## 4 Discussion
Numerous experiments on homogeneous and heterogeneous aqueous-solid systems demonstrated that the impurities (present in the form of soluble salts or suspended materials) are separated from aqueous solution by the so-called dynamic freezing front [60; 61]. Other experiments on the freezing of natural surface waters of Arctic peatlands, similar to the peat porewater of the current study, demonstrated a progressive freezing of water from the reactor edges to the center, accompanied by the relative accumulation of DOM in the remaining liquid [46; 53]. The temporal concentration patterns and elemental correlations demonstrated two contrasting group of solutes--labile elements and insoluble elements. The labile elements were present in essentially ionic forms (alkalis, alkaline-earth metals, oxyanions, and neutral molecules) which did not correlate with DOC, Fe, and Al, and exhibited rather similar concentrations in the fluid prior to freezing and after the entire freeze-thaw sequence. Contrasting to these soluble and labile solutes were divalent metals and insoluble TE\({}^{3+}\) and TE\({}^{4+}\) trace elements, which are known to be carried in the form of organic and organo-mineral (Fe, Al) colloids in the peat porewater (i.e., [15]) and dispersed ice of the peat cores [10]. Therefore, in agreement with previous studies of experimental bi-directional freezing of the entire peat cores (i.e., [44; 45]), we hypothesize that, during freezing front propagation, large size organic-Fe-Al colloids are subjected to preferential exclusion relative to truly dissolved (inorganic) solutes from the ice. As a result, they could form precipitates either in the bulk of the remaining fluid or at the actual interface between the forming ice and the remaining aqueous solution. It is known that high DOM concentrations diminish the proportion of DOC incorporated into the ice phase [20; 62; 63]. In the present study, we observed a positive correlation (R = 0.94, \(p<0.01\)) between the initial DOC concentration in the leachates and the \(K_{d}\) value of DOC. Note however, that a rather high concentration of initial DOM and a fast freezing rate (at \(-20\)\({}^{\circ}\)C) could drive some parts of ionic solutes and even organic colloids to be occluded in the bulk of developing ice in the form of liquid pockets, as is known for sea ice formation [64], and has been recently demonstrated for mineral soils [65; 66]. Some decrease in DOM between the initial leachate and final fluid, after full freezing and thawing, is most likely linked to coagulation, precipitation, and removal (on filters) in the form of amorphous organic-rich solid phases. The detailed molecular mechanisms of this freeze-induced coagulation of ionic solutes and colloids are not known [35; 45; 46]. The chemical nature of coagulated material has not been investigated in the present study, but most likely includes, in addition to organic humic particles (e.g., [67]), organo-mineral composites of Fe and Al (hydr)oxides tightly linked to organic matter (OM). Such complex compounds are primary carriers of trace metal in soils and groundwater [68; 69; 70], as also evidenced by studies of colloidal matter in the surface waters of permafrost peatlands [10; 14; 15; 71]. Despite these measurable, but rather minor, decreases in some component concentrations between the initial leachate and the final fluid obtained after the full freezing and thawing of the reactors, the present study generally evidenced a good recovery of DOC and most solutes over reversible freezing and thawing cycles. The results of the present study thus corroborate those reported earlier regarding the rather minor (i.e., \(<\)10-30%) effects of freeze-thaw cycles on DOC quantity and quality in acidic peat leachates [46]. Overall, this demonstrates the high stability of dissolved (\(<\)0.45 \(\upmu\)m) OC, Fe, Al, and some trace metals to the repetitive freezing and thawing of surface waters in permafrost peatlands.