The use of biosolids in phytostabilization of iron ore tailings in Swaziland

Abstract

Historically, mineral resources (such as tin, gold, iron ore and asbestos) have played an important role in advancing Swaziland economically. Nonetheless, the same mining activities of the past are the cause of a legacy of abandoned mine dumps in a number of places such as Bulembu, Ngwenya and Maloma. On the other hand, the achievement of environmentally sound and economically feasible disposal strategies for biosolids is a major problem in many places, including Swaziland. Currently, there are plans in the country to use biosolids in abandoned mine sites. It is thought that this could improve soil conditions and enhance vegetation reestablishment, and more importantly, serve as a permanent solution to biosolids disposal problems. In order to understand potential problems that this might cause, this study was conducted to investigate the effects of biosolids on iron mine soils with regard to soil conditions, soil enzyme activities, plant growth and metal content, and ecotoxicological effects on earthworms. According to the results obtained, the application of biosolids on iron mine soils followed by planting led to significant increases (p < 0.05) in several parameters related to soil fertility such as Ca2+, PO4 –, organic matter, water holding and cation exchange capacities. Soil Cu, Zn, Cd, Hg and Pb were also significantly increased, but remained lower than soil critical concentrations. Significant improvements were also observed in β-glucosidase, alkaline phosphatase and urease soil enzyme activities and plant biomass. Notably, increases in soil metal concentrations (Cu from 17.00–50.13 mg kg–1; Zn from 7.59–96.03 mg kg–1 after plant trials) did not affect enzyme activities. Biosolids-treated mine soils were also favourable to earthworm behaviour (NR>−80 %), biomass and reproduction. There was no immediate threat of metal bioaccumulation in earthworms because amongst the six heavy metals studied, the highest levels were Zn and Pb, which stood at 33.11 and 13.67 mg kg–1 (respectively). Earthworm tissue Ni, Cd and Hg were generally lower than 1 mg kg–1, while Cu ranged from 0.03 – 3.16 mg kg–1. Soil metal concentrations were significantly higher after metal exposure, with Zn and Ni reaching 108.15 and 138.24 mg kg–1. Higher metal uptake by plants was observed, especially Zn, which reached 346 mg kg−1 (in shoots) and 462 mg kg−1 (in roots). Higher bioavailability was observed at 0 – 50 t ha–1 (TBS+P) and at 0 – 75 t ha–1 (TB+P) treatments. From an environmental management point of view, the application of urban biosolids to iron mine soils generally seem favourable, especially when this would immediately be followed by ploughing (to incorporate biosolids into soil) and planting (to avoid aeolian and water erosion). This is particularly an attractive biosolids management strategy considering that wastewater treatment residues are continuously available in large quantities