Fe(III) bioreduction kinetics in anaerobic batch and continuous stirred tank reactors with acidophilic bacteria relevant for bioleaching of limonitic laterites

In the framework of the H2020 project CROCODILE, the recovery of Co from oxidized ores by reductive bioleaching has been studied. The objective was to reduce Fe(III) to Fe(II) to enhance the dissolution of Co from New-Caledonian limonitic laterites, mainly composed of goethite and Mn oxides. This study focused on the Fe(III) bioreduction which is a relevant reaction of this process. In the first step, biomass growth was sustained by aerobic bio-oxidation of elemental sulfur. In the second step, the biomass anaerobically reduced Fe(III) to Fe(II). The last step, which is not in the scope of this study, was the reduction of limonites and the dissolution of metals. This study aimed at assessing the Fe(III) bioreduction rate at 35°C with a microbial consortium composed predominantly of Sulfobacillus (Sb.) species as the iron reducers and Acidithiobacillus (At.) caldus. It evaluated the influence of the biomass concentration on the Fe(III) bioreduction rate and yield, both in batch and continuous mode. The influence of the composition of the growth medium on the bioreduction rate was assessed in continuous mode. A mean Fe(III) bioreduction rate of 1.7 mg·L-1·h-1 was measured in batch mode, i.e., 13 times faster than the abiotic control (0.13 mg·L-1·h-1). An increase in biomass concentrations in the liquid phase from 4 × 108 cells·mL-1 to 3 × 109 cells·mL-1 resulted in an increase of the mean Fe(III) bioreduction rate from 1.7 to 10 mg·L-1·h-1. A test in continuous stirred tank reactors at 35°C resulted in further optimization of the Fe(III) bioreduction rate which reached 20 mg·L-1·h-1. A large excess of nutrients enables to obtain higher kinetics. The determination of this kinetics is essential for the design of a reductive bioleaching process.

Bioleaching of polymetallic sulphidic mining residues: influence of increasing solid concentration on microbial community dynamics and metal dissolution

Within the European research project NEMO, a bioleaching strategy was developed for efficient metal extraction from bioleach residue currently heap-leached at Sotkamo (Finland) that still contains sulphidic minerals and valuable metals (Ni, Zn, Co, Cu). The strategy of gradually increasing the solid content with 5% steps allowed the adaptation of the consortium up to 20% (w/w) solid content, with efficient metal dissolution and same dominant bacteria. Largest proportions of Sulfobacillusthermosulfidooxidans while Eh increased suggested it to be most involved in iron oxidation. Acidithiobacilluscaldus was rather found when pH stabilized, in line with a production of protons from sulphur oxidation that maintained low pH. 'Acidithiomicrobium' P2 was favoured towards the end of the runs and at 20% (w/w) solids possibly due to its tolerance to Ni. The use of gene abundance to evaluate biomass in the pulp provided complementary results to classical cell counts in the liquid phase, and suggested a key role of bacteria associated to mineral particles in iron oxidation. Scaling-up in 21-L stirred-tank reactor at 20% (w/w) solids had no detrimental effect on bioleaching and confirmed metal extraction rates. 'Acidithiomicrobium' P2 and Sb. thermosulfidooxidans remained main actors. However, the biological activity was considerably reduced at 30% (w/w) solid concentration, which may be due to a too drastic environmental change for the bacteria to adapt to higher solid concentration. Efficient bioleaching of Sotkamo bioleaching residue at high solid concentration was demonstrated, as well as the robustness of the selected moderately thermophilic consortium, at laboratory and pilot scales.

pXRF on printed circuit boards: Methodology, applications, and challenges

In order to develop methods to determine the chemical composition of Waste Printed Circuit Boards (WPCB), this study focused on the analysis of 10 metals (Cu, Fe, Sn, Zn, Pb, Ni, Sb, Cr, Mo and Pd) using portable X-ray fluorescence (pXRF) compared to ICP-MS measurements after aqua regia digestion. Different experimental conditions were tested: 3 particle sizes (200 µm, 750 µm and 2 mm) and 3 sample preparations (tube, cup and loose powder). For each condition tested, 8-16 independent replicates were done. ICP measurements with the 200 µm sample, considered as the reference condition in this study, confirmed the homogeneity of the sample at this particle size and the robustness of the sampling protocol (RSD < 5% for all elements). For this particle size, pXRF has low data dispersion too (Cu, Fe, Sn, Zn, Pb, Sb and Cr showed RSD < 10%) and the use of loose powder seems to be a sufficient preparatory step. Moreover, the deviation of pXRF measurements with the 200 µm sample from the reference condition was acceptable (<20%) for Cu, Sn, Zn, Pb, Ni, Sb and Mo. For coarser samples, i.e. 750 µm and 2 mm, the homogeneity was much more doubtful, which needs to be offset by a larger number of repetitions. For these particles sizes, pXRF set to factory-installed mining mode did not produce accurate measurements but could provide a rapid non-intrusive approach for first-level screening to assess the relative difference of metal contents between WPCB samples.

Bioleaching of E-Waste: Influence of Printed Circuit Boards on the Activity of Acidophilic Iron-Oxidizing Bacteria

Bioleaching is a promising strategy to recover valuable metals from spent printed circuit boards (PCBs). The performance of the process is catalyzed by microorganisms, which the toxic effect of PCBs can inhibit. This study aimed to investigate the capacity of an acidophilic iron-oxidizing culture, mainly composed of Leptospirillum ferriphilum, to oxidize iron in PCB-enriched environments. The culture pre-adapted to 1% (w/v) PCB content successfully thrived in leachates with the equivalent of 6% of PCBs, containing 8.5 g L^–1 Cu, 8 g L^–1 Fe, 1 g L^–1 Zn, 92 mg L^–1 Ni, 12.6 mg L^–1 Pb, and 4.4 mg L^–1 Co, among other metals. However, the inhibiting effect of PCBs limited the microbial activity by delaying the onset of the exponential iron oxidation. Successive subcultures boosted the activity of the culture by reducing this delay by up to 2.6 times under batch conditions. Subcultures also favored the rapid establishment of high microbial activity in continuous mode.

Sampling waste printed circuit boards: Achieving the right combination between particle size and sample mass to measure metal content

The current worldwide expansion of waste PCB (WPCB) deposits represents both a pressing environmental issue and an economic opportunity, fostering the development of numerous recycling processes across the world. An important input for designing such processes is the metallic content of WPCBs, which is assayed by grinding and leaching samples taken from the stack of WPCBs to be recycled. The content values come with substantial uncertainties, arising mainly from the uneven distribution of the metals within the structure of WPCBs. This study aims to quantify the effects on these uncertainties of the particle size, the mass of the sample digested and the number of digestion replicates. It focused on the abundance of six metals in WPCBs: Cu, Fe, Zn, Pb and Ni, and also Co, which is a critical element for the EU. A batch of 485 kg of WPCBs was put through several shredding and splitting steps to produce three fractions: one shredded to 2 mm, and two ground to 750 μm and 200 μm. From each sample, 16 samples of 0.5 g, 2 g or 5 g were digested in hot aqua regia. Bootstrapping of the results allowed the error around the mean content to be estimated, for each metal and for all the experimental conditions. Considering the largest sample masses and three replicated digestions, the uncertainties for Zn (resp. Ni) were reduced from 35% to 10% (resp. from 70% to 10%) when the particle size was reduced from 2 mm to 200 μm.

Recycling-oriented methodology to sample and characterize the metal composition of waste Printed Circuit Boards

As spent printed circuit boards (PCBs) are among the most valuable components in waste electrical and electronic equipment (WEEE), their recovery makes economic and strategic sense. However, their composition varies considerably depending on the location, year and type of appliance in which they were used. Developing new treatment processes requires representative sampling of spent PCBs from large samples and accurate determination of their raw material composition. This study aimed to characterize spent PCBs by milling, sampling and leaching with an appropriate reagent. Sampling was performed on 526 kg of spent PCBs, to obtain different samples milled at 750 µm in order to access the metals. The samples were leached with aqua regia and the metal contents of the leachates were determined. For most metals, the analyses of 40 g-samples of spent PCBs showed limited variation in the composition of the different samples. These results concurred well with other studies reported in the literature.