Phytoremediation of theevaporation ponds at Impala’srefineries in Springs

Impala’s refineries, SpringsImpala’s refineries, Springs

The Implats' Enhanced Evaporation Spray System (EESS) ponds, which are part of the refining operations in Springs are situated a short distance from the Blesbokspruit in the Ekurhuleni Metropolitan Municipality (EMM), one of the most heavily industrialised areas in South Africa. The Blesbokspruit leads into a wildfowl conservation site of international importance under the RAMSAR convention. The soil surrounding the evaporation ponds is degraded and polluted by multiple metal contaminants from the ponds, and requires a succession of treatments for site cleaning and rehabilitation.

Physico-chemical remedies (soil excavation, haulage and burial, or soil washing) were discounted as rehabilitation options owing to high risk, cost and disturbance factors which would have led to further degradation of the area. However, plant-based (phyto) technologies are able to provide in-situ and simultaneous decontamination and rehabilitation of sites. Phytotechnology is a broad concept, which involves the use of plants, algae, soil micro organisms and even non-living biomass to improve the quality of a substrate. Organisms that can tolerate toxic conditions are used in order to degrade, transform, sequester, immobilise or extract pollutants from soil and water. All plants use water which allows them to consume seepage or exert hydraulic control to varying degrees depending on growth form and evapotranspiration rate.

The phytoremediation project is being implemented for Implats by the University of the Witwatersrand, Johannesburg's (Wits) Ecological Engineering and Phytotechnology programme in the School of Animal, Plant and Environmental Sciences (APES). After characterising the EESS site and surroundings, phytoremediation was selected as the technology of choice. Trees have also been used for hydraulic control to prevent leaching of contaminants and to help control movement of any existing plume. Phytoremediation involves the uptake of salts and metals into leaves of specialised plants in order to be cleansed from soil and water. It is through this process that the plants will concentrate contaminants into smaller packages for disposal. The biomass is then harvested for proper disposal or further processing, such as ashing or leaching for metal recovery.

Biotechnologies in general, including phytotechnologies, are lower-risk, lower-cost and in situ processes for site decontamination in comparison to other methods that rely on physical excavation and chemical or thermal methods. Globally, the cost of non-biological technologies for contaminated soils ranges from US$10 to US$4 000 per cubic metre, or from US$100 000 to US$3 million per ha. In contrast, phytotechnology costs range from US$0.02 to US$10 per cubic metre, or US$200 to US$100 000 per ha (Weiersbye, 2007). Estimates from the United States of America indicated that excavation of metal contaminated soil from a one hectare area to a depth of 45cm would result is approximately 5 000 tonnes of soil for treatment or land filling, whereas the use of metal-hyperaccumulator plants retains soil in situ, and results in a mere 25 to 35 tonnes of plant ash for disposal or metal recovery (Ensley, 2000). However, phytotechnologies are slower than physical excavation methods, and the time taken for rehabilitation depends largely on plant type, climate, substrate and the contaminant chemistry.

Over the last 17 years, the Wits research programme has been testing plants and micro organisms from a range of polluted soils or specialised conditions and propagating suitable strains for use in rehabilitation of polluted mine land and tailings. The plants selected are indigenous, and highly tolerant to pollution and local climate conditions. Overall, the EESS pond decontamination and rehabilitation programme will utilise three species of salt- or metal-accumulating plants (a tree and two small bushes), and three non-accumulating tree species which are indigenous to the region.

The EESS ponds phytoremediation project commenced in late 2007, with the first stages of site preparation and planting conducted midway through summer 2008 and into 2009. An area of some 3.5 hectares surrounding the EESS ponds was planted with 4,517 trees.

The project has also provided employment opportunities in local communities and mine labour-sending areas. Site preparation, planting and maintenance is being done by vegetation contractor EMPR Services (Pty) Ltd, who train local community labour to conduct these operations. The same team will be trained in coppice and harvesting techniques. All plants were produced by two community nurseries, Modula Qhowa and Green Eden, specifically for the EESS ponds from selected seeds and cuttings. Sepate, a community nursery in Kungwini, has recently been partnered with the project and will also produce plants for the next growing season.

The first stage of decontamination at the EESS site involves reducing the salt levels so that a wider suite of plants can then be used to remove the metals present in the soil. The salts (sulphates and chlorides) and some metals are being extracted by a specialised tree species which is able to accumulate salts and excrete the crystals from its leaves. The tree will be routinely harvested at intervals for salt and metal removal and be allowed to re-sprout. It is being inter-cropped with non-accumulating evergreen tree species tolerant to the site conditions in order to ensure continuous hydraulic control and minimise leaching of contaminants to groundwater or run-off to the Blesbokspruit. Once soil levels have declined other metal hyper-accumulating plants will be planted to extract selected metals.

Two general approaches for the disposal of the metal-contaminated biomass will be compared. The first involves harvesting the leaves and disposing of the compacted leaves or ash directly in a registered disposal site. The second will assess recovery of salts and some metals from the leaves in order to develop safe artisanal phyto-mining techniques that can be used by mines as a combined approach to job creation, legacy site rehabilitation, metal recovery and in some cases plant products (fuel wood, essential oils, etc) or compost production. The type and potential value of the contaminants extracted in the plant leaves will also determine whether any revenue can be generated through phyto-mining of the EESS soil.

Once the levels of salts and metals in the EESS site soils have declined to acceptable levels, the accumulating plants will begin to die back, as they cannot survive without the contaminant elements in the soil. Consequently, evergreen tree species that are indigenous to the local area are being inter-cropped on site. These in time will grow into a series of thick hedges to provide long-term hydrological control around the EESS ponds in order to minimise run-off to the Blesbokspruit, leaching to groundwater and dust.

The plants, soils and soil-water are being routinely monitored by Wits for performance and chemistry. The tree water-use measurements commence in 2010. Salt excretion is already visible as a "snow" on the leaves of the salt-accumulating tree, and has been identified as mainly containing gypsum using electron microscopy of the crystals. Gypsum is a relatively inert substance (calcium sulphate) with low solubility and low environmental risk.

The first harvests of branches and leaves are expected to commence in late 2010 or 2011 depending on plant growth rates. After harvest the contaminated biomass will be assessed for metal recovery and composting of residue, or, if not economic for proper disposal. The plants resprout after the harvest (coppice) in order to extract further amounts of salts and metals into their leaves. The site is fenced and secure and is under surveillance to prevent trespassers and access by animals.

Reference

Weiersbye, I.M. (2007) Global review and cost comparison of conventional and phyto-technologies for mine closure. Plenary paper in A.B. Fourie, M. Tibbett and J. Wiertz (eds), Mine Closure 2007 - Proceedings of the 2nd International Seminar, Santiago, Chile. Publ. Australian Centre for Geomechanics and the University of Western Australia, Perth, ISBN 978-0-9804 185-0-7, pp 13-31

Ensley, B.D. (2000) Rational use for phytoremediation. In: I. Raskin and B.D. Ensley (eds). Phytoremediation of toxic metals: using plants to clean-up the environment. John Wiley and Sons Inc., New York, pp. 3-12.

Implats Sustainable Development Report 2009