Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
Iron co-precipitation is a known process for arsenic, metals and selenium removal. The technology has been implemented at full scale in various industries, including the mining sector, for arsenic and selenium removal. (MSE 1998, CH2M Hill 2010, EPA 2014) Iron co-precipitation (or ferrihydrite adsorption) is a relevant mechanism also in many passive treatment technologies such as wetland treatment.
Description of the technology
Iron co-precipitation or ferrihydrite adsorption can be used to remove arsenic, metals and selenium from mining waters that have low initial iron concentrations. When iron is added to aqueous solution as ferric salt such as FeCl3 or Fe2(SO4)3 it reacts to form ferrihydrite (Fe2O3·H2O) which is a crystalline form of ferric hydroxide precipitates. Ferrihydrite may be present with variable levels of water of hydration and thus appears with several chemical formulas. (CH2M Hill 2010, EPA 2014)
Simultaneous precipitation (co-precipitation) of certain pollutants takes place together with iron hydroxide. Adsorption is a central mechanism in the process. The floc formation can be enhanced by using polymer flocculants. The precipitates are removed mechanically, e.g. by gravity sedimentation or filtration. (Kauppila et al. 2011, EPA 2014)
In arsenic removal oxidation of As(III) to As(V) is often required in order to improve precipitation and the stability of resulting sludge. Excess iron can also be used to improve the sludge stability. A two-step process is preferred for As removal. In the first precipitation step at pH 4-6 ferric arsenate will form. This is preferred, since the co-precipitation as ferric arsenate results in more stable sludge compared to adsorbed arsenic. In the second precipitation step metals and remaining arsenic are precipitated. (Aube 2004, Kauppila et al. 2011)
For selenium removal pH adjustment is typically also needed, as selenium is best removed at pH 4-6 (EPA 2014).
Iron co-precipitation is applicable for arsenic, selenium and metals removal when the water contains limited initial concentrations of iron.
Advantages of iron co-precipitation (MSE 1998) include e.g.:
- Relatively simple process
- Proven technology with full scale applications
Disadvantages of iron co-precipitation (EPA 2014) are e.g.:
- Process generates sludge requiring appropriate disposal
- Stability of ferrihydrite precipitates is uncertain
- High operational costs
The technology has been applied at industrial scale. The results show that iron co-precipitation is most successful in arsenic removal but also removes selenium and metals. (EPA 2014)
A pilot study by USEPA at Kennecott Utah copper Corporation´s Garfield Wetlands-Kessler Springs site showed that the technology can effectively reduce selenium concentrations (from 1950 µg/l to 90 µg/l with iron dosage 4,800 mg/l). Large-scale application at this site was limited by the high treatment costs. The technology still lacks long-term evidence on selenium removal to regulatory discharge level (5 µg/l). Still, USEPA considers ferrihydrite adsorption as a Best Demonstrated Available Technology for arsenic and selenium removal. (MSE 1998, EPA 2014)
Aube, B. 2004. The Science of Treating Acid Mine Drainage and Smelter Effluents
CH2MHill 2010. Review of available technologies for the removal of selenium from water. Final Report. North American Metal Council. June 2010.
EPA 2014. Reference Guide to Treatment Technologies for Mining-Influenced Water. EPA 542-R-14-001.
Kauppila, P., Räisänen, M.L. & Myllyoja, S. 2011. Best Environmental Practices in Metal Ore Mining. Finnish Environment 29 en/2011.
MSE 1998. Final report—Arsenic removal demonstration project mine waste technology program. Activity iii, Project 9. MSE Technology Applications, Inc. Prepared for: U.S. Environmental Protection Agency. April 1998.