Interface Science in Mineral (Bio)Processing

Dear Colleague,

The metal and energy extractive industries play a strategic role in the economic development of all countries. Conventionally, physicochemical methods are used in mineral processing and extractive metallurgy to recover value minerals and metals from ores. The mineral and metallurgical industries present a serious threat to the environment. Mining waste is one of the largest waste streams and is responsible for 18% of overall waste generation. In particular, one of the most severe ecological problems is associated with the emission of sulphur dioxide to air from the roasting and smelting of sulphide concentrates and with the multidimensional environmental pollution produced in the course of ageing of ore processing tailings and waste rocks. Oxidation of metal sulphides in mines, mine dumps, and tailings (acid mine drainage, AMD) is a notorious source of acidity, heavy metal (e.g., Fe²⁺, Mn²⁺, Al³⁺, Cu²⁺, Pb²⁺, Cd²⁺, Zn²⁺, etc.), and oxyanion (CrO₄^2–, AsO₄^3–, etc.) contamination for streams and groundwater, which poses a serious threat of short- and long-term environmental degradation. Mining tailings can be additionally contaminated by chemical reagents—collectors, modifiers, frothers, flocculants, etc. On the other hand, certain waste contains elements that are useful for the industry, which invokes an important problem of recycling.

The most promising new approach based on integral green-chemistry methods is the biotechnological one. Microorganisms have a tremendous influence on their environment through the transfer of energy, charge, and materials across a complex biotic mineral–solution interface. Effective autotrophic and heterotrophic biosolubilization/bioleaching was observed for sulfide ores, wastes, and low-grade minerals, including soils and muds, filter dust/oxides, lateritic ores, copper converter slag, fly ash, and electronic waste materials. Alternatively, bacteria can immobilize, through a number of mechanisms, various components of solutions/suspensions/emulsions, offering remediation, recovery or detoxification applications. Reduction of Se(VI), Cr(VI), U(VI), and Te(VI) by dissimilatory metal-reducing bacteria that use metals as terminal electron acceptors leads to the precipitation and long-term immobilization of these harmful to humans and wildlife ions. A combined reduction–deacidification approach can be applied to protecting the environment from AMD if one properly exploits the ability of metal- and sulfate-reducing bacteria to suppress oxidative solubilization of minerals and eliminate acidity from the system. Another known biotechnologically promising immobilization mechanism are biosorption and binding of the solution components with peptides, proteins, polysaccharides, and other biomolecules. In addition, several recent investigations have revealed that adapted bacteria associated with ore deposits can selectively be attached to sulphides, thereby essentially modifying the surface properties relevant to bioflotation and bioflocculation processes.

Almost all mineral/metal–microbe interactions have been examined hitherto as a means for removal, recovery or detoxification of inorganic and organic metal or radionuclide pollutants. However, except for some mechanisms, notably bioleaching, which has already been employed at a commercial scale, practical exploitation of the biotechnological potential of most of the above bioprocesses is still far from feasibility. There have been several attempts to commercialize biosorption using microbial biomass, but success has been limited, primarily due to competition with commercially produced ion exchange media. One of the reasons of such a situation is that all the bioprocesses mentioned have been developed empirically, focused either on the particular biochemical aspects or on the process engineering. Remarkably, an understanding of interfacial phenomena at the molecular level remains elusive for all but the simplest systems. As a result, knowledge-based control, optimization, and design of the biotic interfaces for solving a given applied problem from the first principles are still considered as a matter of the future.

A molecule-level understanding of abiotic and biotic interfaces of solids with aqueous media is the key to innovations in biotechnologies that are based on stringent control of interfacial processes, ranging from high-tech to raw material industries and ecosystem services. Among those, ore processing, waste recycling, and environmental protection have a substantial impact on the societal economical, ecological, and political climate.

Thus, a Special Issue on the current state of the knowledge at the mineral/metal–bacteria–water interfaces, with contributions from international experts from universities, research institutions, and industry, will help students and researchers working in this area to improve research and development of economically more efficient “green” chemistry technologies in ore processing, (bio)geochemistry, recycling of mineral and metal industries wastes/by-products, cleaning of ground and river waters, (bio)remediation of contaminants, radioactive storage and disposal, and control of acid mine drainage.

Prof. Dr. Hanumantha Rao Kota
Prof. Dr. Sankaran Subramanian
Guest Editors

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