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

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