Algae growth can be carried out in treated mine waters, providing biomass and helping to achieve the standards for water discharge. However, efficient separation of algae from the aqueous medium is crucial. The present work investigated the stability of Scenedesmus sp. in treated acid drainage from coal mining and assessed the harvesting of microalgae via coagulation/flocculation and dissolved air flotation (DAF). Successful algae growth was achieved, with cells remaining suspended in the water at a wide range of pH values, requiring the use of reagents for destabilization/aggregation. Algae coagulation/flocculation was attained with the use of tannin or ferric chloride associated with an anionic polymer flocculant at a pH of 8.0 ± 0.1. When combined with the flocculant, both tannin and the inorganic coagulant proved effective in enhancing floc stability and hydrophobicity for the DAF process. In summary, this operational approach facilitated algae biomass recovery and significantly reduced turbidity in the treated water. Finally, a schematic diagram illustrating the algae cultivation and harvesting process is presented, offering a practical alternative to acid mine drainage (AMD) treatment refinement associated with algae biomass production.

Engineered nanoparticles of TiO₂ (TiO₂-NPs) are used in the industry for a great number of applications. After their usage, the particles end up in aquatic environments, contaminating supply waters and watercourses. Bench-scale studies report removal of TiO₂-NPs (450 nm, the mean volumetric diameter) by flocculation followed by settling or by dissolved air flotation (4 bar saturation pressure and 30% recycling ratio). Floc formation was conducted after heterocoagulation with iron hydroxide (30–40 mg L⁻¹Fe³⁺⁾ and gelatinized corn starch (10–20 mgL⁻¹) as flocculant, at pH 7. Particle size distribution and zeta potential, removal efficiencies as a function of time and microphotography of flocs were analyzed. Mechanisms involve ferric hydroxide precipitation, heterocoagulation with the nanoparticles and flocculation of the loaded carrier precipitates with gelatinized starch. Best results showed removals between 95–100% of TiO₂-NPs, either by settling or flotation after 5 min. Clear treated waters with low turbidity < 3 nephelometric turbidity units (NTU) and TiO₂-NPs concentrations 100 mg L⁻¹). Conversely, the removal by settling of the flocs increased at high dosages. It is believed that both processes are sustainable in terms of reagents and the removal efficiencies of TiO₂ nanoparticles from water. GRAPHICAL ABSTRACT

A presença de ácidos naftênicos (ANs) em águas produzidas (APs) oriundas do beneficiamento de petróleo proporciona riscos toxicológicos à vida marinha e à qualidade da água. A América do Sul caracteriza-se pela produção de óleos que apresentam altos teores de ANs, constituindo um desafio para o gerenciamento e o tratamento de APs, bem como para o atendimento aos Objetivos de Desenvolvimento Sustentável (ODS). Este estudo avaliou a remoção de ácido ciclohexanocarboxílico (ACHC) de soluções salinas (100 g∙L-1 NaCl) por meio de uma técnica inovadora de floculação combinando o tensoativo aniônico dodecilbenzenosulfonato de sódio (SDBS) e uma poliacrilamida catiônica (PAAC), seguida de filtração (1,2 μm). Razão molar SDBS/ACHC (0,33, 1, 2 e 2,6), pH (3,5 a 9,5) e tempo de mistura lenta (1, 5 e 10 minutos) foram avaliados na remoção de ACHC (concentrações iniciais 150, 200 e 300 mg∙L-1). A remoção de ACHC foi função da razão molar SDBS/ACHC, sendo o melhor resultado obtido (ACHC inicial = 300 mg∙L-1 e residual = 61,2 mg∙L-1, remoção = 79,4%) em razão molar = 2,6, pH = 6,5 e 5 minutos de agitação lenta. Os mecanismos de floculação consistem na precipitação de complexos polímerosurfactantes (PSCs), removidos da solução após filtração. Testes Microtox® indicaram redução na toxicidade da água tratada (FT = 4,4) em relação à solução salina inicial contendo 200 mg∙L-1 ACHC (FT = 142,9). Pode-se concluir que o tratamento de APs em plataformas marítimas usando a combinação de tensoativo aniônico e polímero floculante catiônico tem potencial para remoção de ANs (fração de óleo dissolvida) e redução da toxicidade residual.