Min-North Development, Evaluation and Optimization of Measures to Reduce the Impact on the Environment from Mining Activities in Northern Regions (2016-2018)

In this study, the suitability of natural peat (Nat-Peat) and HCl-modified peat (M-Peat) as a sorbent for purification of mining water was evaluated in two different small-scale pilot systems: a continuous stirred tank reactor (CSTR) and a horizontal flow filter (HFF). The effect of process parameters (peat type, peat dose, mixing time, mixing intensity) on metal (metalloid) removal in the CSTR system was also investigated. In the CSRT, Nat-Peat achieved higher removal of Ni (<80%) and As (∼61%) than M-Peat (72% and 26% for Ni and As, respectively). In the HFF, Nat-Peat achieved slightly lower maximum removal of Ni (<96%) than M-Peat (<98%) and higher removal of As and Sb (<87% and 8%) than M-Peat (<35% and 7%). Thus, chemical modification (HCl) of peat did not improve its affinity for metal and metalloids. Among the process parameters studied, peat dose exerted the strongest effect on residual concentrations of Ni, As and Sb. Higher removal of Ni and As was achieved in treatment combinations involving high peat dose (2 g/L), mixing time (60 min) and mixing intensity (300 rpm), but the effect of increasing level of these factors was not linear. This study showed that peat can be a viable sorbent material in CSTR systems (followed by sedimentation) if sorbent particle removal can be improved. Use of peat in HFF systems is not viable, due to its inability to cope with large water volumes.

The Min-North (Development, Evaluation and Optimization of Measures to Reduce the Environment Impact of Mining Activities in Northern Regions) project was a trans-national cooperative project, with participants from the Geological Survey of Finland (GTK), University of Oulu (UO), UiT The Arctic University of Norway (UiT), Luleå University of Technology (LTU) and SMEs from Sweden, Finland and Norway. The project included four work packages I) Long-term behavior of waste rock piles and performance of cover structures II) Development of methodology for tracing pollution transport by integrating geophysical and geochemical methods, III) Removal of metals and nitrogen from mining wastewater in treatment wetlands and utilization of locally available biomass as sorbent materials, IV) Anticipated effects of climate change on contaminant transport.

Due to the complexity of soil freeze/thaw processes and a variety of factors affecting pollutant removal in treatment wetlands, laboratory pilot systems are powerful tools offering a rare opportunity to observe processes that have a significant impact on year-round purification. This paper describes the design, construction, monitoring and operation of two replicate pilot peat-based wetlands subjected to two simulated freeze-thaw cycles. Undisturbed peat soil and pre-treated gold mine process wastewater were collected from a full-scale treatment wetland operating at a mining site in Northern Finland. The wastewater (pH ~7.8, electric conductivity ~3.6 mS/cm) contained a mix of metals/metalloids (e.g. arsenic 12 µg/L, antimony 19 µg/L) and other contaminants e.g. sulphate (~2 g/L). Fluctuations in removal efficiency of target compounds due to freezing and thawing conditions were observed. Overall, removal of sulphate and arsenic decreased during frost periods, while removal of antimony increased. Monitoring data from the full-scale treatment wetland were used to assess the representativeness of the results obtained. Comparisons of seasonal variations in pollutant concentrations in outflow samples from the full-scale wetland and those measured in the pilot wetlands revealed similar fluctuations in removal efficiency during frost and frost-free periods, suggesting that the pilot wetlands simulated the real system rather well. Carefully designed pilot systems can thus be valuable tools for assessing the effect of harsh winter conditions on wetland processes and operation.

The study was performed to evaluate chemically modified biosorbents, hydrochloric acid treated peat (HCl-P) and citric acid treated sawdust (Citric acid-SD) for their metal removal capacity from dilute industrial wastewater and urban runoff and compare their efficiency with that of commercially available mineral sorbents (AQM PalPower M10 and AQM PalPower T5M5 magnetite). Batch and column experiments were conducted using real water samples to assess the sorbents' metal sorption capacity. AQM PalPower M10 (consisting mainly of magnesium, iron and silicon oxides) exhibited excellent Zn removal from both industrial wastewater and spiked runoff water samples even at low dosages (0.1 g/L and 0.05 g/L, respectively). The high degree of Zn removal was associated with the release of hydroxyl ions from the sorbent and subsequent precipitation of zinc hydroxide. The biosorbents removed Ni and Cr better than AQM PalPower M10 from industrial wastewater and performed well in removing Cr and Cu from spiked runoff water, although at higher dosages (0.3-0.75 g/L). The main mechanism of sorption by biosorbents was ion exchange. The sorbents required a short contact time to reach equilibrium (15-30 min) in both tested water samples. AQM PalPower T5M5 magnetite was the worst performing sorbent, leaching Zn into both industrial and runoff water and Ni into runoff water. Column tests revealed that both HCl-P and AQM PalPower M10 were able to remove metals, although some leaching was witnessed, especially As from AQM PalPower M10. The low hydraulic conductivity observed for HCl-P may restrict the possibilities of using such small particle size peat material in a filter-type passive system.

The effect of cold climate conditions on nitrogen removal in treatment wetlands is not entirely clear, especially in regards to the effect of freeze-thaw conditions on purification processes. Effective monitoring of treatment wetlands in harsh climate regions is difficult. Thus, pilot wetland systems were designed to simulate real freeze-thawing conditions in a controlled environment and purification efficiency achieved in the treatment of real mining water was evaluated. Two freeze-thaw cycles were conducted. In the first freeze-thaw cycle (5 weeks), NH4-N was mostly not removed in both pilot wetlands during the freezing and thawing periods. In contrast to that, the removal of NH4-N was high during the frost-free periods (> 60%). Throughout the second and longer freeze-thaw cycle, consistent and significant higher removal of NH4-N was achieved in both pilots during the freeze and thawing (>80%) and frost-free (>90%) periods when compared to the first cycle. Only careful evaluation of data regarding the concentration of different nitrogen species will provide a better understanding of nitrogen behaviour during the two studied freeze-thawing periods and the factors affecting related processes.

This study investigated the sorption behaviour of natural (N peat) and HCl-acid-modified peat (HCl peat) for contaminants in water collected at a mine site in northern Finland. Batch sorption experiments were conducted at room temperature and at 5 °C. Characterization of the sorbents by FTIR and XPS revealed no substantial change in the peat’s functional groups due to the acid treatment. Generally, the N peat was a more efficient sorbent for the mine water, although the HCl peat exhibited better nickel uptake capacity (21 mg Ni/g) than the N peat (16 mg Ni/g) from synthetic water. This is attributed to the lower equilibrium pH in samples treated with the HCl peat as well as the water’s different chemical composition. At room temperature, the N peat removed As(V) (80%) and Ni (85%) at low dosage (1–2 g/L), whereas the HCl peat presented good removal of As(V) (80%) at low dosage (1 g/L) but did not achieve satisfactory removal of Ni, even at a higher dosage (4 g/L). The performance of both sorbents was significantly affected by contact time. Ni removal by N peat increased substantially with contact time whereas removals achieved by HCl peat increased slightly up to 60 min, but decreased significantly at 24 h. Unlike with HCl peat, the N peat was less efficient in the experiments conducted at 5 °C. Overall, for both sorbents, As(V) and Ni were the most efficiently removed contaminants from the mine water. HCl peat had slightly better settling properties, however, both products settled poorly, thus rendering the studied mixing and settling system unsuitable for the proposed application. Nevertheless, both peat products, and especially the N peat, exhibited high contaminant removal potential and could represent a cost-effective and sustainable option for mine water treatment.

Increased N load on recipient water bodies typically deteriorate water quality, especially if N is the limiting nutrient in an aquatic ecosystem. In this study, constructed wetlands as an N removal step in mine wastewater treatment were studied with the focus of winter time purification efficiencies and hydraulic wetland designing. The results showed that N removal is clearly lower during winter time than in frost-free period even though N removal processes happened also at low temperatures in the wetlands. It seems that temperature is not the only factor controlling N removal processes but also oxygen availability is important while snow/ice cover restrict oxygen transportation. The results of this study increase the understanding of N removal processes in winter conditions and can be used to design constructed wetlands for mine waters.

Peat is an inexpensive and biodegradable sorbent material with good capacity to sorb cationic ions such as metals and metalloids. The aim of this study was to evaluate metal removal efficiency of natural and chemically treated (HCl) peat when applied as sorbent media in small scale pilot filter systems. Based on the results obtained, purification efficiency e.g., removal of nickel and arsenic, was good but decreased with time. Leaching of aluminium and iron occurred in both pilots, but residual concentrations of leaching elements were mainly lower than the Finnish drinking water quality recommendations.