Contents lists available at ScienceDirect Environmental Research journal homepage: www.elsevier.com/locate/envres Remediation of chromium and mercury polluted calcareous soils using nanoparticles: Sorption –desorption kinetics, speciation and fractionation Mohamed Moharem a , Elsayed Elkhatib b, ⁎ , Mohamed Mesalem b a Regional Center for Food and Feed, Agricultural Research Center, Alexandria, Egypt b Department of Soil and Water, College of Agriculture (Elshatby), Alexandria University, Alexandria 21545, Egypt ARTICLE INFO Keywords: Sorption/desorption Speciation Kinetics Sequential extraction Chromium and mercury polluted soil ABSTRACT Stabilization is an emerging technology for the cost-effective remediation of heavy metals polluted soils. To evaluate the potential of water treatment residual nanoparticles (nWTR) in reducing Hg and Cr mobility in contaminated calcareous soil, sorption-desorption kinetics; speciation and fractionation experiments were per- formed. Application of nWTR strongly enhanced Cr and Hg sorbed in the calcareous soil, whereas the released amount of both metals through 6 successive desorption steps dramatically decreased. The power function model best described the desorption kinetic data of Cr and Hg from nWTR amended and non-amended calcareous soil. Fractionation experiment data demonstrated that nWTR amendment significantly increased metals concentra- tion in the residual fraction (RS) and simultaneously decreased the more accessible forms of Hg and Cr. Addition of nWTR at a rate of 0.3% to the contaminated calcareous soil significantly increased Hg and Cr in the RS fraction from 69.27% and52.62% to 93.89% and 90.05% respectively. Additionally, the formation of stable Hg and Cr species such as Hg(OH) 2 amor, CrSO 4 . xH 2 O and Cr(OH) 2 ) were increased as a result of nWTR appli- cation. These findings jointly indicate the enhancement of Hg and Cr immobilization in the nWTR amended calcareous soil. FTIR spectroscopy analysis indicated the contribution of OH group and Al-O-Si of nWTR in Hg and Cr sorption process and suggests chemo-sorption reaction between both metals and the nWTR surface functional groups. Overall, the final results confirm the strong capability of nWTR application in reducing Hg and Cr risks in highly contaminated sites of the calcareous soil. 1. Introduction Soils polluted with heavy metals can potentially lead to severe en- vironmental problems due to the hazardous effects of heavy metals on human and environmental health (Jiang et al., 2017; Burges et al., 2015). Increasing heavy metals content in agricultural soils can reduce soil fertility and increase toxic metals accumulation in food chain which may cause health problems to the consumers (Xiong et al., 2016; Pierart et al., 2015). Several disorders and major public health issues in hu- mans including respiratory, reproductive, immune, and digestive sys- tems can be associated with chromium and mercury exposure (Jarup, 2003). Thus, restoration of soils contaminated with chromium and mercury is urgently needed. The chemical form and species of heavy metals can govern the transport and fate of metals in soil environment. Once heavy metals enter the soil system, they undergo initial fast sorption reaction, fol- lowed by slow sorption reaction; consequently be fractionated into various chemical forms with different mobility, bioavailability, and toxicity (Buekers, 2007; Shiowatana et al., 2001). Investigation of the chemical forms of heavy metals in soils could furnish valuable in- formation necessary regarding assessing their potential environmental impacts and remediation strategies action (Wu et al., 2016; Liu et al., 2013). In general, reducing heavy metals availability in contaminated soils could be achieved by using low cost amendments with the cap- ability in converting metal into stable geochemical phases through sorption, complexation and precipitation processes (Elkhatib et al., 2013; Hashimoto et al., 2009). The use of such approach in remediation of heavy metals contaminated sites is more sustainable than the con- ventional techniques due to their high cost, time consuming and en- vironmentally-destructive nature (Khalid et al., 2016). Recently, low cost waste materials have been frequently used in order to sequester heavy metals in polluted soils (Lu et al., 2017; Bolan et al., 2014; Ok et al., 2011). One of such materials is the waste by- product of drinking water industries which known as water treatment residuals (WTRs). Iron or aluminum salts are commonly used for water clarification via the processes of colloid destabilization, flocculation https://doi.org/10.1016/j.envres.2018.12.054 Received 8 November 2018; Received in revised form 15 December 2018; Accepted 21 December 2018 ⁎ Corresponding author. E-mail address: selkhatib1@yahoo.com (E. Elkhatib). Environmental Research 170 (2019) 366–373 Available online 27 December 2018 0013-9351/ © 2018 Elsevier Inc. All rights reserved. T