Improving phytoremediation efficiency in metal-polluted wastes

Human activities are responsible for the release of pollutants into the environment, and soil contamination by metals and metalloids is of primary concern. Currently, the laws covering environmental management and protection of several countries are focusing on the less expensive and more environmentally friendly biological methods of restoration, compared with the more invasive physico-chemical ones. The present study regards the first attempt in Italy to apply in situ phytoremediation of a metal-polluted, markedly degraded area located at Torviscosa (Udine, NE Italy) and included in the national priority list of sites for restoration. Within the site, pyrite cinders, derived from mineral roasting for sulphur extraction, had been severely contaminated by various metals such as arsenic, cadmium, copper, lead and zinc. The wastes were discharged in the Thirties and covered with an unpolluted 0.15-m layer of gravelly soil. Because of the particular substrate composition, poor physical structure, nutrient deficiency, and extreme hydrological conditions, the site was colonised by a rare and inadequate vegetation cover. Application of phytoremediation with biomass species required particular care in soil management and adjustment of the cultivation method. In this regard, ploughing was compared with subsoiling, in order to dilute the cinders with the unpolluted top layer and to maintain a shallow favourable layer for plant establishment, respectively. In situ, trials were carried out to compare woody species (Populus alba L., P. nigra L., P. tremula L., Salix alba L.) and crops (Helianthus annuus L., Lolium multiflorum Lam., Medicago sativa L., Raphanus sativus L. var. oleiformis Pers.) in terms of above-ground productivity and metal accumulation, and to establish the role of root colonisation. Greater root lengths were generally correlated with metal concentrations in plant tissues but not with mass removals. The excessively high metal concentrations, both total and DTPA-extractable, of the wastes in ploughing cause worse rooting than subsoiling in almost all species. In both woody and crop species, the mass balance of phytoextraction mainly involved Zn and was relatively poor because of generalised impaired growth. However, poplar and willow were able to stabilise a considerable fraction of trace elements at below ground, making the application of phytostabilisation (coarse roots) through short-rotation coppicing a reliable complementary strategy. Instead, this possibility seems less practicable in herbaceous crops through their finer roots, although more precise quantification of root production and turnover is required for insights into long-term metal stabilisation in tap-rooted species. Results obtained at the Woolston New Cut Canal in the UK, which are an integral part of this thesis, indicate that stabilisation of several metals can also be achieved by adding soil with a small percentage of cement (1%) or lime (1%). This would reduce risks of metal movements through windblow and leachate transfer, and allow plants to improve their above-ground productivity. Among crops, Raphanus sativus var. oleiformis showed the highest tolerance to pyrite wastes, providing interesting phytoextraction balances, thanks to less altered growth rather than increased metal concentrations. The consistency of these results with past experiments in another polluted site suggested that this species can be conveniently exploited by testing various methods of improvement in metal uptake (e.g., auxins, humic acids, chelators). In pot trials, repeated soil applications of indolebutyric acid (IBA) have shown detrimental effects on plant growth and phytoextraction, regardless of doses of 0.1 and 1 mg kg-1. These negative effects were not observed for foliar spraying at 10 IBA mg L-1. Conversely, significant increases in root length and metal removal were seen after soil application of humic acids (HA) at small doses (0.1 g HA kg-1 of substrate), probably because of their hormone-like and chelating (weak) effects. Higher doses of HA (1 g kg-1), an extremely expensive option on a large scale, were fortunately not effective, as worsening of shoot biomass greatly exceeded improvements in metal concentrations. The application of ethylene-diamine-disuccinic acid (EDDS), a less persistent and more easily degraded chelator than the traditional ethylene-diamine-tetraacetic acid (EDTA), was experimented in both Raphanus sativus var. oleiformis and Brassica carinata A. Braun, but only occasionally were good phytoextraction results observed. EDDS always improved metal concentrations, regardless of doses of 2.5 and 5 mmol kg-1 (applications close to harvest), but biomass reduction was a prevailing effect. Repeated low-dosage applications of EDDS (1 mg kg-1 × 5 times) during the cycle, another possible strategy of chelator management, was less effective because of considerable phytotoxitoxicity and undesirable marked increases in metal leaching. Like those found for other chelators, these results suggest that, for maximum effectiveness and absence of environmental risks, EDDS should be applied one week before harvest at a moderate dose (2.5 mmol kg-1), regardless of species. It is concluded that phytomanagement of metal-polluted sites with field crops and coppices undoubtedly has a landscape value, but may only seldom become a reliable phytoextraction strategy in extreme contamination conditions like those due to pyrite wastes. In these circumstances, phytostabilisation is an option which should be evaluated more carefully as additional complementary mean. For both phytoextraction and stabilisation, the proper choice of species and adjustment of its cultivation method, as well as substantial soil amelioration and identification of specific treatments, can significantly improve these green technologies.