Contents lists available at ScienceDirect Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv Bioavailability and sequential extraction of mercury in soils and organisms of a mangrove contaminated by a chlor-alkali plant Paula Renata Muniz Araújo, Caroline Miranda Biondi * , Clístenes Williams Araújo do Nascimento, Fernando Bruno Vieira da Silva, Alfredo Montero Alvarez Department of Agronomy, Federal Rural University of Pernambuco, Dom Manuel de Medeiros Street, S/n - Dois Irmãos, 52171-900, Recife, PE, Brazil ARTICLE INFO Keywords: Trace element heavy metal biogeochemistry estuary soil fractionation ABSTRACT Botafogo river estuary poses the highest Hg concentration reported for mangrove soils in Brazil. Such high contamination took place owing to the improper waste disposal for 24 years from a chlor-alkali plant nearby the estuary. Here we determined Hg concentrations in soils, mangrove plants (Rhizophora mangle and Laguncularia racemosa), and an aquatic organism (the oyster species Crassostrea rhizophorae) to assess Hg bioavailability. Besides, a sequential extraction procedure was used to separate soil Hg into five fractions: water-soluble; ‘human stomach acid’ soluble; organically bound; elemental Hg; mercuric sulfide. Results showed that environmentally available concentration of Hg in the mangrove soils were up to 150-fold higher than a pristine mangrove under the same geological context used as a reference. Additionally, Hg concentration in soils was also above sediment quality guidelines and Hg adverse effects towards sensitive estuarine organisms are likely. Mercury con- centrations in oysters were the highest reported in Brazil, but within safety standards according to the country food security agency. It seems that Hg stocks in the studied soils are governed by organic matter and redox conditions, but changing on environmental conditions and land use can alter this balance and convert these mangrove areas from sink to source of Hg to the environment. 1. . Introduction Mercury (Hg) is one of the most concerning global pollutants owing to its high toxicity, environmental persistence, and bio-accumulation. Production of chlorine and caustic soda is a significant source of Hg contamination as the metal is widely used as a catalyst in chlor-alkali plants and discharged in effluents or emitted in the gaseous elemental form (Hg 0 )(Hisler and Probost, 2006; Song et al., 2018). Thus, chlor- alkali plants can potentially contaminate surrounding soils and water bodies through effluent disposal and atmospheric deposition (Esbrí et al., 2015; Gordeeva et al., 2017). A mercury-cell chlor-alkali plant that eliminated residues for 24 years in the vicinity of the Botafogo River (northeastern Brazil) caused a widespread Hg contamination. The amount of Hg emitted from this plant into the Botafogo estuary is estimated between 22 and 35 tonnes (CETESB, 1984; Meyer, 1996). Mercury concentration in mangrove sediments of Botafogo estuary is one of highest reported for the country, being 15-fold higher than the average value (0.185 ± 0.3 mg kg -1 ) found in the Brazilian coast (Marins et al., 2004). In this scenario, the understanding of the chemical and biological processes occurring at the solid-solution interface of these soils and sediments is essential to pre- dict Hg solubility and bioavailability in the estuary. Soils play a fundamental role in the mangrove Hg cycle, acting both as a sink and source of Hg to biota and adjacent waters (Reis et al., 2016; Zhu et al., 2018). Mercury retention in soils can occur on ad- sorption sites of both organic matter (SOM) and mineral surfaces (Fe, Mn, or Al oxides and silicate minerals), with the Hg availability to plants and organisms being determined by Hg +2 and Hg complexes (Jing et al., 2007; Reis et al., 2016; Zhu et al., 2018). It is also well known that the distribution of a metal in the solid phase of soils or sediments is crucial to determine its environmental mobility and bioavailability (Bloom et al., 2003; Oliveri et al., 2016; Ramasamy et al., 2012). Data on the distribution of Hg in mangrove soils are scarce in Brazil (Araujo et al., 2015) and nonexistent so far to Botafogo es- tuary. Such studies are essential not only to assess the environmental impacts due to the transfer of Hg to organisms but also to support human health risk assessments. Mangroves are regarded as hotspots for methylmercury (MeHg) https://doi.org/10.1016/j.ecoenv.2019.109469 Received 5 May 2019; Received in revised form 19 July 2019; Accepted 22 July 2019 * Corresponding author. Tel.: +55 81 3320 6222; fax: +55 81 3320 6220. E-mail addresses: paula.cienciasolo@gmail.com (P.R.M. Araújo), caroline.biondi@ufrpe.br (C.M. Biondi), cwanascimento@hotmail.com (C.W.A. do Nascimento), ferbruno01@yahoo.com.br (F.B.V. da Silva), amlopez3000@hotmail.com (A.M. Alvarez). Ecotoxicology and Environmental Safety 183 (2019) 109469 0147-6513/ © 2019 Elsevier Inc. All rights reserved. T