Harnessing the power of Microalgae and Daphnia for bioremediation of nutrients, pharmaceuticals, and heavy metals in wastewater.

Abstract

Excess nutrients in aquatic ecosystems promote eutrophication, which significantly affects oxygen-dependent organisms. Furthermore, toxic microalgae, such as microcystin, cylindrospermopsis, etc., thrive during eutrophication, releasing poisonous compounds harmful to human health and other aquatic organisms. Pharmaceutical compounds and heavy metals in aquatic environments further exacerbate these global concerns. Thus, addressing such problems is paramount and aligns with Sustainable Development Goals (SDGs) 6 - Clean Water and Sanitation, 12 - Responsible Consumption and Production, and 14 - Life Below Water. Bioremediation of wastewater with biological microorganisms such as microalgae and daphnia provides an excellent solution due to their remarkable properties that enable them to efficiently eliminate many contaminants, including heavy metals, nutrients, and pharmaceuticals. So, this study explored the novel approach to nutrient removal by combining Chlorella spp. and Daphnia magna (D. magna). Chlorella spp. completely removed nitrate and nitrite from wastewater by converting them into compounds like amino acids, proteins, and lipids. When D. magna was employed alone, it faced significant challenges due to the absence of primary producers like bacteria and microalgae, which they mainly feed on. However, combining it with Chlorella spp. proved exceptionally effective, as 100% removal was obtained for nitrate and nitrite, possibly driven by D. magna grazing on Chlorella spp. that had assimilated nitrate and nitrite into their biomass. Challenges arose for ammonia and phosphate removal as they achieved up to 27% removals. This can be ascribed to nutrient release, Chlorella’s saturation capacity, and environmental changes such as pH. For pharmaceuticals, the study successfully developed and validated the LC-PDA method for separating sulfamethoxazole, nevirapine, and efavirenz. The optimum conditions were a mobile phase (90:10% acetonitrile: water with 0.1% formic acid), run time (8 minutes), flow rate (0.4 mL/min), and wavelength (220 nm). The study also developed and validated the solid-phase extraction (SPE) method for extracting analytes of interest in water matrices. The optimum conditions were conditioning solvent (mixture of acetonitrile and methanol in a 70:30 (v/v) ratio), sample volume (50 mL), and pH 7. The study then assessed Chlorella's capacity to remove sulfamethoxazole, nevirapine, and efavirenz. The obtained removal efficiencies for efavirenz, nevirapine, and sulfamethoxazole ranged from 0–60%, 5–51%, and 10–50%, respectively. Furthermore, the results exhibited lower removal efficiency (up to 15%) for higher concentrations (1 and 5 mg/L), whereas lower initial concentrations (0.5 and 0.25 mg/L) showed higher removal rates (up to 60%). Low removals could be due to factors like toxic metabolite accumulation and pharmaceutical toxicity. This study also explored copper, lead, and zinc adsorption capacity on Chlorella spp. biomass. Batch cultures were assessed in triplicate at 150 rpm in an orbital shaker under different biomass dosages, pH levels, contact times, and metal concentrations. The optimum conditions were pH 7, 60 minutes contact time, biomass dosage of 12.5 mg, and 0.5 m/L concentration. The optimal conditions yielded complete removal of lead and zinc, with copper reaching up to 80% removal. The study also assessed the adsorption of target heavy metals employing Freundlich, Langmuir, and Temkin isotherms, with the Langmuir isotherm better fitting copper (R2 = 0.9888) while the Freundlich isotherm best-fitted lead (R2 = 0.976) and zinc (R2 = 0.968). Lead and zinc favoured the pseudo-first-order kinetic model, whereas copper favoured the pseudo-second-order kinetic model. Thermodynamic studies exhibited an endothermic and spontaneous process for copper and zinc. The results of this PhD underscored Chlorella's potential as an environmentally safe and effective option for removing nutrients, pharmaceutical compounds, and heavy metals. Mechanisms for removal included surface adsorption, photodegradation, bioaccumulation, and enzymatic degradation. The Fourier transform infrared spectroscopy (FTIR) confirmed the existence of functional groups like alkene, amide, carbonyl, carboxyl, ethers, hydroxyl, and methyl, which participate in the adsorption of these contaminants through various interactions. Surface morphology analysis through scanning electron microscopy (SEM) shows changes in Chlorella spp. cells after exposure to target compounds (nutrients, pharmaceutical compounds, and heavy metals), suggesting the possibility of interaction that aids their removals. Thus, this study contributed valuable insights for improving wastewater treatment strategies and addressing water scarcity concerns. Additionally, it promotes a circular economy as Chlorella spp. and daphnia biomass can be harvested at the end of the treatment process for diverse uses, including biogas production, organic fertiliser, animal feed, etc. Going forward, future research should focus on optimising bioremediation by exploring different combinations of microalgae and other biological agents to enhance the removal efficiencies of heavy metals and pharmaceuticals. Moreover, genetic modification of Chlorella spp. to improve resilience and uptake capacity is crucial, and integrating advanced monitoring technologies like biosensors are promising directions. In-depth studies on removal mechanisms, such as adsorption, photodegradation, bioaccumulation, and enzymatic degradation, are essential. Also, scaling up to pilot and full-scale applications is crucial for evaluating feasibility and economic viability. Lastly, collaboration with industrial partners and policymakers can help develop regulatory frameworks and incentives. These efforts can advance bioremediation and support global SDGs related to clean water, responsible production, and life below water.