The aim of this review was to summarize information and scientific data from the literature dedicated to the fate of polyacrylamide (PAM)-based flocculants in hydrosystems. Flocculants, usually composed of PAMs, are widely used in several industrial fields, particularly in minerals extraction, to enhance solid/liquid separation in water containing suspended matter. These polymers can contain residual monomer of acrylamide (AMD), which is known to be a toxic compound. This review focuses on the mechanisms of transfer and degradation, which can affect both PAM and residual AMD, with a special attention given to the potential release of AMD during PAM degradation. Due to the ability of PAM to adsorb onto mineral particles, its transport in surface water, groundwater, and soils is rather limited and restricted to specific conditions. PAM can also be a subject of biodegradation, photodegradation, and mechanical degradation, but most of the studies report slow degradation rates without AMD release. On the contrary, the adsorption of AMD onto particles is very low, which could favor its transfer in surface waters and groundwater. However, AMD transfer is likely to be limited by quick microbial degradation.

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International audienceThis study presents an innovative method for concrete waste up-cycling based on concrete weakening through microwave heating before impact crushing. Two series of tests were conducted in order to assess the influence of the aggregate properties (size distribution, mineralogical nature) and the influence of the operating conditions of the microwave heating pretreatment on concrete fragmentation; and thus to evaluate the feasibility and the robustness of this process. Experiments were carried out on lab-made, cylindrical concrete specimens and on no-slump concrete waste with a multimode cavity microwave equipment (2.45 GHz, 6 kW) and an impact crusher. Results showed that microwave heating always induced an embrittlement of concrete samples which resulted in lower fracture energy, higher fragmentation of samples and higher liberation of aggregates (i.e. free of cement paste). A microwave-assisted comminution process is therefore an effective recycling technique for the recovery of high-quality aggregates from concrete waste

Bioleaching of sulfidic mining wastes enables to produce lixiviant solution that can be used further for e-wastes leaching. Biological re-oxidation of iron greatly enhances metals dissolution kinetics and yields during PCBs leaching. Copper extraction above 90% was achieved in 24 h of PCBs leaching. Microbiologically assisted leaching of waste PCBs is a promising way for metals recycling. Decoupling the lixiviant production from the leaching process enables to avoid toxicity issues.

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Ultra-High Performance Fibre-Reinforced Concrete (UHPFRC) such as LafargeHolcim Ductal® is a new concrete product that incorporates large amounts of fine metal fibres, and is designed to have multiple advantages over traditional concrete products. These fibres, while providing additional strength, represent a new recycling challenge as they may block or increase wear of conventional mechanical apparatus, or be broken during processing rendering them unusable. High voltage electric-pulse fragmentation (EPF) systems such as those produced by SELFRAG AG use repeated electric discharges to selectively fragment composite materials along phase boundaries, overcoming compressive strength and preventing damage to metallic fibres. Initial tests in a laboratory scale system at a range of specific energy levels up to 60 kWh/t showed that Ductal® sample with a compressive strength of 170 MPa was amenable to EPF with good recovery rate of the steel fibres, which were fully liberated in the 0/2 mm product size fraction. Upscaled tests were performed on two Ductal® samples with compressive strengths of 170 and 210 MPa respectively using the 'Pre-Weakening Test Station' (PWTS), a continuous EPF system. Tests with specific energy levels up to 27 kWh/t showed similar results for both Ductal® samples: fibre liberation correlates with increasing specific energy input up to a plateau at about 13 kWh/t where increased energy produces little to no additional breakage. About 60% of fibres were recovered after just one treatment step performed at 13.4 kWh/t. These promising results obtained at pilot-scale indicate that this technology is suitable for UHPFRC recycling and fibre recovery, and that scaling-up the process to a commercial level is technically feasible.

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