Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
In an adsorption process substances from a fluid are attached onto a solid surface. Adsorption processes can be divided into physical adsorption, chemisorption and biosorption on biological materials which exhibits features of both physical and chemical interactions as well as metabolic uptake. Adsorption can be utilized in the treatment of mining influenced waters to remove pollutants such as metals, metalloids and nitrogen (ammonium). Adsorption as an active treatment technique is not yet established in full-scale for mine water treatment (Haanpää 2013) but possesses future potential.
Description of the technology
The central removal mechanisms in adsorption processes include ion exchange, chelation as well as precipitation of metals on the sorbent. The regeneration (desorption) of the sorbent is possible for some sorbent materials to allow metal recovery and repeated usage of sorbent. However, less attention in the R&D has been paid on this aspect. (Acheampong et al. 2009)
Removal of metals in AMD can be readily achieved by activated carbon adsorption. However, the price of activated carbon is high and there is a need to find low cost materials for AMD treatment. There are several studies suggesting alternative sorbents such as natural or synthetic zeolites, clay, natural clinker, fly ash, zero valent iron, agricultural wastes, microbial biomass, activated sludge, collophane and lignite. (Mohan & Chander 2006, Rios et al. 2008, Motsi et al. 2009)
Adsorption can also be applied to removal of nitrogen and arsenic from mine water. For arsenic removal sorption materials such as activated alumina (AA), granular activated carbon, granular ferric hydroxide, iron oxide coated sand and iron filings have been applied. In a process called ferrihydrite adsorption (or iron co-precipitation) a ferric salt is added to water with low iron concentration to form ferric hydroxide and ferrihydrite precipitates capable of adsorbing arsenic, selenium and metals from water. Sedimentation may be needed to separate the precipitates. (Acheampong et al. 2009, EPA 2014).
Zero valent iron (ZVI) can be applied to neutralize acid and to capture pollutants such as arsenic, selenium and radionuclides from mine water. Iron may be present in the treatment system in several different forms, e.g. filings, steel wool or micro-nano-scale particle. (EPA 2014)
Adsorption is technically well suited for the treatment of metal containing mine water.
General advantages of adsorption (e.g. García et al. 2013) include:
- Possibility for metal recovery
- Suitable for dilute solutions
- Some sorbent materials remove acidity from/produce alkalinity to the treated water
- Design, scale-up and operation are relatively simple
Disadvantages of adsorption:
- Adsorbent loses its efficiency over time
- Material need for mining water treatment may be large
- Requires disposal of spent sorbent and/or regeneration solutions
- Desorption of pollutants may not be successful
- The novel nanomaterials are still at development stage
High removal and recovery rates have been obtained for metals in lab/pilot scale (e.g. Mohan & Chander 2006, Rios et al. 2008, Motsi et al. 2009). However, the lack of full scale performance data is evident and limits the possibility of technology evaluation.
pH has a significant effect on adsorption process and the optimum for each sorbent/feed combination can be found. Sorption of metals is often declined at low pH. Other factors to be considered include temperature, initial metal concentration, presence of competing ions, sorbent dosage and particle size. Elevated ionic strength often reduces the metal sorption due to competing ions. The rate of adsorption varies by species. (Acheampong et al. 2009)
Acheampong, M.A., Meulepasa, R. J.W. & Lensa, P.N.L. 2009. Removal of heavy metals and cyanide from gold mine wastewater. J Chem Technol Biotechnol, 85: 590–613.
EPA 2014. Reference Guide to Treatment Technologies for Mining-Influenced Water. EPA 542-R-14-001.
García, V., Häyrynen, P., Landaburu-Aguirre, J., Pirilä, M., Keiski, R.L. & Urtiaga, A. 2013. Purification techniques for the recovery of valuable compounds from acid mine drainage and cyanide tailings: application of green engineering principles. J Chem Technol Biotechnol 89: 803–813.
Haanpää, K. 2013. Kaivosvesien hallinta ja käsittelymenetelmät. Pöyry Finland Oy. Kestävä kaivostoiminta –tutkimusseminaari. 21.11.2013, Mutku ry.
Mohan, D. & Chander, S. 2006. Removal and recovery of metal ions from acid mine drainage using lignite—A low cost sorbent. Journal of Hazardous Materials B137:1545–1553.
Motsi, T., Rowson, N.A. & Simmons, M.J.H. 2009. Adsorption of heavy metals from acid mine drainage by natural zeolite. International Journal of Mineral Processing 92:42–48.
Rios, C.A., Williams, C.D. & Roberts, C.L. 2008. Removal of heavy metals from acid mine drainage (AMD) using coal fly ash, natural clinker and synthetic zeolites. Journal of Hazardous Materials 156: 23–35.
Simate, G.S. & Ndlovu, S. 2014. Acid mine drainage: Challenges and opportunities. Journal of Environmental Chemical Engineering, 2:1785-1803.