Recirculating aquaculture systems (RAS) represent a new way to farm fish in land-based aquaculture, in which the water is filtered and cleaned for recirculating through the fish culture tanks. However, the current RAS facilities still lack a fit-for-purpose digital control of the water chemical parameters to adjust/tailor the treatment trains and quickly identify problems related to the quality of the recirculated water [2].
In this work, a novel analytical platform for monitoring of chemical species in RAS is presented, namely nitrate and nitrite ions to better understand if the treatment of recirculated water is effective and adequate. The platform combines the powerful features of electroanalytical sensors with technological advances in terms of microfluidics and electronics to provide the control of water quality. It is divided into different modules, namely the sample processing and detection module which are controlled by an electronic module (Figure 1).
The sample processing module enables the collection of the water and the performance of pre-treatment steps to overcome interfering matrix effects, such as solid particles, salinity, and pH. For this purpose, the sample filtration is followed by two serial microfluidic chambers fabricated in poly(methyl methacrylate) (PMMA). The first includes a cation-exchange membrane and a pair of electrodes (Ag as working and Ag/AgCl as reference/counter electrodes) in which an anodic potential is applied to oxidize the Ag and remove the chloride ions by the formation of AgCl at the surface of the working electrode. The applied anodic potential, silver thickness, sample volume, and reusability are under optimization to achieve the best efficiency of the desalination cell. The second chamber enables the mixing of the desalinated sample plug with an acidic media to reach the required pH for analysis.
The detection module comprises solid-contact ion-selective electrodes for nitrate and nitrite ion as well as a custom-made Ag/AgCl reference electrode, both integrated into a PMMA microfluidic cell. Commercial carbon screen-printed electrodes modified with graphene oxide were used to prepare the sensors by drop-casting the corresponding polymeric ion-selective membrane on top (Figure 2). The proposed potentiometric sensors were evaluated in terms of analytical response, selectivity, robustness, and durability. Nitrate-sensors showed a sensitivity of 52.0 mV/decade within the linear range of 0.2-612 mg/L and a limit of detection of 0.1 mg/L at 0.1 M phosphate buffer background (pH 5.0). A fast response time (<20s), good reproducibility (RSD<1.4%), potential stability (0.3 mV/h), and durability of four weeks are some of the remarkable properties. Likewise, the nitrite-selective electrodes provided a sensitivity of 45.4 mV/decade over the linear range from 0.05 to 454 mg/L and a limit of detection of 0.04 mg/L within the same conditions. A response time <50s, excellent reproducibility (RSD<0.4%), potential stability of 2.0 mV/h but a shorter lifespan of about a week were observed, mainly attributed to the leaching of the sensing element from the polymeric membrane. Additionally, the proposed nitrogen sensors showed great selectivity to the target species against common interfering ions present in seawater. Cations do not impose any interference due to the perm-selectivity of potentiometric sensors while the most interfering anions were iodide and perchlorate, however their common low levels in water are not a cause of concern.
The applicability of nitrate sensors was assessed, first, by the analysis of different natural water samples. Appropriate recovery percentages (88-108%) and an excellent agreement with a commercial nitrate probe (difference<8.5%) in the analysis of freshwaters confirmed the great reliability of the proposed sensors. Nevertheless, the presence of chloride at high levels in seawater worsened the accuracy, justifying the use of the aforementioned cell for the decrease in salinity.
Future work envisage the tailoring of the proposed analytical platform for on-site monitoring of chemical parameters in RAS. This innovative technology brings major advantages when compared to conventional techniques such as cost-effectivity, autonomous performance, and improved spatial/temporal analytical resolution, offering new opportunities to the aquaculture sector.
Acknowledgments
This works was funded by EEA (European Economic Area) Grants Portugal (EEA.BG.Call4.023.2020) through funded project OPTIRAS.
References
1. Halvorson, H.O. and R. Smolowitz, Aquaculture, in Encyclopedia of Microbiology (Third Edition), M. Schaechter, Editor. 2009, Academic Press: Oxford. p. 17-22.
2. Martins, C.I.M., et al., New developments in recirculating aquaculture systems in Europe: A perspective on environmental sustainability. Aquacultural Engineering, 2010. 43(3): p. 83-93.