Metal distribution and spectroscopic analysis after soil washing with chelating agents and humic substances

Abstract

Biodegradable chelating agents ([S,S]-ethylenediamine-N,N-disuccinic acid (EDDS) and glutamic-N,N-diacetic acid (GLDA)) and natural humic substances (lignite-derived, standard, and commercially available humic acids) are potentially useful for enhancing soil remediation of timber treatment sites. This study integrated macroscopic and spectroscopic analyses to assess their influence on the distribution and chemical speciation of the remaining metals as well as their interaction with the soil surface after 48-h washing of a field-contaminated soil. The results demonstrated that EDDS and GLDA were an appealing alternative to non-biodegradable ethylenediamine-tetraacetic acid, but the three humic substances were less effective. As shown by sequential extractions, Cu was primarily extracted from the carbonate fraction while Cr and As extraction resulted from (co-)dissolution of the oxide fraction. As a result, the relative proportion of strongly bound organic matter and residual fractions increased by 7-16 %. However, it was noteworthy that the exchangeable fraction also increased by 5-11 %, signifying that a portion of the remaining metals was destabilized by chelating agents and transformed to be more labile in the treated soil. The X-ray photoelectron spectroscopy spectra confirmed the substantial removal of readily accessible Cu from the soil surface, but Cr maintained its original chemical forms of trivalent chromium oxides and iron-chromium coprecipitates, whereas As remained as arsenic trioxide/pentoxide and copper arsenate precipitates. On the other hand, the absence of characteristic peaks of adsorbed carboxylate groups in the Fourier-transform infrared (FTIR) spectra inferred that the extent of adsorption of chelating agents and humic substances on the bulk soil was insufficient to be characterized by FTIR analysis. These results suggested that attention should be paid to the exchangeable fraction of Cu and oxides/coprecipitates of As prior to possible on-site reuse of the treated soil.

Integrating EDDS-enhanced washing with low-cost stabilization of metal-contaminated soil from an e-waste recycling site

“…1b). This was consistent with our previous findings that EDDS complexation with Cu was preferred over Zn and Pb (Tsang Lo et al, 2011a), such that the remaining Cu species in the washed soil were shifted to the exchangeable fraction as indicated by sequential extraction (Tsang et al, 2013a, b;Tsang and Hartley, 2014). Such labile fraction was prone to leaching out and the effect was more discernible in terms of SPLP mobility than TCLP leachability, because the former test employed a less aggressive condition (simulating metal leaching under acid rainfall) where the strongly bound metals leached out by a much smaller extent.…”

“…In chelantenhanced extraction, residual metal-chelant complexes would shift metal distribution to weakly bound fractions and hinder (co-) precipitation in the treated soils, arousing concerns about longterm metal release and on-site reuse (Tsang and Hartley, 2014;Yang et al, 2015). On the other hand, for stabilization approach, short-term leaching potential and long-term stability/bioaccessibility of the immobilized metals in the amended soils would require further investigation (Komarek et al, 2013;Bolan et al, 2014).…”

A laboratory test of NOM-assisted remediation of arsenic and copper contaminated soils

“…Human activities such as wood impregnation and mining cause 7 arsenic (As) and copper (Cu) contamination of soils, which must be 8 removed as As and Cu are non-degradable and toxic posing a threat to 9 human health and ecosystem functioning [1][2][3]. As and Cu can be 10 removed in-situ by phytoremediation on moderately contaminated 11 soils or ex-situ by soil washing of strongly contaminated soils.…”

“…12 Because As and Cu are often strongly bonded to soil solids, both 13 methods often use a strong complexant to increase contaminant 14 solubility, e.g. ethylenediaminetetraacetic acid (EDTA), nitrilotri- 15 acetic acid (NTA) or similar compounds [4][5][6][7][8]. However, although 16 EDTA, NTA and other synthetic aminopolycarboxylates are effective 17 cleaning agents, they are environmentally problematic [9][10][11] 18 suggesting a need for their replacement with cheap natural organic 19 matter (NOM) compounds such as soluble humic substances (HS) 20 and citrate forming strong soluble complexes with Cu and other 21 heavy metals [2,4,5,8,9,12,13].…”

“…ethylenediaminetetraacetic acid (EDTA), nitrilotri- 15 acetic acid (NTA) or similar compounds [4][5][6][7][8]. However, although 16 EDTA, NTA and other synthetic aminopolycarboxylates are effective 17 cleaning agents, they are environmentally problematic [9][10][11] 18 suggesting a need for their replacement with cheap natural organic 19 matter (NOM) compounds such as soluble humic substances (HS) 20 and citrate forming strong soluble complexes with Cu and other 21 heavy metals [2,4,5,8,9,12,13]. In fact, recent studies have shown 22 that NOM compounds such as HS-like materials formed by oxidative 23 hydrolytic degradation of cow slurry, leonardite HS and Sigma- 24 Aldrich HS as well as citrate can extract As, Cu and some other heavy 25 metals but they are clearly less effective than EDTA and NTA [2,8,13].…”