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Petrochemical plants constitute the main sources of rare earth elements (REEs) in the Musa estuary. Under different redox conditions, the accumulated REEs in sedimentary beds might be released into the water column. An automated biogeochemical microcosm system was utilized to simulate reducing conditions. The effect of redox potential (Eh) on the release of REEs in an estuarine sediment sample was investigated. REEs were released from sediments by reducing Eh, and the average amount of REEs mobilization was 31.03%. When Eh was decreased from 120 to −150 mV, a decrease in pH (from 8.08 to 6.66), an increase in dissolved organic carbon (DOC) (from 18.93 to 704.84 mg/L), and an increase in the dissolution of Fe–Mn oxy-hydroxides occurred. Lu, Sm, and Pr were the most mobile elements with over 40% release rate, while Ce showed the lowest amount of release. The cluster analysis and principal component analysis exhibited that the indirect effect of Eh via the linked changes of DOC and chemistry of Fe–Mn influenced the solubility and release of REEs. Enrichment factors (EFs) revealed that the Musa estuary is threatened by Ce, Dy, La, Nd, Pr, and Sm. Since the ecotoxicological effects of dissolved REEs cause environmental risk and health issues, serious consideration needs to be given to the high potential mobility of REEs. [ABSTRACT FROM AUTHOR]

The study aims to establish denitrification potential of the Northern Arabian Gulf (NAG), as nitrogen critically affects the ocean productivity, obliterates acidity, oxidative capacity and radiative transfer capability of atmosphere. The experimental study was conducted by taking cores from intertidal zones from two different sites in North and South, referred as sites N and S; representing two distinct environmental milieu. The experiment was conducted in controlled laboratory conditions simulating the tidal cycles. Multiple cores were taken and loaded with seawater with different N concentrations, the redox potential was established for each condition. Redox potential was significantly lower at 10 cm depth compared to the surface in all cores ( P < 0.001). The redox potential at surface and at 10 cm depth was significantly lower at site S compared to site N ( P < 0.001; F = 714.2), suggesting anaerobic sediments at site S. Effects of nitrate spiked seawater on denitrification under nonflooded and flooded conditions at the two sites were also studied. Three-way ANOVA analysis indicated that site, nitrate concentration, and flooding had significant main and interactive effects on the rate of denitrification. The results suggest that under ambient nitrate concentrations (0.03 mg NO-N l), 6.3 ± 2.1 g NO-N ha day can be denitrified by inter-tidal zone sediments. At a nitrate concentration of 1 mg NO-N l, 92 ± 16 g NO-N ha day may be denitrified whilst at a very high nitrate load of 10 mg NO-N l, the sediments may attain a rate of denitrification close to 404 ± 78 g NO-N ha day. [ABSTRACT FROM AUTHOR]

Sediments are capable of adsorbing and desorbing heavy metals (HMs) under various environmental conditions. This study investigated the impact of pre-set redox potential (Eh) on the release dynamics of HMs (Co, Cr, Cu, Ni, Pb, V, and Zn) from sediment in an automated biogeochemical microcosm. The release of Co, Pb, and V under reducing conditions increased that may increase the potential risks in the aquatic environment. This phenomenon could be attributed to the decrease in pH, the reductive dissolution of Fe Mn oxides, and the complex of HMs with dissolved organic carbon (DOC). However, the soluble Cr, Cu, Ni, and Zn decreased at redox potentials as low as −150 mV. Co, Ni, Pb, and Zn were observed in mobile fractions while Cu primarily existed in the residual fraction (indicating lithogenic source). HPI and HEI indexes showed that water quality concerning HMs would become more unsuitable for aquatic life by reducing Eh. [Display omitted] • Release of heavy metals from sediments enhanced in reducing conditions • Redox potential showed a converse relation with DOC and direct relation with pH. • Indexes signified unsuitable water quality for aquatic life. [ABSTRACT FROM AUTHOR]