Silicone membrane contactor for selective volatile fatty acid and alcohol separation

The effect of pH and extraction temperature on flux, recovery, mass transfer coefficient and separation factor of volatile fatty acids (VFAs) and alcohols from synthetic solutions and cheese whey fermentate was investigated using a silicone membrane contactor with water as extractant. The silicone membrane allowed extraction of undissociated acids only, resulting in substantially higher recovery efficiencies at pH 3 than at pH 5. Furthermore, the non-porous silicone membrane favoured extraction of longer chain over shorter chain acids. Caproic acid was extracted with the highest flux of 1.30 (± 0.02) g m−2 h−1 in short time (32 h), with a 41.5 % recovery efficiency at pH 3 and 20 °C, indicating the feasibility of its selective separation from the VFA mixture. A similar trend was observed for alcohols, with butanol being extracted with a 39 % recovery efficiency at 40 °C, against 32 % and 19 % of propanol and ethanol, respectively, while the mass transfer coefficients were not affected by temperature. When applying the silicone membrane contactor to real cheese whey fermentate at pH 3, butyric and acetic acid were extracted with 21.5 % and 7% recovery efficiency, respectively, suggesting the feasibility of the contactor for VFA recovery from real fermentate

Trends and perspectives in the use of organic acids for critical metal recycling from hard-metal scraps

Hard-metal sector, strategic for the industrial economies, is suffering from the reduced availability and price volatility of its main feedstock: critical W and Co. In 2021, a 73.5 kt W and 9.2 kt Co demand for hard-metal production (65% and 5.3% of global demand, respectively), was recorded. Hard-metal scrap recycling is hence desirable for both environmental and economic reasons. A significant recovery of W and Co from manufacturing by-products and scraps is already good practice in the hard-metal industry (42% for W and 22% for Co). However, there is still a lot to do to meet the technical-economic-environmental sustainability in materials and energy enhancement for pursuing a green economy model. Indeed, Chemical Modification and Direct Recycling, which are the most widely employed industrial approaches, typically involve energy and/or harsh chemicals-intensive treatments which require expensive equipment and skilled workers. In the last decade, research efforts have been spent on implementing alternative materials reclamation processes from hard-metal scraps based on the use of bio-based organic acids with the view to increase the rate and quality of the recycled materials exploiting their peculiar metal complexing action as well as to preserve natural resources and prevent the disposal of potentially toxic/polluting substances. Despite the preliminary stage of the research, organic acids were demonstrated to be powerful but gentle agents for the selective leaching of cobalt from WC-Co-based materials as well as promising agents for WO3 dissolution. Indeed, thanks to their acid and complexing properties, they can stabilize metals in their oxidized form giving soluble products and preventing passivation phenomena. Furthermore, organic acids can be obtained by renewable biomass transformation, limiting the request for high-impact industrial chemicals. Hence they points out key features making them promising for the design of eco-friendly recovery processes. In this context, the different industrial approaches to the recovery and recycling of Hard-metal wastes, with specific reference to the role of bio-derived organic acids in hydro- and solvo-metallurgical processes, will be critically reviewed with the view of opening a discussion on the perspectives of their use in designing circular economy models in HM manufacturing as economically, technically and environmentally sustainable as possible

A comparison among bio-derived acids as selective eco-friendly leaching agents for cobalt: the case study of hard-metal waste enhancement

Peculiar chemical, mechanical, and magnetic properties make cobalt a key metal for a variety of “hot” applications like the cathode production of Li-ion batteries. Cobalt is also the preferred metallic binder for tungsten carbide tool manufacturing. The recent increasing criticality of cobalt and tungsten is driving the interest of manufacturers and researchers toward high-rate recycling of hard-metal (HM) waste for limiting the demand for raw materials. A simple and environmentally friendly hydrometallurgical route for Co-selective dissolution from HM wastes was developed by using weak, bio-derived, and biodegradable organic acids (OAs). In this study, OAs, namely, acetic (HAc), citric (H3Cit), maleic (H2Mal), lactic (HLac), succinic (H2Suc), lactobionic (HLB), and itaconic (H2It) acids, were selected for their pKa1 values spanning from 1.8 to 4.7 and systematically tested as selective cobalt leaching agents from WC-Co-based wastes in water, isolating the formed complexes in the solid state. Thereby, all of them seemed to be efficient in selective Co leaching, achieving almost quantitative Co dissolution from HM by-products still at low concentration levels and room conditions in a short time, leaving the residual WC unreacted and ready to be re-employed for industrial purposes. Nevertheless, two main categories of organic acids were distinguished depending on their oxidizing/complexing behavior: class 1 OAs, where the metal oxidation is carried out by H+, and class 2 OAs, where oxidation is carried out by an external oxidant like O2. A combined experimental/theoretical investigation is described here to show the reasons behind this peculiar behavior and lay the foundation for a wider discussion on the leaching capabilities of OAs toward elemental metals. Due to the demonstrated effectiveness, low cost, eco-friendliness, and large availability through biotechnological fermentative processes, particular attention is devoted here to the use of HLac in hydrometallurgy as an example of class 2 OA. WC-Co materials recovered by HLac mild hydrometallurgy demonstrated a metallurgical quality suitable for re-employment in the HM manufacturing process

Controlling dark fermentation of agro-industrial waste for valued organic acids production

In recent years, our planet has been facing severe environmental degradation and climate change with consequences for the health of people and repercussions on the social and economic balance. Organic waste contains valuable compounds, worth recovering. Traditional waste management methods are typically based on aerobic treatment and anaerobic digestion (AD) of the organic substrate, or a combination of them. From these traditional processes, it is possible to obtain compost and biogas that have a relatively low economic value. In a context characterized by finite resources, the wastes represent both an environmental issue to be properly managed and a potential resource of secondary raw materials. In this context, the present Ph.D. thesis will pay attention to the possibility to produce valuable mixtures rich in organic acids through controlled dark fermentation (DF) of sheep cheese whey (CW). The main goal is the identification of the operating parameters which affect the reactions occurring during DF to engineer the process itself to produce specific organic acids (such as lactic, acetic, propionic, and butyric acid) useful in several applications. For this purpose, two specific applications were investigated: polyhydroxyalkanoates (PHA) production n and metal leaching. 1) Polyhydroxyalkanoates are biopolymers synthesized by several microorganisms as carbon and energy reserves in the event of carbon excess and nutrient deficiency. Industrial production of PHA remains constrained by production costs up to five times higher than those of petroleum-derived plastics. In this framework, to reduce production costs, PHA can be produced starting from waste material as feedstock and using mixed microbial cultures (MMC). 2) Critical metal leaching and recovery. Besides PHA production, organic acids produced through DF were used to recover metals from waste from electrical and electronic equipment (WEEE) through leaching process. Electrical and electronic equipment (EEE) is increasingly part of our life and contributes to improving the quality of life by providing benefits and opportunities in various sectors. At the same time, EEE production requires a high number of resources. Specifically, printed circuit boards (PCBs) and small IT equipment samples were treated for metal recovery (i.e., lead, tin, iron, nickel, zinc) using a leaching mixture produced from an appropriately controlled DF of CW. These scraps, indeed, contain a large amount of base, noble, and "rare earth" metals that can have a high value for the market or can be dangerous for human health and the environment when improperly managed