Abstract
Nitrates are a ubiquitous and universal pollutant in surface waters. They are associated with algal blooms and eutrophication of watercourses as well as direct toxicity to aquatic organisms. Dyes are another common anionic water pollutant. Many remediation techniques exist, however they are often expensive and highly technical. Adsorption is an alternative remediation method that can be easier and cheaper to implement. Here, spent coffee grounds (SCG) were used as a low-cost biosorbent. The aim was to develop a biosorbent that would remove a significant amount of these pollutants from water, be safe to use in the environment and divert significant amounts of waste from landfills.
In initial proof of concept experiments, SCG were immobilized on sodium alginate cubes and examined for their capacity to remove Evans Blue dye from synthetic solution (2 μM). Six types of cubes were prepared from two coffee roast types (medium and dark) and three particle sizes (small <240 μm, mixed <500 µm and large 240-500 μm). Roast and particle size did not significantly influence adsorption. All SCG cubes removed over 80% of the dye in 72 h.
Thus, the biosorbent was then examined for its ability to remove nitrates. SCG cubes were unable to adsorb nitrates therefore, loose SCG were investigated. Additionally, the loose SCG were quaternized using a choline chloride : urea deep eutectic solvent following the principles of green chemistry to add positively charged quaternary amines on the surface of SCG and specifically target anionic compounds. The chemical transformation was confirmed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). SEM-EDS showed that the nitrogen content on the surface of quaternized SCG (QCG) increased by 4.2 wt%. Adsorption of nitrates was first tested in a laboratory set up with 50 mL synthetic nitrate solutions in Erlenmeyer flasks. Quaternization, agitation, biosorbent quantity, pH, contact time and temperature were all examined. Nitrate concentrations were determined by a Brucine enzymatic quantification assay. Because loose coffee grounds are impractical for field applications, containment systems were trialed in adsorption experiments. Once the conditions for optimal adsorption were determined in synthetic solutions, the biosorbent was tested in nitrate-spiked (25 mg/L) stream water. The most effective containment system was a 90 µm-mesh pouch closed by a zip tie. In 24 h, pouched QCG removed 90.5% of the nitrates from nitrate-spiked (25 mg/L) stream water. To gauge the impact of varying water qualities on the performance of QCG, surface water samples for 3 sources were spiked with nitrates (25 mg/L) and compared to a lab-made solution. In 24 h, QCG successfully adsorbed 88.3, 73.3, 59.3 and 47.1% of nitrates from synthetic water, urban stream water, pristine stream water, and eutrophic lagoon water, respectively.
In order to examine the utility of the pouched QCG in a more field-relevant system, a simplified microcosm experiment was designed. The effectiveness of QCG was assessed in the presence of sediments that were collected from a local lagoon, with and without nitrate-spiked (25 mg/L) stream water. Microcosm results showed that in spiked stream samples, sediments alone, QCG alone and sediments + QCG removed 35.1%, 52.0% and 86.2% of nitrates, respectively.
To investigate if there was any ecotoxicity elicited by the biosorbent, QCG leachate was used in toxicity bioassays using microalga Pseudokirchneriella subcapitata (Algaltoxkit F™) and Daphnia magna (Daphtoxkit F™). 72-h algal growth inhibition experiments were conducted in 10-cm long spectrophotometer cells with QCG leachate solutions of 4, 10, 20 and 40 g/L. Optical density was measured at 670 nm. 48-h daphnia immobilization experiments were conducted with the same QCG leachate concentrations as the algae, as well as 4 and 40 mg/L. The 3 lowest concentrations were also used to assess the effect of additional centrifugation (1 h, 14 000 rpm) of the QCG leachate. Tests on P. subcapitata showed that QCG leachate inhibited growth at 72 h at all concentrations tested (4, 10, 20 and 40 g/L) likely due to a significant increase in turbidity and decrease in light availability. Studies on D. magna showed 100% immobilization at 48 h of exposure to QCG leachate concentration higher or equal to 4 g/L. Microscopic observation (Nikon Ti inverted microscope, bright field, 4x and 20x) showed that particulate matter in the leachate aggregated and coated D. magna. Centrifugation of the leachate lessened the accumulation of residues qualitatively but still caused 100% immobilization at 4 g/L. This effect was concentration-dependent as lower concentrations of 4 and 40 mg/L leachate did not cause any adverse effect on D. magna.
In conclusion, SCG immobilized on sodium alginate effectively adsorbed EB dye but failed to remove nitrates from solution. Loose SCG were effectively quaternized using non-toxic and biodegradable chemicals. Quaternized grounds are a cheap, widely available, and highly effective biosorbent with the ability to remove nitrates from surface water. However, the current design produces leachate that is toxic to aquatic organisms. A modification of the washing process could reduce the leaching which would justify testing the QCG in a more comprehensive mesocosm system. This biosorbent has great potential but will need to be improved and rigorously tested in specific local contexts to ensure that it is safe to the ecosystem.