitation3 4 5 6 7 8 12 followed by sedimentation.13 This system accomplishes the end-of-pipe metals removal from all common metals-bearing wastewater streams that are present at a facility. The recovery of precious metals, the... 

Metal removal in SSFCWs has been recently focused on metal elimination from synthetic water and different wastewaters,66-86 on the evaluation of the effects of season, temperature, plant species, and chemical oxygen demand (COD) loading on metals removal,87 and on the accumulation of metals in wetland plant species and sediments.88-89 Recent reviews on heavy metal phytoremediation wetlands are also available.48... 

Maine, M.A., Sune, N., Hadad, H., Sanchez, G., and Bonetto, C., Nutrient and metal removal in a constructed wetland for wastewater treatment from a metallurgic industry, Ecological Engineering, 26, 341-347, 2006. 

In a separate study, Igwe and Abia46 determined the equilibrium adsorption isotherms of Cd(II), Pb(II), and Zn(II) ions and detoxification of wastewater using unmodified and ethylenediamine tetraacetic acid (EDTA)-modified maize husks as a biosorbent. This study established that maize husks are excellent adsorbents for the removal of these metal ions, with the amount of metal ions adsorbed increasing as the initial concentrations increased. The study further established that EDTA modification of maize husks enhances the adsorption capacity of maize husks, which is attributed to the chelating ability of EDTA. Therefore, this study demonstrates that maize husks, which are generally considered as biomass waste, may be used as adsorbents for heavy metal removal from wastewater streams from various industries and would therefore find application in various parts of the world where development is closely tied to affordable cost as well as environmental cleanliness.46... 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

HPT Research, Inc., has developed the ionic state modification (ISM) process for the treatment of acid mine drainage (AMD). ISM is an ex situ treatment technology that uses magnets, electricity, and proprietary chemical to precipitate heavy metals, remove sulfate ions, and neutralize acidity from AMD and industrial wastewaters. The end products of the process are a metal hydroxide sludge, a calcium sulfate sludge, and treated liquid effluent. The vendor claims that the metal hydroxide sludge may have some value as an ore, the calcium sulfate may be used as an agricultural additive to soils, and the liquid effluent is free of metal contamination and has low sulfate concentrations. 

RO is widely used for desalinization of brackish water to produce a potable water source. Special membranes have been developed for industrial uses and for purifying wastewater. Metal compounds are readily removed. RO is a commercially mature technology available for many special applications including the treatment of process water from metal finishing, pulp and paper, semiconductor, and electroplating industries. 

The Unipnre Environmental, Unipnre process technology is a unique iron co-precipitation method for removal of heavy metals from waste streams or groundwater. It can act as a primary metal-removal system or as a polishing step to an existing treatment system. The reactor mod-nle replaces the nentrahzation tank in a conventional wastewater treatment system. The process prodnces solids that are extremely insolnble in water and mild acid solutions. 

Iron (III) hydroxide waste Especially for heavy metals removal from wastewater, iron (III) hydroxide waste and waste slurry from the fertilizer industry, xanthate, rice husk, carbon, and coconut shell have been studied and can be considered as alternatives. 

Metal removal from surface water, groundwater or wastewater streams is more straightforward than that from soils. Typically, removal is achieved by concentration of the metal within the wastestream using flocculation, complexation, and/or precipitation. For example, the use of lime or caustic soda will cause the precipitation and flocculation of metals as metal hydroxides. Alternatively, ion exchange, reverse osmosis, and electrochemical recovery of metals can be used for metal removal (Chalkley et al., 1989 Moore, 1994). Unfortunately, these techniques can be expensive, time-consuming and sometimes ineffective, depending on the metal contaminant present. 

Metal reclamation from acid mine drainage and contaminated surface- and groundwater and wastewaters has been extensively studied. Technologies for metal removal from solution are based on the microbial—metal interactions discussed earlier the binding of metal ions to microbial cell surfaces the intracellular uptake of metals the volatilization of metals and the precipitation of metals via complexation with microbially produced ligands. 

Lime or caustic precipitation is the most commonly used method for treating aqueous wastes with non-complexed metals. Alkaline reagents such as lime, or caustic raises the pH of the wastewater and causes metals to precipitate out of the solution as metal hydroxides. For many metals there is a specific pH at which the metal hydroxide is least soluble, as shown in Figure 1. Because several metals co-exist in a waste in most cases, it is not possible to operate a treatment system at a single pH value that is optimum for all metal removals. As a compromise, a pH between 9.5 and 12.0 is maintained for typical mixed metals removal. A typical precipitation reaction is as follows ... 

Wastewaters containing complexed metals with a strong complexing agent such as EDTA, ammonia, or citrates require a two step precipitation for the metal removal. A continuous process using ferrous sulfate or ferrous chloride is as follows ... 

Zero-valent iron can also be used for heavy-metal removal. Applications for cadmium and chromate removal have already been shown to be successful. When combined with chloride ions, iron has been shown to be a simple and inexpensive method to remove mercury from wastewater (Grau and Bisang, 1995). Nitrates also degrade in the presence of zero-valent iron, but the application of treating nitrate-contaminated water has not been extensively studied (Siantar et al., 1996). 

A potential and very attractive practical application of reduction by semiconductor photocatalysis technology is the removal of harmful toxic metals and the recovery of noble metals in wastewater. Metal species, such as Hg(II), Pb(II), Cd(II), Ag(I), Ni(II) and Cr(VI), are generally nondegradable and they are very toxic when present in the environment. 

The application of natural zeolites in heavy metal removal is described now. The methodology consists of the development of a process for heavy metal removal from wastewater using dynamic ion exchange in natural zeolite columns [38,53],... 

For designing a canister system for heavy metal removal (see Figure 7.12) [38,53], a simple phenomenological description of dynamic ion exchange in zeolite bed reactors was worked out, which allows for the design of modular canister ion-exchange bed reactors for applications in heavy metal removal from wastewater. 

---