Clean Water the Clean Way
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The main sources of mercury emissions to land, water and air are the processes of ores mining and smelting (in particular Cu and Zn smelting), burning of fossil fuels (mainly coal), industrial production processes (Hg cell chlor-alkali processes for the production of Cl and caustic soda) and consumption related discharges (including waste incineration).
This section provides details on the latest developments and efforts in general mercury removal from wastewater.
We have discussed the following:
- Current Wastewater Treatment Process - Mercury Removal
- New Technologies in Mercury Removal from Wastewater
- Blue PRO®
- NUSORB® MERSORB® Family of Adsorbents
- Removal of Mercury from Chloralkali Electrolysis Wastewater by a Mercury-Resistant Pseudomonas putida Strain
- Mercury Removal from Wastewater Using Porous Cellulose Carrier Modified with Polyethyleneimine
- Kinetics of Mercury Adsorption from Wastewater Using Activated Carbon Derived from Fertilizer Waste
- Removal of Mercury from Chemical Wastewater by Microoganisms in Technical Scale
Current Wastewater Treatment Process - Mercury Removal
Mercury can be precipitated to low levels using carbonate, phosphate or sulfide. When mercury is precipitated as the sulfide and high mercury residuals are often observed. This effect is due to the reduction of the mercury to the metallic mercury by the sulfide. Once in the metallic form, the mercury cannot form the insoluble sulfide. Metallic mercury is soluble in water at about 25 ug/l, which is above the regulatory limits. It may be visible as a lake floating on the surface of the reactor during the settling step. The residual mercury in the treated water must by oxidized to mercury 2 and then retreated to achieve low residual concentrations.
When treating mercury to form mercury phosphate, the oxidation step should be done prior to the precipitation step. Following the initial precipitation step, the residual phosphate must be precipitated by the addition of calcium ion. Mercury can also be removed by ion exchange. Again an oxidation step is required ahead of the ion exchange column and either a chelating resin or a mercury specific resin should be used. Once the resin is spent it is virtually impossible to regenerate, so the resin must be disposed of as a hazardous waste.
Mercury can be reduced to low concentrations by reducing agents. Granulated carbon is often used to polish treated mercury solutions with varying success. A multi-step process is required to reduce mercury concentrations to very low levels.
New Technologies in Mercury Removal from Wastewater
Blue Water’s patented Blue PRO® reactive filtration process is able to lower mercury to very small concentrations for relatively little capital and operating cost by employing multiple removal mechanisms in a high flow system. Blue PRO is able to accomplish co-precipitation and adsorption, overcome diffusion limitations within the moving sand bed, and also filter particulates. The process has been shown to have efficacy for mercury removal in both treatability studies and pilot-scale wastewater treatment projects. Recently Blue Water has successfully piloted Blue PRO for mercury removal at municipal wastewater plants. Third-party laboratory results confirmed that the 1.3 ppt total mercury goal was achieved by Blue PRO.1
NUSORB® MERSORB® Family of Adsorbents
The NUCON family of mercury removal adsorbents is available to remove mercury from gases, water and wastewater and liquid hydrocarbons. MERSORB®adsorbents for gas phase applications are available as coal based pellets. The pelleted carbon offers superior performance when compared with granular carbon especially in terms of more uniform bed packing which results in a lower pressure drop. The inherent hardness of the pelleted carbon provides for long bed life. The unique NUCON sulfur impregnation used to produce these adsorbents provides a technical advantage allowing their use for mercury removal from higher temperature streams then competitive products. MERSORB®LW adsorbent and Granular coal based version of this adsorbent are also available. These adsorbents provide a reliable and cost effective means to reduce the mercury content of water and waste water streams to below acceptable levels.2Removal of Mercury from Chloralkali Electrolysis Wastewater by a Mercury-Resistant Pseudomonas putidaStrain
A mercury-resistant bacterial strain which is able to reduce ionic mercury to metallic mercury was used to remediate in laboratory columns mercury-containing wastewater produced during electrolytic production of chlorine. Factory effluents from several chloralkali plants in Europe were analyzed.Pseudomonas putida Spi3, was isolated and biofilms of P. putida Spi3 were grown on porous carrier material in laboratory column bioreactors. The bioreactorswere continuously fed with sterile synthetic model wastewater or nonsterile, neutralized, aerated chloralkali wastewater. About 90 and 98% of mercury retention efficiency were obtained. Thus, microbial mercury removal is a potential biological treatment for chloralkali electrolysis wastewater. Similar reactions with pure cultures of seven other mercury resistant strains of Pseudomonas were also studied by Irene Wagner-Döbler et al.3
Mercury Removal from Wastewater Using Porous Cellulose Carrier Modified with Polyethyleneimine
An adsorbent for heavy metal was synthesized by introducing polyethyleneimine (PEI) into porous cellulose carriers. Evaluations of synthesis results and adsorbent properties were conducted. Elementary analysis of the adsorbent had revealed extensive crosslinking of PEI with the modified matrix. Batch adsorption tests showed the ability of cell-PEI to selectively remove mercury even at acidic regions. At low concentration ranges, mercury adsorption by cell-PEI can be interpreted by the Langmuir isotherm. With this model, an adsorbent capacity and Hg-ligand stability constant of approximately 288.0 mg g−1and 12.91 mg−1, respectively, were obtained. From adsorption rate experiments, diffusivity of Hg in the carrier was found to be approximately equal to 7.30 × 10−14 m2 s−1. Extensive crosslinking of PEI chains that restricts ligand mobility was cited as the foremost factor contributing to these observed properties.4
Kinetics of Mercury Adsorption from Wastewater Using Activated Carbon Derived From Fertilizer Waste
The waste slurry generated in a fertilizer plant was converted into a carbonaceous material and used as an adsorbent for the uptake of Hg (II) from wastewater. The kinetics of adsorption depends on the adsorbate concentration, and the physical and chemical characteristics of the adsorbent. Studies were conducted to delineate the effect of pH, temperature, initial absorbate concentration, particle size of the adsorbent and solid to liquid ratio. The adsorption of Hg (II) increased with the decrease in pH and the process was exothermic. On the basis of these studies, various parameters such as mass transfer coefficient, effective diffusion coefficient, activation energy and entropy of activation were evaluated to establish the mechanisms. It was concluded that the adsorption occurs through a film diffusion mechanism at low concentrations, and particle diffusion at higher concentrations.5
The enzymatic reduction of Hg(II) to water insoluble Hg(0) by mercury resistant bacteria has been used for removal of mercury from wastewater in technical scale. Pure cultures of seven mercury resistant strains of Pseudomonas were immobilized on carrier material inside a 700 L packed bed bioreactor. Neutralized chloralkali electrolysis wastewater with a mercury concentration of 3−10 mg/L was continuously fed into the Bioreactor (0.7 m3/h up to 1.2 m3/h). A mercury retention efficiency of 97% was obtained within 10 h of inoculation of the bioreactor. At optimum performance, bioreactor outflow concentrations were below 50 μg Hg/L, which fulfill the discharge limit for industrial wastewater. In combination with an activated carbon filter, outflow concentrations below 10 μg Hg/L were always obtained. The retention efficiency of the bioreactor was not affected by fluctuations in inflow conductivity (between 20 and 105 mS/cm), pH (between 6.5 and 7.5), or mercury concentration (between 3 and 10 mg/L) and was between 95% and 99%. Temperature increases up to 47 °C did not impair bioreactor performance. Standby periods up to 6 h could be tolerated without loss in activity. A simple, effective, and robust biotechnology for remediation of mercury polluted wastewater is thus demonstrated. 6
3Von Canstein H, Y. Li, Timmis K. N, Deckwer W.-D, and Wagner-Döbler I., 1999. Removal of Mercury from Chloralkali Electrolysis Wastewater by a Mercury-Resistant Pseudomonas putida Strain. Applied and Environmental Microbiology, 65 (5279- 5284).
4Ronald R. Navarro, Katsuhiro Sumi, Naoyuki Fujii and Masatoshi Matsumura, 1996. Mercury removal from wastewater using porous Cellulose carrier modified with polyethyleneimine. Water Research, 30 (2488-2494).
5Dines Mohan, Gupta V. K, Srivastava S. K and Chander S, 2000. Kinetics of mercury adsorption from wastewater using activated carbon derived from fertilizer waste. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 177 (169- 181).
6Irene Wagner-Döbler, Harald von Canstein, Ying Li, Kenneth N. Timmis, and Wolf-Dieter Deckwer, 2000. Removal of Mercury from Chemical Wastewater by Microoganisms in Technical Scale. Environ. Sci. Technol., 34 (21), pp 4628–4634