The effect of phytostabilization on Zn speciation in a dredged contaminated sediment using scanning electron microscopy, X-ray fluorescence, EXAFS spectroscopy, and principal components analysis

Abstract

Running Title: Effect of phytostabilzation on Zn speciation in dredged sediment 2 ABSTRACTThe maintenance of waterways generates large amounts of dredged sediments, which are deposited on adjacent land surfaces. These sediments are often rich in metal contaminants and present a risk to the local environment. Understanding how the metals are immobilized at the molecular level is critical for formulating effective metal containment strategies, such as phytoremediation. In the present work, the mineralogical transformations of Zn Seven Zn species were identified at the micrometer-scale : sphalerite, gahnite, franklinite, Zncontaining ferrihydrite and phosphate, (Zn-Al)-hydrotalcite, and Zn-substituted, kerolite-like, trioctahedral phyllosilicate. Bulk fractions of each species were quantified by LSF of the powder EXAFS spectra to linear combinations of the identified Zn species spectra.In the untreated and unvegetated sediment, Zn was distributed as ~50 % (mole ratio of total Zn) sphalerite, ~40 % Zn-ferrihydrite, and ~10-20 % (Zn-Al)-hydrotalcite plus Znphyllosilicate. In unvegetated but amended sediments (AP and TS), ZnS and Zn-ferrihydrite each decreased by 10 to 20 % and were replaced by Zn-phosphate (~30-40 %). In the presence of plants, ZnS was almost completely dissolved and the released Zn bound to phosphate (~40-60 %) and to Zn phyllosilicate plus (Zn,Al)-hydrotalcite (~20-40 %). Neither the plant species 3 nor the co-addition of mineral amendment affected the Zn speciation in the vegetated sediment.The sediment pore waters were supersaturated with respect to Zn-containing trioctahedral phyllosilicate, near-saturation with respect to Zn-phosphate, and strongly undersaturated with respect to (Zn,Al)-hydrotalcite. Therefore, the formation of (Zn,Al)-hydrotalcite in slightly alkaline conditions ought to result from heterogenous precipitation on mineral surface. 4

Is phytoextraction a suitable green treatment for metal contaminated sediments?

“…This result was consistent with the identification of pyrite (FeS) and sphalerite (ZnS) in similar sediments (ISAURE et al, 2002;PANFILI et al, 2005). CdS was also identified in marine sediments (O'DAY et al, 1998).…”

“…The sediment after 5-month culture with A. halleri is currently being studied. PANFILI et al (2005) showed that plants favour the oxidation of ZnS and the formation of Zn-phosphate, so a similar phenomenon might take place for Cd. Open questions include the stability of the Cd,Zn,Fe-sulphide and the nature of the distribution of possible secondary species.…”

“…Before culture, Cd was present as mixed Zn, Cd and Fe sulphide. Based on previous work on Zn in a similar sediment (PANFILI et al, 2005), one can expect the oxidation of sulphides and the redistribution of metals in new phases. A precise characterization of substrate and Cd chemical forms and phytoavailability after culture is needed to test these hypotheses.…”

Development and application of a methodology for screening, on the basis of the carcinogenic (H7), toxic for reproduction (H10) and mutagenic (H11) criteria of the latest European legislation, hazardous vs. non-hazardous sediments to be disposed of on land

“…If the measured total trace elements concentrations correspond only (or mainly) to these dangerous substances, a hypothesis maximizing the potential hazard, the linear correlations between the concentrations (mg/kg) of pairs of trace elements should be highly significant (0.01 level) with a slope of the correlation (or median ratio of concentrations) equal (or very close) to the ratio of atomic mass of the two elements considered, i. e.1.26 for the correlation Zn/Cr, 0.8 for Cr/Zn, 2.77 for Pb/As, 0.36 for As/Pb, 3.99 for Pb/Cr, and 0.25 for the correlation Cr/Pb. Such an indirect estimation of the possible presence of dangerous inorganic substances is made necessary by the fact that neither chemical techniques for operationally-defined speciation of metals (Tessier et al 1979), nor more specific spectroscopic techniques (O'Day et al 2000;Panfili et al 2005;Peltier et al 2005) enable the identification of specific substances.…”

Chemometrics and its applications to x‐ray spectrometry

“…X-ray fluorescence, scanning electron microscopy, extended x-ray absorption fine structure spectroscopy and principal component analysis were applied to the evaluation of the effect of phytostabilization on Zn speciation in a dredged contaminated sediment. 104 Kohonen artificial neural network was applied to classify energy dispersive x-ray spectra of 12 mineralogical samples. By extracting 12 rules and seven element intensities out of 15, 100% correct classification of samples was obtained.…”