A greenhouse trial to investigate the ameliorative properties of biosolids and plants on physicochemical conditions of iron ore tailings: Implications for an iron ore mine site remediation

• We investigated the influence of biosolids and plants on iron ore mine soils.
• Biosolids and plants are able to improve conditions related to soil fertility.
• Wastewater treatment residues are contaminated with heavy metals.
• Biosolids application may increase soil metal content.
• Recolonization of reclaimed sites may be affected by higher metal concentrations.

An iron ore mine site in Swaziland is currently (2015) in a derelict state as a consequence of past (1964–1988) and present (2011 – current) iron ore mining operations. In order to control problems associated with mine wastes, the Swaziland Water Services Corporation (SWSC) recently (2013) proposed the application of biosolids in sites degraded by mining operations. It is thought that this practice could generally improve soil conditions and enhance plant reestablishment. More importantly, the SWSC foresees this as a potential solution to the biosolids disposal problems.In order to investigate the effects of biosolids and plants in soil physicochemical conditions of iron mine soils, we conducted two plant growth trials. Trial 1 consisted of tailings that received biosolids and topsoil (TUSB mix) while in trial 2, tailings received biosolids only (TB mix). In the two trials, the application rates of 0 (control), 10, 25, 50, 75 and 100 t ha−1 were used. After 30 days of equilibration, 25 seeds of Cynodon dactylon were sown in each pot and thinned to 10 plants after 4 weeks. Plants were watered twice weekly and remained under greenhouse conditions for 12 weeks, subsequent to which soils were subjected to chemical analysis.According to the results obtained, there were significant improvements in soil parameters related to fertility such as organic matter (OM), water holding capacity (WHC), cation exchange capacity (CEC), ammonium (NH4+)(NH4+), magnesium (Mg2+), calcium (Ca2+) and phosphorus (PO4−PO4−). With regard to heavy metals, biosolids led to significant increases in soil total concentrations of Cu, Zn, Cd, Hg and Pb. The higher concentrations of Zn and Cu in treated tailings compared to undisturbed adjacent soils are a cause for concern because in the field, this might work against the broader objectives of mine soil remediation, which include the recolonization of reclaimed sites by soil-dwelling organisms.Therefore, while biosolids contain important nutrients that may greatly improve physicochemical conditions and enhance vegetation reestablishment in mined soils, the threat of the build-up of higher levels of trace elements in treated tailings compared to surrounding adjacent soils must not be underestimated.