Preliminary investigations of farm residues for the recovery by sorption of critical metals from seawater in a novel flow reactor

Technical Summary: Preliminary Investigations of Farm Residues for the Recovery by Sorption of Critical Metals from Seawater in a Novel Flow Reactor

Problem Statement
The global transition to low-carbon technologies has intensified the demand for critical minerals such as uranium (U), vanadium (V), nickel (Ni), and others, which are currently sourced primarily from declining geological ore reserves. While seawater represents a vast renewable reservoir of these dissolved elements, recovery remains challenging due to trace concentrations and the high costs associated with synthetic sorbents (e.g., amidoxime-functionalized polymers). Previous research has largely relied on batch systems, which do not accurately simulate the dynamic mass transfer and long-term stability requirements of industrial-scale deployment. Furthermore, the specific mechanisms governing metal uptake by biomass in natural seawater—particularly the interplay between passive sorption and biological/redox processes—require further elucidation under continuous-flow conditions.

Methodology
This study evaluated the efficacy of four agricultural residues (birch, garlic powder, peanut shell, and potato skin) as low-cost biosorbents for recovering critical metals from natural seawater using a novel continuous-flow reactor system.

• Experimental Setup: Natural seawater from the Irish Sea was pumped at a controlled rate (50 mL h⁻¹) through contact tanks containing biomass samples encapsulated in nanocellulose and cotton bags. The system operated for 28 days to simulate prolonged environmental exposure.
• Materials: Biomass samples were dried, ground, and sieved to <0.2 mm. Duplicate untreated controls were retained to establish background elemental concentrations.
• Analytical Protocol: Supernatant samples were collected at 14 and 28 days to monitor pH and dissolved metal concentrations. Post-exposure, biomass samples were air-dried, ashed, and digested in nitric acid. All samples were analyzed via Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for target elements: V, Mn, Ni, Cu, Mo, Ag, Au, and U.
• Performance Metrics: Sorption performance was quantified by calculating the net accumulation (subtracting control background) and the partition coefficient ($K_d$) to determine the efficiency of metal concentration from the liquid phase to the solid biomass.

Key Results

• Material Performance: Potato skin emerged as the most effective biosorbent, demonstrating the highest $K_d$ values for vanadium (reaching ~15,663 mL g⁻¹ at 14 days) and nickel, alongside the most consistent reduction in dissolved uranium concentrations. While the text notes garlic showed "exceptional enrichment" for V, Ni, Mo, and U, the quantitative data in Table 5 reveals a net surface decrease (negative accumulation) for V (-0.06 µg g⁻¹), Ni (-0.06 µg g⁻¹), and Mo (-0.67 µg g⁻¹) relative to controls, with only U showing a positive surface increase (0.43 µg g⁻¹). Birch and peanut shell exhibited moderate to weak performance, with some elements showing lower surface enrichment than controls.
• Flow vs. Batch Comparison: A critical finding was the significantly reduced metal accumulation under continuous-flow conditions compared to previous batch studies. While batch systems often showed extreme depletion of supernatant metals, the flow reactor maintained higher dissolved concentrations, suggesting that the high uptake observed in static systems is driven not only by passive sorption but also by biological and redox-mediated processes (e.g., biofilm development) that are less prevalent or different in dynamic flow.
• Operational Stability: Potato skin maintained sufficient structural integrity throughout the 28-day period. In contrast, garlic powder dispersed, and peanut shell and birch showed mass increases attributed to biofilm growth. The study noted that the nanocellulose containment method reduced absorption efficiency by approximately 50% compared to previous findings, and biofilm formation initially caused pH instability until flow rates were adjusted.
• Economic Viability: The study calculated that a partition coefficient greater than $10^5$ mL g⁻¹ is required to achieve "ore-grade equivalent" concentrations. Only potato skin achieved this threshold for vanadium and nickel in the 14-day period, though rates dropped by day 28.

Significance and Claims
The authors assert that this research demonstrates the technical feasibility of using agricultural waste to accumulate economically valuable trace elements from natural seawater under continuous-flow conditions. However, the paper maintains a modest stance regarding immediate commercial application. The primary significance lies in:

1. Process Validation: Confirming that agricultural residues can function as biosorbents in dynamic flow environments, a necessary step beyond static batch testing.
2. Mechanistic Insight: Highlighting that the high uptake rates seen in static systems may be partially attributed to biological and redox mechanisms, implying that reactor design must account for these factors to optimize performance.
3. Pathway for Optimization: Identifying potato skin as a promising candidate while acknowledging that current partition coefficients generally fall short of the levels required for economically significant concentration. The authors conclude that future work must focus on improving biomass containment, optimizing reactor hydraulics, and enhancing biologically mediated uptake pathways to bridge the gap between laboratory feasibility and economic viability.