Fabrice Amisi Muvundja

Current processes for tantalum (Ta) and niobium (Nb) leaching are based on fluoride media, which pose significant environmental challenges. This work focuses on upgrading Ta and Nb oxides from coltan ore retrieved in South Kivu (Democratic Republic of the Congo) mines. The overall process includes alkali roasting of coltan ore with potassium carbonate, water leaching, precipitation of Ta and Nb from the solution using guanidine carbonate and calcination of the precipitate. The effect of roasting process parameters was studied using the Response Surface Method based on the Box–Behnken design. The optimal alkali roasting parameters were found to be: temperature of 950°C, particle size of −150 + 106 µm, roasting time of 140 min and a mass ratio of K ₂ CO ₃ to ore of 1.32/1. Results from the optimised design showed that only 46.6% of Ta and 69.8% of Nb could be recovered from the ore. The overall design optimisation study showed that the mass ratio of K ₂ CO ₃ to ore has a high influence on Ta and Nb recoveries. Hence, a mass ratio was increased up to 1.98/1 to improve these recoveries, with other parameters kept at their optimal levels, resulting in recoveries of 64.92% for Ta and 78.43% for Nb. The leached Ta and Nb were recovered from the pregnant leaching solution by precipitation with guanidine carbonate, followed by calcination of the precipitates. The results indicate that a Ta-Nb mixed oxides concentrate of about 77% was obtained from a coltan ore containing 34% of the Ta-Nb mixed oxides. This process is a promising one regarding the low cost and environmentally friendly characteristics of the process. The residual metals can still be recycled by further extraction in a multiple-extraction process. Despite the relatively modest grade obtained, this technique can be a practical solution to address the conformity of the coltan industry to the mining law in Democratic Republic of Congo, which forbids any exportation of raw ore materials.

The contact details of the author Alidor Shikika have been updated

In total, 28 of the 29 fish species reported from the Lake Kivu basin occur in the littoral zone of the lake, but information about their structure, occurrence, and the habitats affecting their distribution is largely lacking. The lake’s inshore area is poorly heterogenous, with rock and macrophyte habitats representing the major habitats. The lack of heterogeneity in the habitats is probably an important factor influencing species richness, abundance, and the association between the species and habitats. We evaluated the fish diversity, abundance, and habitat parameters across 14 sites representing the major habitats of the lake, using data collected between April 2018 and October 2019. We calculated Hull and Jaccard indices and applied uni- and multivariate statistical approaches to the collected data. We identified 18 fish species in the lake. In the north, 17 species were found, with high abundance in rocky sites but low abundance in sandy habitats. In the south, 15 species were reported. We identified 12 species in the Ishungu Basin where the site with rock substrate had high abundance and 13 species in the Bukavu Basin with weak species richness and low abundance across degraded sites. Lamprichthys tanganicanus, a non-native species, was abundant in rocky (north) and macrophyte sites (south). Sand, rock, conductivity, depth, dissolved oxygen, and water transparency were significant parameters that could explain the fish distribution in the north while dissolved oxygen, vegetation cover, depth, and conductivity were significant in the south. A monitoring programme of the fish fauna in Lake Kivu is needed.

Hydroelectric power (HP) represents the main source of electricity in Africa, including the Democratic Republic of Congo. The demand for new dam construction is high, and major projects are currently progressing through planning and implementation stages. New HP dams should comply with both past and emerging environmental requirements. River systems need water to maintain hydraulic and ecological functions. Flow regime disturbance can prevent rivers from providing their ecosystem services and disrupt riparian communities. Most dammed rivers in Africa are understudied, however, in terms of their environmental flow requirements. This study analysed the hydrological regime and water quality of the Ruzizi River. The research investigated conditions of minimum water flow and hydropeaking at the Ruzizi I HP dam in terms of land management constraints and ecological impacts. According to Gumbel’s hydrological model, a discharge of ∼130 m³/s showed the longest return period (12 years) among the most recurrent flows. By contrast, the maximum recorded discharge of 143 m³/s showed a return time of 76 years. Any discharge between 46 and 120 m³/s could occur at any time within three years. The discharge–hydropower production relationship for the power plant provided a possible minimum environmental flow of 28 m³/s (i.e., 25%). Drinking water quality was assessed according to WHO water quality index (WQI) standards. Turbidity (i.e., total suspended solids) upstream and downstream of dams correlated strongly with rainfall (r = 0.8; n = 12) and land use. WQI values observed in excess of WHO drinking water standards indicate that the Ruzizi River is currently unsuitable for drinking water purposes.

A novel approach for Ta and Nb extraction consisting of the pre-treatment of a coltan-bearing ore with an ammonium bifluoride sub-molten salt and subsequent acid leaching has been studied. The effects from ore granulometry, ammonium bifluoride (ABF) to ore mass ratio, temperature and duration of fluorination on the degree of Ta and Nb extraction were examined. The ABF to ore ratio and process temperature were found to have the most pronounced impact on extraction efficiency. The following optimal process conditions were determined: ore granulometric fraction (−75 + 45 µm), ABF-ore (5/1), fluorination temperature (200 °C) and fluorination time (2.5 h). Maintaining these parameters enabled about 94% of Ta and 95% of Nb to be brought into solution during the sulfuric-acid-leaching stage. A comparison of the proposed method with previously reported studies suggests that due to the effects of mechanical agitation and the recirculation of the HF-containing gaseous phase back into the process, the dosage rate of ABF at the fluorination stage could be reduced significantly without sacrificing the overall recovery of Ta and Nb. In such a way, the approach could offer added environmental benefits since release of fluoride-containing effluents into the environment could be limited.

Inappropriate land management leads to soil loss with destruction of the land’s resource and sediment input into the receiving river. Part of the sediment budget of a catchment is the estimation of soil loss. In the Ruzizi catchment in the Eastern Democratic Republic of the Congo (DRC), only limited research has been conducted on soil loss mainly dealing with local observations on geomorphological forms or river load measurements; a regional quantification of soil loss is missing so far. Such quantifications can be calculated using the Universal Soil Loss Equation (USLE). It is composed of four factors: precipitation (R), soil (K), topography (LS), and vegetation cover (C). The factors can be calculated in different ways according to the characteristics of the study area. In this paper, different approaches for calculating the single factors are reviewed and validated with field work in two sub-catchments of Ruzizi River supplying the water for the reservoirs of Ruzizi I and II hydroelectric dams. It became obvious that the (R)USLE model provides the best results with revised R and LS factors. C factor calculations required to conduct a supervised classification using the Maximum Likelihood Procedure. Different C factor values were assigned to the land cover classes. The calculations resulted in a soil loss rate for the predominantly occurring Ferralsols and Leptosols of around 576 kt/yr in both catchments, when 2016 landcover and precipitation are used. This represents an area-normalized value of 40.4 t/ha/yr for Ruzizi I and 50.5 t/ha/yr for Ruzizi II due to different landcover in the two sub-catchments. The mean value for the whole study area is 47.8 t/ha/yr or even 27.1 t/ha/yr when considering land management techniques like terracing on the slopes (P factor). This work has shown that the (R)USLE model can serve as an easy to handle tool for soil loss quantification when comprehensive field work results are sparse. The model can be implemented in Geographic Information Systems (GIS) with free data; hence, a validation is crucial. It becomes apparent that the use of high resolution Sentinel 2a MSI data as the basis for C factor calculations is an appropriate method for considering heterogeneous Land Use Land Cover (LULC) patterns. To transfer the approach to other regions, the calculation of factor R needs to be modified.

During this study, we investigated the seasonal variability of the concentration and the stable isotope composition of several inorganic and organic matter (OM) reservoirs in the large, oligotrophic and deep tropical Lake Kivu (East Africa). Data were acquired over 1 year at a fortnightly temporal resolution. The δ¹³C signature of the dissolved inorganic carbon (DIC) increased linearly with time during the rainy season, then suddenly decreased during the dry season due to vertical mixing with ¹³C-depleted DIC waters. The δ¹³C signature of the particulate organic carbon pool (POC) revealed the presence of a consistently abundant methanotrophic biomass in the oxycline throughout the year. We also noticed a seasonal shift during the dry season toward higher values in the δ¹⁵N of particulate nitrogen (PN) in the mixed layer and δ¹⁵N-PN was significantly related to the contribution of cyanobacteria to the phytoplankton assemblage, suggesting that rainy season conditions could be more favourable to atmospheric nitrogen-fixing cyanobacteria. Finally, zooplankton were slightly enriched in ¹³C compared to the autochthonous POC pool, and the δ¹⁵N signature of zooplankton followed well the seasonal variability in δ¹⁵N-PN, consistently 3.0 ± 1.1 ‰ heavier than the PN pool. Together, δ¹³C and δ¹⁵N analysis suggests that zooplankton directly incorporate algal-derived OM in their biomass, and that they rely almost exclusively on this source of OM throughout the year in general agreement with the very low allochthonous OM inputs from rivers in Lake Kivu.