minerals Review The Cr(VI) Stability in Contaminated Coastal Groundwater: Salinity as a Driving Force Ioannis-Porfyrios D. Eliopoulos 1, George D. Eliopoulos 2 and Maria Economou-Eliopoulos 3,* 1 Department of Pharmacy, School of Health Sciences, National University of Athens, GR-15771 Zografou, Greece; [email protected] 2 Department of Chemistry, University of Crete, GR-70013 Heraklion, Greece; [email protected] 3 Department of Geology and Geoenvironment, National University of Athens, GR-15784 Athens, Greece * Correspondence: [email protected] Abstract: Chromium concentrations in seawater are less than 0.5 µg/L, but the Cr(VI) in contaminated coastal groundwater affected by Cr-bearing rocks/ores and/or human activities, coupled with the intrusion of seawater may reach values of hundreds of µg/L. A potential explanation for the stability of the harmful Cr(VI) in contaminated coastal aquifers is still unexplored. The present study is an overview of new and literature data on the composition of coastal groundwater and seawater, aiming to provide potential relationships between Cr(VI) with major components in seawater and explain the elevated Cr(VI) concentrations. It is known that the oxidation of Cr(III) to Cr(VI) and the subsequent back-reduction of Cr(VI) processes, during the transport of the mobilized Cr(VI) in various aquifers, facilitate the natural attenuation process of Cr(VI). Moreover, the presented positive trend between B and Cr(VI) and negative trend between δ53Cr values and B concentration may suggest that seawater components significantly inhibit the Cr(VI) reduction into Cr(III), and provide insights on the role − of the borate, [B(OH)4] ions, a potential buffer, on the stability of Cr(VI) in coastal groundwater. Citation: Eliopoulos, I.-P.D.; Therefore, efforts are needed toward the prevention and/or minimization of the contamination by Eliopoulos, G.D.; Cr(VI) of in coastal aquifers, which are influenced by the intrusion of seawater and are threatened by Economou-Eliopoulos, M. The Cr(VI) changes in sea level, due to climate change. The knowledge of the contamination sources, hotspots Stability in Contaminated Coastal and monitoring of water salinization processes (geochemical mapping) for every coastal country may Groundwater: Salinity as a Driving contribute to the optimization of agricultural management strategies. Force. Minerals 2021, 11, 160. https://doi.org/10.3390/min11020160 Keywords: groundwater; contamination; chromium(VI); salinization; borate; chromium isotopes Academic Editor: María Ángeles Martín-Lara Received: 29 December 2020 1. Introduction Accepted: 29 January 2021 Published: 3 February 2021 The scientific interest on the sustainable management of water resources and the effect on groundwater aquifers by either nature processes and/or extensive application Publisher’s Note: MDPI stays neutral in industry, is an attractive and fundamental subject to global food security. Among with regard to jurisdictional claims in heavy metals, chromium (Cr) has become widespread in the environment. It appears published maps and institutional affil- in several oxidation states, the trivalent Cr(III) and hexavalent forms [Cr(VI)] being the iations. most thermodynamically stable Cr forms in nature [1,2]. Cr(III) is necessary for lipid and sugar metabolism, and it is an essential trace element for human and animal health, whereas Cr(VI) in the food chain (groundwater, soil and plants), has created an alarming situation for human life and ecosystems [3–5]. Potential sources for the relatively high Cr contents in soil and groundwater may be the widespread Cr-bearing peridotites, which Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. are parts of ophiolite complexes, covering more than 1% of the earth, along orogenetic This article is an open access article zones [6], intense ore mining/smelting, industrial and agriculture activities (fertilizers and distributed under the terms and pesticides) [7,8]. The weathering processes of ultramafic rocks, depending mainly on the conditions of the Creative Commons climate conditions and morphology, may result in the formation of laterites, both releasing Attribution (CC BY) license (https:// significant amounts of Cr among other heavy metals (Ni, Co, Mn, Fe). Groundwater from creativecommons.org/licenses/by/ sites characterized by the extensive presence of ultramafic rocks contain more than 10 µg/L 4.0/). Cr(VI), reaching values up to hundreds of µg/L of Cr(VI) [6,9–16]. The contamination of Minerals 2021, 11, 160. https://doi.org/10.3390/min11020160 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 16 Minerals 2021, 11, 160 sites characterized by the extensive presence of ultramafic rocks contain more than2 of 1510 μg/L Cr(VI), reaching values up to hundreds of μg/L of Cr(VI) [6,9–16]. The contamination of groundwater by Cr may be derived from industrial activities, such as in the Czech Re- groundwaterpublic (a highly by Cr industrialized may be derived country from industrialin Central activities,Europe) [17], such at as the in thearea Czech of Friuli Republic Vene- (azia highly Giulia industrialized (northern Italy) country [18], the in Assopos Central Basin Europe) (Oinofyta [17], at or the Inofyta, area of near Friuli the VeneziaAssopos Giuliariver) (northernin Greece, Italy)exhibiting [18], as the high Assopos as 8000 Basin μg/L (OinofytaCr(VI) in shallow or Inofyta, groundwater near the Assopos [19]. Ad- river)ditionally, in Greece, ferrochromium exhibiting (FeCr), as high which as 8000 is aµ g/Lcritical Cr(VI) alloy in in shallow the production groundwater of stainless [19]. Additionally,steel, is produced ferrochromium in several European (FeCr), which countries is a critical (Finland, alloy France, in the productionItaly, Norway, of stainless Sweden, steel,the nations is produced formerly in several comprising European Yugoslavia, countries Germany, (Finland, Italy, France, Switzerland, Italy, Norway, and Sweden,the U.K.) thegenerating nations formerlyCr-bearing comprising wastes [7,20]. Yugoslavia, Germany, Italy, Switzerland, and the U.K.) generatingIn coastal Cr-bearing groundwater wastes [that7,20 ].is contaminated by either Cr-bearing rocks/ores or in- dustrialIn coastal wastes, groundwater Cr(VI) often that exceeds is contaminated the maximum by acceptable either Cr-bearing level for rocks/ores Crtotal in drinking or in- dustrialwater (50 wastes, μg/L) Cr(VI) [21]. Such often groundwater, exceeds the maximum at the interface acceptable zone level between for Cr totallandin and drinking sea, is watercontinually (50 µg/L) influenced [21]. Such by both groundwater, marine and atterre thestrial interface processes, zone and between it often land contains and sea, ele- isvated continually concentrations influenced of Na, by bothCl, B, marine Li, Se, andAs, S, terrestrial Ca, and Mg, processes, which andis characteristic it often contains of the elevatedseawater concentrations composition [22,23]. of Na, Although Cl, B, Li, Se, about As, one S, Ca, quarter and Mg, of the which global is characteristicpopulation lives of thein the seawater vicinity composition of the world’s [22 ,coastlines23]. Although [24], the about potential one quarter role of of the the presence global population of seawater livescomponents, in the vicinity which ofmay the significantly world’s coastlines facilitate [24 the], theCr(VI) potential stability, role inhibiting of the presence the Cr(VI) of seawaterreduction components, to Cr(III), is unexplored which may in significantly groundwater. facilitate Additionally, the Cr(VI) because stability, high inhibitingtechnology themetals Cr(VI) such reduction as the rare to Cr(III), earth elements is unexplored (REEs in) have groundwater. become contaminants Additionally, in because the environ- high technologyment [25,26], metals this study such aspresents the rare new earth data elements on the composition (REEs) have of become coastal contaminants water, including in theREEs, environment from the industrial [25,26], this zone study of presentsthe Assopos new and data other on the Neogene composition Basins. of They coastal are water, com- includingbined with REEs, available from data the industrial from previous zone ofstudies, the Assopos and the and delineated other Neogene relationships Basins. are They pre- aresented. combined A major with aim available was the data interpretation from previous of studies,the Cr(VI) and stability, the delineated as a function relationships of the aresalinity presented. in groundwater A major aim resulting was the from interpretation the intrusion of the of Cr(VI) seawater, stability, the asprevention a function of ofgroundwater the salinity infrom groundwater further degradation, resulting from and theprotection intrusion of the of seawater, valuable thewater prevention resources. of groundwater from further degradation, and protection of the valuable water resources. 2. Chromium Background 2. Chromium Background StructureStructure of of Cr(III) Cr(III) and and Cr(VI) Cr(VI) and and Soluble Soluble Products Products AssumingAssuming that that Cr(III) Cr(III) forms forms hexa-coordinate hexa-coordinate complexes complexes with with the the octahedral octahedral arrange- arrange- ment of ligands, [Cr(H2O)3+63+] is the main Cr species in solutions of inorganic Cr(III) salts ment of ligands, [Cr(H2O)6 ] is the main Cr species in solutions of inorganic Cr(III) salts under strongly acidic pH, while at pH ≥ 4, Cr(III)-bound H2O molecules undergo