Are indigenous sedges useful for phytoremediation and wetland rehabilitation?

Abstract

Toxic metal pollution is a side effect associated with mining (van der Merwe et al., 1990), sewage pollution (Muchuweti et al., 2006) and agricultural practices (Jadia and Fulekar, 2009). South Africa, a rapidly developing country, increasingly faces this kind of pollution in catchments such as the Olifants River (Dabrowski et al., 2010). Future development plans of the mining industry, such as in the Waterberg area (Schachtschneider et al., 2010), will put additional catchments at risk of metal contamination. It is necessary to investigate options for remediation in already polluted catchments, and to properly plan for future developments. Phytoextraction is emerging as a cost-effective and long-term method of removing metal pollutants from soil, especially in widespread areas with low to medium contamination levels (US EPA, 2000; Singh and Jain, 2003; Paquin et al., 2004; Jadia and Fulekar, 2009). In phytoextraction, selected plants accumulate and stabilise metals in their tissue, making it possible to harvest and sometimes even use them as a metal resource (USEPA, 2000; O’Niell and Nzengung, 2004). Plants are reported to accumulate up to 5% of their dry weight in nickel (Baker, 1995). Shoot accumulators are favoured for harvesting and removal from site (USEPA, 2000). Several wetland plant species have been investigated internationally for their accumulative properties, including Lemna minor, Eichhornia crassipes, Typha latifolia and Typha capensis, Juncus effusus, Cladium mariscus, Arundo donax and Phragmites australis (van der Merwe et al., 1990; USEPA, 2000; Deng et al., 2004; Komosa et al., 2006). Their introduction into catchments for phytoextraction purposes may however pose a whole set of alien invasive problems. Hence it is valuable to investigate the accumulation capacity of species indigenous to contaminated areas. Sedges (Cyperaceae) are typical wetland plants that are still understudied in South Africa, and hence their potential as phytoextractors is unestablished. The aim of this brief, ongoing study is to determine whether sedges and other wetland graminoid species, common to the wider Limpopo catchment, accumulate the metals Al, Fe, Mg and Mn – which are common wetland pollutants in northeastern parts of South Africa. Plant specimens were selected from the relatively pristine Mokolo and Lephalale rivers as well as the heavily polluted Olifants catchment. The sedges Schoenoplectus corymbosus and Cyperus haspan were collected, as well as Phragmites australis and Juncus effusus. The latter two species have previously shown to accumulate metals (Pb, Zn, Cu) (Deng et al., 2004) and also occur naturally in the area.