Sequential extraction of labile and recalcitrant fractions of soil organic matter: A case study focusing on antimony (Sb) in humic acids, fulvic acids and humin fractions of long-term aged contaminated soils.

Soil organic matter (SOM) plays a key role in environmental chemistry of macro and micro nutrients as well as heavy metal (loids). In this research, a modified sequential extraction scheme was used to isolate labile and recalcitrant SOM from organic rich soils after 18 months of ageing with antimony. Humic substances were extracted with a mixture of 0.5 M sodium hydroxide +0.1 M sodium pyrophosphate solution from soils. Then soils deprived of humic substances were sequentially subjected to extraction with glycerol, citric acid, pre-treated with acid and extracted with boiling alkali mixture. The humic acids (HA) and fulvic acids (FA) of isolated SOM fractions were separated and HAs been characterized using FTIR, 1H NMR, and UV-VIS. Acid-alkali treatment of the most recalcitrant SOM fraction (A1-ROM) led to the extraction of sparingly soluble, highly aromatic compound with considerable amounts of N (44% of the extractable N), possibly due to the breakdown of bounds between aromatic rings and amine functional groups. Nevertheless, the highest content of C and TOC was associated with the glycerol extractable SOM. Substantial amounts of Fe and Al were extracted with glycerol, resulting in a dramatic rise of Sb in SOM extracts. The largest increase (60%) in Sb concentrations was observed after the removal of Fe with citric acid. The humic substances (HS) were responsible for 63% of extractable Sb, whereas even after exhaustive alkali extractions 22% of the total Sb remained in the residual humin fraction. Within the HS fraction, 95% of antimony was associated with the low molecular weight FAs. Antimony concentrations in organic fractions correlated significantly with TOC and N contents, possibly due to the role of amine functional groups in Sb complexation. The results of this research highlight the importance of Fe-Al-SOM bridging and humin fraction in sequestration of Sb in recalcitrant SOM pools.

Removal of antimonate (Sb(V)) from aqueous solutions and its immobilization in soils with a novel Fe(III)-modified montmorillonite sorbent.

Over the past decades, contamination of terrestrial environments with antimony (Sb) has aroused a great deal of public concern. In this research, the efficacy of Fe(III)-modified montmorillonite (Mt) (Fe-Mt) for the removal of Sb(V) from aqueous solutions with Sb(V) concentration in the range of 0.2-1 mmol L-1 and immobilization of Sb(V) in soils spiked with 250 mg Sb(V) kg-1 was investigated. The immobilizing mechanisms of the modified clay were assessed by fitting the experimental sorption data with the Langmuir and Freundlich sorption models and a series of single and sequential extraction studies. The results showed that the adsorption data had a better fit with the Langmuir equation (R2: 0.99) and Fe-Mt could efficiently remove up to 95% of Sb(V) at lower concentration ranges. The concentrations of Sb(V) in exchangeable fraction of modified Community Bureau of Reference (BCR) sequential extraction and distilled water extracts of the amended soils decreased dramatically by up to 60% and 92%, respectively. Furthermore, the bioaccessibility of Sb(V) in simulated human gastric juice reduced remarkably by 52% to 60%, depending upon the soil fraction sizes. The results confirmed that Fe-Mt could be a promising candidate for the removal of Sb(V) from aqueous solutions and immobilization of Sb(V) in terrestrial environments.

The influence of different antimony (Sb) compounds and ageing on bioavailability and fractionation of antimony in two dissimilar soils.

Assessing the bioavailability of various Sb substances plays a crucial role in human health and the ecological risk assessment of contaminated soils. However, fate, behaviour and bioavailability of different Sb compounds in soils are insufficiently known. Therefore, in this present study, the effects of soil properties and ageing on bioavailability of four different Sb compounds (C8H4K2O12Sb2, Sb2S3, Sb2O3 and Sb2O3 nanoparticles) were evaluated during 120 days ageing time. A black soil (BS) with approximately 12% organic matter (OM) and a red soil (RS) with less than 1% OM were amended with 1000 mg Sb kg-1 of different Sb compounds and subjected to single extractions with distilled (DI) water, 2M HNO3, Simplified Bioaccessibility Extraction Test (SBET) and a modified Community Bureau of Reference (BCR) sequential extraction method. The results revealed that there are substantial variations in dissolution rate of various Sb sources, depending upon soil type and Sb compound. The amounts of DI water extractability of Sb during the incubation time varied between <1% and 2%, whereas HNO3 extractable fractions and Sb bioaccessibility at the end of ageing time ranged between about 1%-3% and <1%-9% of the total Sb, with maximum bioaccessibility observed in BS contaminated with C8H4K2O12Sb2. The residual and labile fractions accounted for 77-93% and 0.1-4% of the total Sb, respectively, indicating that Sb is mostly associated with recalcitrant fractions of the soils. The results of single and sequential extraction studies revealed that source of Sb, ageing time and soil properties can greatly affect the bioavailability of Sb in soils. The findings of this research provide a deeper understanding of the potential risks associated with Sb compounds and highlights the role of site-specific considerations for improving the robustness of toxicity guidelines and long-term management of Sb contaminated sites.

In situ stabilization of As and Sb with naturally occurring Mn, Al and Fe oxides in a calcareous soil: bioaccessibility, bioavailability and speciation studies.

This study investigated the effectiveness of 6 different types of naturally occurring manganese, aluminum and iron oxides for stabilization of As and Sb in a calcareous soil spiked with 50mgkg(-1) of As or Sb and two dosages of treatments (2% and 5%). The resulting contaminated soils were subjected to a series of chemical extraction studies including sequential extraction, single step extraction with DTPA and Simplified Bioaccessibility Extraction Test (SBET) for estimation of bioaccessible fraction of As and Sb in soil and a greenhouse experiment using barley as the test crop. The results showed that Fe-associated and carbonate-bound fraction of As and Sb were predominant fractions. However, the amounts of labile fractions were higher in As contaminated soils, whereas the percentage of Sb associated with crystalline Fe-oxide and residual fractions were higher. The results revealed that application of natural metal oxides reduced DTPA and SBET extractable amounts and plant uptake of As and Sb. After application of amendments, the exchangeable fraction of As decreased dramatically by up to 82% while Sb exchangeable fraction decreased by up to 60% depending upon the additive. The results of chemical extractions and plant uptake confirmed that Sb had lower bioavailability, compared with As.