Machine learning-based prediction of seasonal hypoxia in eutrophic estuary using capacitive potentiometric sensor.

A hypoxia occurred in eutrophic estuary was predicted using long short-term memory (LSTM) model with prediction time steps (PTSs) of 0, 1, 12, and 24 h. A capacitive potential (CP), which provides quantitative information on dissolved oxygen (DO) concentration, was used as a predictor along with precipitation, tide level, salinity, and water temperature. First, annual changes in DO concentration were clustered in three phases of annual DO trends (oversaturation, depletion, and stable) using k-means clustering. CP was the most influential variable in clustering the DO phases. The LSTM was implemented to predict the DO phases and hypoxia occurrences. In the simultaneous prediction of the depletion phase and hypoxia occurrence with a 12 h PTS, the accuracy was 92.1% using CP along with other variables; it was 3.3% higher than that achieved using variables other than CP. In the case of predicting the depletion phase and hypoxia non-occurrence using CP along with other variables, the accuracy was 61.1%, which was 5.5% higher than that when CP was not used. When using CP along with other variables, the total accuracy was highest for all PTS. Overall, the utilization of CP and machine learning techniques enables accurate predictions of both short-term and long-term hypoxia occurrences, providing us with the opportunity to proactively respond to disasters in aquaculture and environmental management due to hypoxia.

Effect of sediment deposition on phosphate and hydrogen sulfide removal by granulated coal ash in coastal sediments.

Granulated coal ash (GCA) is a strong in-situ capping material for removing PO4-P and H2S-S in contaminated coastal sediments. Although GCA performance is weakened by sediment deposition, related research is rare. To evaluate sediment deposition effects on PO4-P and H2S-S removal by GCA, GCA was placed on the top of sediment (C-GCA), was partially mixed with sediment (M-GCA), and was fully covered by sediment (N-GCA). Effective PO4-P and H2S-S removal from sediments occurred in the order of C-GCA > M-GCA > N-GCA. C-GCA and M-GCA significantly decreased PO4-P and H2S-S concentrations by 84- 90% and 100%, respectively, through calcium phosphate and iron sulfide precipitation. N-GCA was less effective in PO4-P and H2S-S removal than the control after 2.5 months, as fine sediment particles blocked the GCA pores, decreasing calcium and iron elution. The results provide a better understanding of how sediment deposition negatively impacted GCA performance.

High-resolution monitoring of seasonal hypoxia dynamics using a capacitive potentiometric sensor: Capacitance amplifies redox potential.

Hypoxia is a long-standing environmental problem in coastal areas worldwide, but technical and economic difficulties impede accurate and continuous spatiotemporal monitoring. This study aims to monitor seasonal hypoxia dynamics at high-resolution by developing a novel capacitive potentiometric sensor. The underlying hypothesis of this study was that (1) the capacitive carbon electrode charges redox energy and creates an overvoltage; (2) the overvoltage reflects the redox energy as an amplified signal. A viability of the capacitive potentiometric sensor for seasonal hypoxia was investigated from summer to autumn in Fukuyama inner bay, Japan. The study area was a brackish water with strong stratification of upper fresh water and lower saline water. In the water surface, which is a redox-equilibrium environment, the capacitive potential increased to 0.7 V with overvoltage, which corresponds to amplifying the redox energy of dissolved oxygen by 35 times. In contrast, in the bottom layer, the capacitive potential responded in a Nernstian manner, confirming that diffusion of hydrogen sulfide was the direct cause of the hypoxic water mass in the bottom of the study area. The vertical discontinuity layer of the redox reactions was defined as 0.05 V of the capacitive potential. This threshold value intuitively illustrates the spatiotemporal dynamics of the seasonal hypoxia. A principal component analysis confirmed that dissolved oxygen concentration was a major determinant of the capacitive potential. Furthermore, this novel potentiometric sensor overcomes the limitations of conventional redox potential sensors, which fail to capture weakly poised redox couples. The capacitive potential exhibited that the stratification protected environments for photosynthesis (surface water temperature and aerobic condition), thus regularly supplies dissolved oxygen to seabed with tide and suppressed full-depth hypoxia. In conclusion, the capacitive potential provides spatiotemporal information on the chemical activity of dissolved oxygen, which is a novel approach to elucidate the mechanisms of hypoxia dynamics.

Nutrient salt removal by steel-making slag in sediment microbial fuel cells.

Applying sediment microbial fuel cells (SMFCs) into sediment can remediate the sediment; however, nutrient salts cannot be removed by SMFCs effectively. In this study, sediment mixed with steel-making slag is used a fuel in SMFCs for understanding the potential of steel-making slag in nutrient salt removal in SMFCs. To the best of our knowledge, no report related to the use of steel-making slag in SMFCs is found in the literature. The combination of SMFCs with steel-making slag is expected to make the individual efficiency of SMFCs and steel-making slag more reactive, and is another way to increase the benefit of using steel-making slag. Experimental results showed that steel-making slag was more effective for adsorbing nutrient salts in SMFCs. Interestingly, the combination of SMFCs with steel-making slag can increase the individual efficiency of SMFCs and steel-making slag.

Relaxing the formation of hypoxic bottom water with sediment microbial fuel cells.

The method of improving bottom water environment using industrial wastes to suppress diffusion substances from bottom sediment has recently captured the attention of many researchers. In this study, wastewater discharge-derived sediment was used to examine an alternative approach involving the use of sediment microbial fuel cells (SMFCs) in relaxing the formation of hypoxic bottom water, and removing reduced substances from sediment. Concentrations of dissolved oxygen (DO) and other ions were measured in overlying water and sediment pore water with and without the application of SMFCs. The results suggest that SMFCs can markedly reduce hydrogen sulfide and manganese ion concentrations in overlying water, and decrease the depletions of redox potential and DO concentration. In addition, SMFCs can dissolve ferric compounds in the sediment and thereby release the ferric ion available to fix phosphate in the sediment. Our results indicate that SMFCs can be used as an alternative method to relax the formation of hypoxic bottom water and to remove reduced substances from the sediment, thus improving the quality of both water and sediment environments.

Variation in properties of the sediment following electrokinetic treatments.

Many studies have reported variation in properties of the sediment within electrokinetic treatments (EKTs). However, we aim to reveal the variation in properties of the sediment following EKTs through laboratory experiments. We collected sewage-derived sediment from a littoral region, and passed it through a 2-mm sieve. We used a potentiostat to cause electrical current in EKT. We measured the sediment properties such as pH, redox potential (ORP), and hydrogen sulphide (H2S) concentration at the end of EKT and at 30 days following EKT. Results showed decreases in pH, increases in ORP, and decreases in H2S concentration at the end of EKT. Compared with the sediment without EKT, the decrease in ORP for the sediment within EKT was higher at 30 days following EKT. These suggest that anaerobic digestion of organic compounds occurs in the sediment following EKT, of which the oxidants produced by EKT serve as electron acceptors and organic compounds serve as electron donors. Furthermore, we found that EKT can remove H2S from the sediment and reduce H2S production in the sediment within EKT when compared to the case without EKT. These ensure that EKT can be used to remove H2S and control H2S production in the sediment.

A pilot study on remediation of muddy tidal flat using porous pile.

In order to prove that porous piles are effective in remediating muddy tidal flat sediments and increasing the biomass, field experiments were carried out at the tidal flat of a brackish river located in Hiroshima City, Japan. Porous piles with a diameter of 16cm and height of 50cm were installed in the muddy sediment that covers the sand layer of the tidal flat. After installation, concentrations of dissolved oxygen in interstitial water in and around the porous piles increased to a maximum concentration of 6mg/l due to enhancement of the groundwater flow. The increase of dissolved oxygen in the interstitial water produced a decrease in the concentration of ammonia and an increase in the individual number of benthos at the porous pile site. From these results, we concluded that the porous pile is an effective technology for remediation of muddy tidal flats.

Field experiments on remediation of coastal sediments using granulated coal ash.

Field experiments were carried out to evaluate the effect of Granulated Coal Ash (GCA) on remediation of coastal sediments in terms of removing phosphates and hydrogen sulfide. Phosphate concentrations in the sediment were kept below 0.2 mg/l after the application of GCA, whereas those in the control sites increased up to 1.0 mg/l. The concentration of hydrogen sulfide in the sediment was maintained at almost zero in the experimental sites (GCA application sites) for over one year, whereas it ranged 0.1-2.4 mg S L(-1) in control sites. Meanwhile, individual number of benthos increased in the experimental sites by several orders of magnitude compared to the control sites. The major process involved in hydrogen sulfide removal by GCA was thought to be the increase in pH, which suppresses hydrogen sulfide formation. From our findings, we concluded that GCA is an effective material for remediating organically enriched coastal sediment.

Characteristics of electricity generation and biodegradation in tidal river sludge-used microbial fuel cells.

The electricity generation behavior of microbial fuel cell (MFC) using the sludge collected from the riverbank of a tidal river, and the biodegradation of the sludge by the electricity generation are evaluated. Although the maximum current density (150-300 mA/m(2)) was higher than that of MFC using freshwater sediment (30 mA/m(2)), the output current was greatly restricted by the mass transfer limitation. However, our results also indicate that placing the anode in different locations in the sludge could reduce the mass transfer limitation. After approximately 3 months, the removal efficiency of organic carbon was approximately 10%, demonstrated that MFC could also enhance the biodegradation of the sludge by nearly 10-fold comparing with the natural biodegradation. We also found that the biodegradation could be identified by the behavior of oxygen consumption of the sludge. Importantly, the oxygen consumption of the sludge became higher along with the electricity generation.

Remediation of muddy tidal flat sediments using hot air-dried crushed oyster shells.

In order to prove that hot air-dried crushed oyster shells (HACOS) are effective in reducing hydrogen sulfide in muddy tidal flat sediments and increasing the biomass, field experiments were carried out. The concentration of hydrogen sulfide in the interstitial water, which was 16 mg SL(-1) before the application of HACOS, decreased sharply and maintained almost zero in the experimental sites (HACOS application sites) for one year, whereas it was remained at ca. 5 mg SL(-1) in the control sites. The number of macrobenthos individuals increased to 2-4.5 times higher than that in the control site. Using a simple numerical model, the effective periods for suppression of hydrogen sulfide were estimated to be 3.2-7.6 and 6.4-15.2 years for the experimental sites with 4 and 8 tons per 10 × 10 × 0.2m area, respectively. From these results, it is concluded that HACOS is an effective material to remediate muddy tidal flats.