Bioaccessibility of As and Pb in orchard and urban soils amended with phosphate, Fe oxide and organic matter

Abstract

Soils historically contaminated in urban and orchard environments by Pb and As were amended separately with organic matter, soluble Ca phosphate, and Fe oxide to determine whether these materials could lower Pb or As bioaccessibility. After 5 years of equilibration in the laboratory, the amended soils and control were tested for bioaccessibility using the standard physiologically based extraction test (PBET). Bioaccessibilities of Pb and As were not substantially reduced relative to the unamended controls after the 5-year period by any of the soil amendments. Gastric bioaccessibility (GB) of Pb was in all cases much greater than gastrointestinal bioaccessibility (GIB) regardless of soil treatment, whereas GB and GIB of As were similar in magnitude for all soils. Both GB and GIB of Pb were greater in the orchard than the urban soil. Electron microprobe investigations identified discrete particulate forms of Pb in the soils by elemental mapping, and energy dispersive spectrometry (EDS) revealed a frequent spatial association of Pb-rich particles with phosphorus. It is suggested that Pb-rich particles in anthropogenically contaminated soils resist chemical transformation into less labile forms despite thermodynamic favorability because of their low surface area and low solubility. This kinetic effect could explain the observed ineffectiveness of amendments in reducing metal bioaccessibility.

Phytoavailability of Lead for Vegetables in Urban Garden Soils

“…17−19 In addition, the association of Pb with phosphates had been observed in urban and orchard environments using a scanning electron microscope equipped with energy dispersive X-ray detector. 20 In general, the forms of Pb in urban soils contaminated due to industrial and residential activities possess relatively lower bioaccessibility (∼2−24%) compared to that in mine-impacted soils as determined by in vitro methods at pH 2.5. 17 −19,21 However, variation of contamination sources and inherent soil properties requires site-specific studies to properly assess the risk associated with urban gardening in a contaminated site.…”

Lead Speciation and In Vitro Bioaccessibility of Compost‐Amended Urban Garden Soils

“…Soils amended with P fertilizer and compost have shown promising results in reduced bioaccessibility when tested against nonamended soils; however, the bioaccessibility of soil Pb did not change or changed very little when the contact time of soil to amendment increased (Hettiarachchi et al, 2000, 2001; Cai et al, 2017). We observed the same in our field studies.…”

Geochemical processes during the stabilization of potentially toxic elements (PTEs) in contaminated soil by using mineral amendments

“…Some recent studies employed a more holistic approach in order to evaluate the stabilization process and determine the factors controlling it by determining the residual, labile, phytoavailable and bioaccessible metal fractions in the treated soils. These studies include a variety of amendments, such as sepiolite (Sun et al, 2013;Abad-Valle et al, 2016), bentonite (Sun et al, 2015), limestone and red mud (Gray et al, 2006;Lee et al, 2009), carbonates and phosphates (Siebielec & Chaney, 2012), Fe-oxides or organic matter (Cai et al, 2017), and Mn-oxides (Sonmezay et al, 2012). The aim is mainly to compare the stabilization efficiency of different amendments regarding a certain contaminated soil.…”

“…The presence of such phases in Stratoni and Lavrion sample which contain high sulfur concentrations in their porewaters can not be excluded although they were not identified during examination by SEM-EDS, possibly due to overlapping spectral peaks of Pb Mα and S Kα at 2.28-2.35 keV region (Newbury, 2009). The impact of the clay amendments in the bioaccesibility of Pb was compared to the effectiveness of other amendments such as organo-clays, limestone, red mud, bauxite, organic matter and phosphates (Gray et al, 2006;Lee et al, 2009;Sarkar et al, 2012;Siebielec & Chaney, 2012;Sonmezay et al, 2012;Sanderson et al, 2015;Cai et al, 2017). None of these treatments could reduce significantly the bioaccessible Pb fraction in the applied soils, even after a considerable aging time had passed (Gray et al, 2006;Cai et al, 2017).…”

“…The impact of the clay amendments in the bioaccesibility of Pb was compared to the effectiveness of other amendments such as organo-clays, limestone, red mud, bauxite, organic matter and phosphates (Gray et al, 2006;Lee et al, 2009;Sarkar et al, 2012;Siebielec & Chaney, 2012;Sonmezay et al, 2012;Sanderson et al, 2015;Cai et al, 2017). None of these treatments could reduce significantly the bioaccessible Pb fraction in the applied soils, even after a considerable aging time had passed (Gray et al, 2006;Cai et al, 2017). Sanderson et al (2015), who applied different types of amendments in shooting ranges (lime, phosphoric compounds, red mud) concluded that the reduction in the bioaccesibility of Pb depended upon the type of soil, as well as the presence of soil colloidal phases and organic matter that may enhance or inhibit the stabilization properties of the amendments.…”