# Organic carbon, major and trace element release from and adsorption onto particulate suspended matter of the Ob River, western Siberia

Artem G. Lim, Ivan V. Krickov, Oleg S. Pokrovsky

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Artem G. Lim, Ivan V. Krickov, Oleg S. Pokrovsky. Organic carbon, major and trace element release from and adsorption onto particulate suspended matter of the Ob River, western Siberia. Science of the Total Environment, 2024, 948, pp.174735. 10.1016/j.scitotenv.2024.174735. insu-04831463

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# Organic carbon, major and trace element release from and adsorption onto particulate suspended matter of the Ob River, western Siberia

Artem G. Lim<sup>a</sup>, Ivan V. Krickov<sup>a</sup>, Oleg S. Pokrovsky<sup>b,c,*</sup>

<figure><img src="image_3.png" /></figure>

## HIGHLIGHTS

* High capacity of particulate suspended matter (PSM) to release DOC and nutrients and adsorb riverine solutes

* Adsorption of aromatic, high molecular weight organic C and colloids onto PSM

* Nutrients (P, Si, Zn, V) and toxicants (As, Cd) from tributary and fen waters are adsorbed onto PSM.

* PSM of a large river as a hydrochemical composition regulator for floodplain water bodies

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

Editor: Ouyang Wei

**Keywords:**

Trace metals

Micronutrient

Sorption

River water

Chemical composition

## ABSTRACT

Particulate suspended matter (PSM) of rivers is a significant factor for carbon, nutrient, and trace metal transfer from land to ocean. Towards better understanding the role that PSM exerts on major and trace elements in riverine systems, here we report the results of an experimental study which utilizes a two-fold approach to assess interaction between PSM and riverine solutes. First, we measured element leaching (via desorption and dissolution in distilled water, simulating snow melt) from PSM of the largest Siberian river, the Ob River. Second, we quantified the capacity of PSM to adsorb dissolved organic carbon (DOC), macro- and micronutrients and trace elements from organic-rich waters of the river floodplain. We documented sizable desorption of organic carbon, some major and trace metals, oxyanions and insoluble elements from PSM; the majority (>50 %) of elements were released over the first hour of reaction. In contrast, PSM of the Ob River was capable of removing 20 to 90 % of dissolved OC, nutrients (Si, P), and trace elements from the tributary and floodplain fen. Our experiments demonstrated preferential adsorption of aromatic compounds large molecular size colloids. Taken together, the adsorption of solutes by PSM can sizably decrease the concentration and modify the molecular size distribution, and therefore the potential bioavailability of major (DOC, P, Si) and trace micronutrients. Overall, the PSM of the Ob River exhibited high reactivity with respect to natural waters and was capable of modifying the elemental composition of the tributary and floodplain fen waters. This transfer of organic carbon and nutrients in the

<footnote><sup>a</sup> BIO-GEO-CLIM Laboratory, Tomsk State University, Tomsk, Russia</footnote>

<footnote><sup>b</sup> Geosciences and Environment Toulouse, UMR 5563 CNRS, 14 Avenue Edouard Belin, 31400 Toulouse, France</footnote>

<footnote><sup>c</sup> Institute of Ecological Problems of the North, N. Laverov Federal Center for Integrated Arctic Research, Arkhangelsk, Russian Academy of Science, Russia</footnote>

<footnote>* Corresponding author at: Geosciences and Environment Toulouse, UMR 5563 CNRS, 14 Avenue Edouard Belin, 31400 Toulouse, France.
E-mail address: oleg.pokrovsky@get.omp.eu (O.S. Pokrovsky).</footnote>
surface-adsorbed (particulate) form is especially important during spring flood and requires specific consider-
ation in short-term biogeochemical cycles of elements in continental waters.

**1. Introduction**

Modification of the export of major and trace components by Arctic and subarctic rivers under climate warming and permafrost thaw is an important factor controlling carbon cycle and nutrient exchange between the land and the Arctic Ocean (Bring et al., 2016; Drake et al., 2018; Beel et al., 2020; Saros et al., 2023). Within this context, significant research has focused on riverine macro- and micronutrient transfer in high latitude regions (Hobbie et al., 1999; Holmes et al., 2012; Guo et al., 2004; Frey and McClelland, 2009; Heinze et al., 2015; Toohey et al., 2016). Primary interest in these investigations was typically centered on dissolved elemental concentrations (Gordeev et al., 1996; Cooper et al., 2008; Vonk et al., 2015; O'Donnell et al., 2016; Tank et al., 2016; Kaiser et al., 2017), because the majority of solutes in Arctic rivers, including C, are carried as dissolved rather than particulate forms (Lobbes et al., 2000). The dominance of dissolved over particulate export is due mostly to the relatively low runoff and high concentration of the main carrier of many lithogenic elements, micro-nutrients and toxicants – dissolved organic matter (DOM). The latter provides high stability of organic and organo- Fe-Al colloids, which are responsible for elemental flux of nutrients and trace elements in large and small rivers of the Eurasian Arctic (Pokrovsky et al., 2010; Krickov et al., 2019; Chupakov et al., 2023).

At the same time, the importance of particulate riverine flux is widely recognized on a global scale (i.e., Schlesinger and Melack, 1981; Meybeck, 1982; Gebhard et al., 2004; Viers et al., 2009; Oelkers et al., 2012; McClelland et al., 2016). Notably, due to their low solubility, the particulate flux of limiting nutrients like Si, P and Fe exceeds their corresponding dissolved flux by factors of 50, 100 and 350 (Oelkers et al., 2011; Jeandel and Oelkers, 2015). The supply of mineral particles to the ocean is predicted to alter significantly with climate change (Gislason et al., 2009; Passow et al., 2014). In some geologically active regions of the world, or those impacted by glacial melting, riverine particulate flux is far more climate-sensitive compared to the corresponding dissolved flux (Gislason et al., 2006). For these reasons, the importance of particulate flux should not be underestimated for Arctic and subarctic regions, especially in regards to climate warming and permafrost thaw, enhancing solid material delivery to rivers. Other consequences of climate change in high latitudes include active layer thickening, coastal abrasion, thaw slumps and massive floods (Krickov et al., 2019; Lafrenière and Lamoureux, 2019; Lim et al., 2019; Karja-lainen et al., 2020; Gibson et al., 2022).

In decades past, considerable progress has been made assessing the elemental composition of river particles in Artic and sub-arctic regions, including the European part of Russia (Shevchenko et al., 2010; Savenko et al., 2004; Pokrovsky et al., 2010), Siberian Rivers (Gordeev et al., 2004; Pokrovsky et al., 2005; Pokrovsky et al., 2006a, b; Kutscher et al., 2017), Canada (Gaillardet et al., 2003; Javed and Shotyk, 2018), the Mackenzie Basin (Millot et al., 2010), and, most extensively, the 6 large Arctic rivers sampled across seasons in the PARTNERS and ARCTIC GRO programs (Holmes et al., 2002, 2012). To date, only a few studies in western Siberia characterized the mineralogical and total chemical composition of PSM with high seasonal resolution (Krickov et al., 2018, 2020, 2023).

In addition to the physical delivery of organic and inorganic matter from land to ocean, particulate suspended matter (PSM) plays two potentially important roles in the river system. Firstly, some labile components of PSM, which are freshly delivered to the river from coastal abrasion or via tributaries, can be subject to leaching, thereby enriching the river in dissolved forms of C and major and trace elements, including macro- and micronutrients. Secondly, when the PSM of a large river is

delivered and spread over its floodplain, there could be physico-
chemical interactions between various solutes including DOC and
riverine PSM surfaces. Indeed, the high reactivity of PSM in oceanic
coastal waters is fairly well known (Jeandel and Oelkers, 2015) as
riverine particles can release elements to seawater by desorption, ex-
change or dissolution (Jones et al., 2012a, b, Jones et al., 2014). The role
of PSM as a regulator of trace metals in temperate and subtropical rivers
is also well known (Lu and Allen, 2001; Han and Zeng, 2021; Cao et al.
2023; Liu et al., 2024). However, in contrast to the sizable number of
experiments centering on PSM biogeochemistry in marine systems, little
is known about the reactivity of suspended particles in freshwater sys-
tems, especially those within boreal regions where waters are rich in
dissolved organic matter.

To assist in filling this research gap, we collected PSM from the Ob River, the largest Arctic river in Siberia. We developed an experimental approach to 1) quantify the capacity of PSM to release major and trace elements (< 0.22 µm) into aqueous solution and 2) assess the adsorption potential of PSM with respect to solutes, including colloidal forms, of water bodies within the Ob River floodplain. We hypothesized a non-negligible role of PSM as a potential regulator of main river water chemistry, via release of organic and inorganic solutes using short-term incubation experiments. We further suggested that PSM is capable of adsorbing some amount of DOC and macro- and micronutrients present in the water of a tributary, fen zone, or coastal grass leachates of the floodplain, all of these are subjected to intense flooding and PSM deposition during massive spring freshet in this region. We anticipate that experimental methods developed here will improve understanding of the factors controlling C and major and trace element interaction between riverine particles and surface waters in the boreal subarctic zone.

**2. Sampling and analyses**

2.1. Suspended matter from the Ob River and surface waters of the floodplain

Particulate suspended matter has been collected from the middle course of the Ob River, of the Western Siberian Lowland, which is the largest reservoir of northern peatlands in the world (Kremenetski et al., 2003). Discharge of this river and the concentration of PSM are at maximal during May's spring flood period (i.e., Vorobyev et al., 2019; Krickov et al., 2023). The Ob River floodplain forms a system of interconnected shallow ephemeral lakes and primary and secondary water channels which extend 10–30 km from the main water channel (i.e., Vorobyev et al. (2015, 2019) and Krickov et al. (2021). Dominant alluvial soils of the floodplain are overlain by a thin (~30 cm) humic layer. Regional vegetation is represented by trees (Betula, Salix and Populus, etc.), shrubs (Rosa canina, Viburnum, Ribes, etc.) and grasses (Calamagrostis, Phleum, Veronica longifolia, etc.).

During the high-water period in spring, Ob River water that carries large amount of PSM which, upon flooding, enters the floodplain and interacts with surface waters of various origins. The most common waters present in the floodplain during this period of the year are 1) shallow fens/lakes subjected to temporary flooding, such as the Ishtan fen located at the floodplain of the Ob River middle course, near INTERACT station Kaibasovo (Krickov et al., 2021), and 2) products of snowmelt that accumulate in grass (essentially *Carex acuta*) covered depressions or puddles, i.e., grass leachates. A third water type present, also subjected to the influence of PSM, is from small tributaries along terraces that is being discharged directly into the main stem of the Ob River. A tributary of the Ob River, the Andreva River which is located
within 12 km of the main stem sampling point, was chosen to represent the third water type. It is a small river originating from upland peat bogs that drain through taiga forests with minimal residence times on the floodplain. The Andreva River (watershed area of 257 km²; length of 38 km; mean annual discharge is 2.3 m³ s⁻¹) transitions from dense pristine taiga forests to birch and willow forests along the narrow floodplain (Krickov et al., 2023). Based on GIS assessment of the land cover (i.e., Krickov et al., 2023), its watershed is covered by birch-pine forest on wetlands (31 %), low pine forest on peatlands (25 %), larch trees (20 %), meadows (19 %), and fir pine forest (5 %). Dominant soils are peatland histosols (88 %) and alluvial (9 %) and hydromorphic podzols (3 %). The rather transparent (low PSM), organic-rich waters of the Andreva River are discharged into murky, PSM-rich waters of the Ob River at the mouth zone of the tributary.

## 2.2. Sampling and characterization of PSM

On May 14, 2021, during intese spring flood, large water sample was taken from the middle of the, Ob River at 0.5 m depth in several pre-cleaned polypropylene barrels (30 to 50 L). PSM was collected via direct filtration of large volumes (20 to 30 L) of river water with an Inox (AISI 304) Teflon® PTFE-coated filtration unit (Fisher Bioblock) equipped with 142 mm acetate cellulose Sartorius membranes (0.45 µm) and operated at 5–7 bars. An average flow rate of 1–2 L h⁻¹ was created by a peristaltic pump (MasterFlex B/T) with Teflon tubing. Immediately after collection, the PSM was freeze-dried and stored in the refrigerator prior the analyses and experiments.

Elemental composition of the PSM was assessed via full acid leaching (HF, HCl and HNO₃ attack) in a clean room (class A 10,000) following ICP-MS (Agilent 7500 ce) analyses using methods for organic-rich natural samples employed by Géosciences *Environment Toulouse* (GET) laboratory as described in detail elsewhere (Krickov et al., 2020, 2023). The mineralogical composition of the particulate fraction was studied on a D8 ADVANCE (Bruker AXS) X-ray diffractometer (Cu-Kα radiation, Ni filter, at 40 kV and 40 mA) equipped with a LYNXEYE linear detector (see Krickov et al., 2023 for details). Identification of clay minerals after thermal and ethylene-glycol treatment followed standard procedures (Moore and Reynolds, 1989). The B.E.T. specific surface area (SSA) of suspended material was measured by multi-point N₂ adsorption (Auto-sorb-1, Quantachrome Instr.) with an uncertainty of 10 %.

## 2.3. Experimental setup

A cartoon of experiments with riverine particulate matter is presented in Fig. 1. Desorption of major and trace elements from the PSM was studied in 50-mL aseptic, single-used polypropylene reactors with Milli-Q water (assuming analogy with low-mineralized snowmelt, Krickov et al., 2022), where different amounts of PSM (1, 3, and 10 g dry weight L⁻¹) were added. The reactors were shaken using a ping pong shaker at 5 °C (a typical temperature of the Ob River during spring flood; Krickov et al., 2021) and with fluid being sampled after first hour, 5 h, 12 h, 1 day, 3 days and 10 days of exposure. Samples were then centrifuged (3000 g, 15 min) and filtered through a 0.22 µm Sartorius acetate cellulose, syringe-driven filter unit.

Adsorption experiments were performed at 1, 3 and 10 g PSM L⁻¹ with 0.22 µm-filtered fen water (Ishtan), a tributary (Andreva) and the leachate of Carex grass, collected at the shore of the Ob River. The grass leachate was prepared via reacting 10 g L⁻¹ of dry grass with Milli-Q water during 3 days at 5 °C in the dark, and sterile-filtered through 0.22 µm Nalgene disposable filter unit. Experimental reactors (50 mL polypropylene vials) were shaken during 3 days at 5 °C in the dark.

Both release and adsorption experiments were run in duplicates and for each sampling, two entire reactors were sacrificed. Sampling consisted in measuring pH and syringe-filtering through a 0.22 µm acetate cellulose Sartorius filter unit. Experimental control reactors were run in Milli-Q without PSM and natural water filtrates, and processed exactly in the same way as leaching or adsorption samples. All samples were stored in a refrigerator for several days prior to analyses.

## 2.4. Analyses

The pH was measured via combined Schott glass electrode with an uncertainty of 0.01 pH units. DOC and Dissolved Inorganic Carbon (DIC) were determined by a Shimadzu TOC-VSCN Analyzer (Kyoto, Japan) with an uncertainty of 3 % and a detection limit of 0.1 mg L⁻¹. Specific UV-absorbance (SUVa₂₅₄), which is a proxy for aromatic C and dissolved organic matter (DOM) source (Weishaar et al., 2003; Ilina et al., 2014), was measured using 10 mm quartz cuvettes in a CARY-50 UV-VIS spectrophotometer (Bruker, UK). The UV and visual absorbance of non-acidified filtered and ultrafiltered samples was measured between 200 and 700 nm. This allowed for assessment of spectral slopes (S₂₉₀–₃₅₀, S₂₇₅–₂₉₅, S₃₅₀–₄₀₀), slope ratio (S<sub>R</sub>), aromaticity index (C<sub>Ar</sub>), molar absorptivity (MA) and weight average molecular weight index (WAMW), as reviewed in Kurashev et al. (2024). Note that the MA is defined via

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Scheme of conducted experiments on solute release from riverine particulate suspended matter (PSM) and adsorption of the dissolved load from a tributary (Andreva River), grass leachate and floodplain fen onto PSM of the Ob River.</figcaption></figure>
absorbance at 280 nm as (A<sub>280</sub> × 1000) / (DOC/12) and used to
calculate the WAMW as (3.99 × MA) + 490 according to Chin et al.
(1994). The SUVA<sub>254</sub> (L mg C<sup>-1</sup> m<sup>-1</sup>) can also be used to estimate the
level of dissolved aromatic carbon (C<sub>Ar</sub>) following Weishaar et al. (2003)
and Pugach et al. (2018): C<sub>Ar</sub> = 6.52 × SUVA<sub>254</sub> + 3.63, where C<sub>Ar</sub> is the
percentage of total carbon present as aromatic carbon.

Filtered solutions for major and trace element analyses (including P and Si) were acidified (pH ~ 2) with ultrapure HNO₃ and stored in pre-cleaned HDPE bottles. Blanks were performed to control the level of pollution induced by sampling and filtration. In addition to conventional (< 0.22 µm) filtration, filtrates of 10 gPSM L⁻¹ of adsorption experiments were processed for colloidal/Low Molecular Weight size separation using Amicon Ultracell (50 mL) centrifugal/ultrafiltration single-use cartridges. The Amicon polypropylene vials were fitted with 3 kDa membranes (regenerated cellulose). They were initially treated with 1 % HNO₃ and then rinsed 3 times via passing 50–100 mL of MilliQ water through the membrane. The first 15-mL portion of the sample ultrafiltrate (15 mL) that passed through the membrane was rejected.

Major cations (Ca, Mg, Na and K), Si, P, and ~ 40 trace elements were determined using an ICP-MS Agilent ce 7500 with In and Re as internal standards and 3 external standards (1, 10 and 100 µg L⁻¹). For all major and most trace elements analyzed by ICP MS, concentrations in experimental control reactors were below analytical detection limits, determined as three times signal intensity of 2 % HNO₃ (≤ 0.1–1 ng L⁻¹ for Cd, Ba, Y, Zr, Nb, rare earth elements (La-Lu), Hf, Pb, Th and U; 1 ng L⁻¹ for Ga, Ge, Rb, Sr and Sb; ~ 10 ng L⁻¹ for Ti, V, P, Cr, Mn, Fe, Co, Ni, Cu, Zn and As). The SLRS-6 (Riverine Water Reference Material for Trace Metals certified by the National Research Council of Canada) was used to check accuracy and reproducibility of analyses (Yeghicheyan

et al., 2019).

3. Results and discussion

3.1. Element release from PSM of the Ob River

The concentration of PSM in the Ob River typically ranges from 20 to 100 mg L⁻¹ (mean 60 mg L⁻¹) during the spring flood (Krickov et al., 2023). The mean chemical composition of the PSM was dominated by Si (20 %), Al (7.8 %), Fe (5.1 %) and organic carbon (5.8 %) as obtained by total chemical analysis (Table S1). The mineralogical composition of the PSM was dominated by quartz (35 %), albite (10 %), illite (16 %), chlorite (17 %), smectite (12 %), and carbonate minerals (< 5 %) and its SSA B.E.T. was equal to 37.5 m²/g. Relatively high Fe concentration in the PSM suggests that main aluminosilicates minerals are coated by Fe oxy(hydr)oxides. Note that hetero-aggregation of Fe hydroxide particles with the surface of aluminosilicates minerals of PSM is widely known in temperate rivers (i.e., Baalousha et al., 2019).

The Ob River receives a sizable amount of clastic mineral particles (quartz, feldspars, pyroxenes and amphiboles) due to physical erosion of podzol soil mineral horizons. In addition to mineral soil particles, we do not exclude the possibility of peat erosion from the upland bogs (Stepanova et al., 2015) as a source of minerals in the PSM of the Ob River and its tributaries. Indeed, a large variety of minerals have been identified in western Siberian peat cores from detrital, clay, and authigenic origins (i.e., Rudmin et al., 2018).

Regardless of PSM concentration, the pH of the suspension increased from ca. 7.0–7.5 at the beginning to 7.8–8.0 and remained stable after the first day of reaction. Leaching experiments demonstrated a sizable

<figure><img src="image_6.png" /><figcaption>Fig. 2. Kinetics of solute (< 0.22 µm) release (desorption and dissolution) from PSM of the Ob River at three different concentrations (1, 3 and 10 g L⁻¹) interacting with distilled water. Error bars are s.d. of duplicates. Solid lines are for eye guide. Note log X scale.</figcaption></figure>
release of organic carbon, some major (Na and K) and trace metals (V, Cr, Co, Ni, Cu and Zn), oxyanions (P, As and Se) and insoluble elements (Al, Ti, Ga, Y and Zr) from PSM; the majority (> 50 %) of elements were released over the very first hour of reaction (Fig. 2). More specifically, kinetics of elements leaching from the PSM into the fluid phase demonstrated a complex pattern of element mobilization as seen on the temporal dependence of element concentrations in solution (Fig. S1) and PSM mass-normalized concentrations (Fig. 2). First, there was fast desorption, occurring over the first few hours of reaction, which essentially controlled the behavior of DOC, labile cations (Na and K), low solubility hydrolysates (Be, Al, Ti, Cr, Ga, Y, Zr and REEs), Ge and divalent metals (Mn, Co, Ni, Cu, Zn, Cd and Pb). Second, there was a gradual increase in concentration, which has been detected for labile oxyanions and neutral molecules (B, Si, P, V, As, Se, W) and some alkali and alkaline-earth metals (Li, Mg, Ca, Sr, Rb, Ba), as illustrated for several examples in Fig. 2 and Fig. S1.

To quantify the attribution of elements to different groups depending on the pattern of their release from PSM, we calculated the percentage of release during the first hour of reaction relative to the total amount released over 72–240 h of reaction (Table S2). The first hour of reaction represents exclusively desorption phenomena, because this period is typically sufficient to achieve mineral surface – aqueous solution equilibrium (90 ± 10 % completeness of reaction), given the fast rates of adsorption/desorption processes (Stumm, 1992; Pokrovsky et al., 2006a, 2012). The period of 72 to 240 h depended on the identity of the element and represented the maximal steady-state concentration in contact with PSM. Based on the data listed in Table S2, 3 groups of solutes could be distinguished: 1) labile, soluble elements which exhibited <20 % release during first hour of reaction, 2) DOC and low-soluble lithogenic elements, likely associated with organo-ferric colloids in aqueous solution, and finally, 3) elements strongly complexed to DOM and affected by redox processes, oxyanions, and elements associated with HMW colloids. In the 1st group, elements were represented essentially by labile anions and cations, likely reflecting dissolution of carbonate minerals or some labile silicate amorphous or crystalline phases. Fast desorption of these elements from PSM was not the main factor controlling their release to the aqueous solution; for this reason, the majority of these elements were gradually released due to dissolution of their host mineral phases. Note that in addition to soil clay minerals, these elements may originate from leaching of snowmelt particles, notably of highly reactive Ca/Mg carbonates, which are known to occur in western Siberian snow coverage (Shevchenko et al., 2017; Krickov et al., 2022). It should also be noted that gradual increase in Si concentration during PSM reaction with aqueous solution (Fig. 2, Fig. S1) likely reflects Si leaching from biogenic silica pools (diatom frustules and/or plant phytoliths; Fraysse et al., 2009). The lack of concomitant Al release (Fig. 2, Fig. S1) suggests a negligible role of short-range ordered aluminosilicates and allophanes (i.e., Lim et al., 2023) in overall Si release from PSM. The 2nd group was represented by elements that were chiefly affected by fast desorption from PSM particles as they showed 50–100 % release during the first hour of reaction. This group combined DOC and low soluble elements that were complexed to DOM (Fe, Zn and Cd) or associated with organo-ferric colloids in aqueous solution (Be, Ti, Cr, Ga, Y, Zr and REEs). These elements likely desorbed from the PSM surface into the aqueous solution in the form of organic complexes and organo-mineral colloids. The latter are known to dominate surface waters of western Siberia (i.e., Pokrovsky et al., 2016) including in the Ob River’s middle course region (Vorobyev et al., 2015; Kolesnichenko et al., 2021). Note that there was an approximate two times higher release of light REE (LREE, such as La, Ce, Pr, Nd) during the first hour of reaction compared to heavy REE (HREE, such as Er, Yb). This is consistent with the affinity of LREE to adsorb to mineral surfaces and HREE capacity to complex with organic ligands in the aqueous solution (Bau, 1999; Vasyukova et al., 2010). The 3rd group is transitional (20 to 50 % of element released over the first hour of reaction) with elements strongly complexed to DOM (Al, Ni and Cu),

associated with high molecular weight (i.e., 30 kDa – 0.22 µm) colloids (some HREEs and Pb), affected by redox processes (V, Mn and Co), and oxyanions (Mo, As, Se, Sb and W). Presumably, the release of these elements was controlled by both desorption and dissolution, because they combined the pattern of the colloidal-bound, fast desorbing low soluble hydrolysates and heavy metals of the 2nd group with slower dissolving labile oxyanions of the 1st group.

The mass-balance estimation of the role of PSM in providing solutes to the Ob River water demonstrated that, at maximal PSM concentrations of 100 mg L⁻¹ during spring flood in the main stem of the Ob River (i.e., Krickov et al., 2023), element release from the PSM, via either desorption or dissolution, cannot add >20–30 % to the current concentrations of most solutes (Fig. 3). The effect of PSM is mostly pronounced for P, Mn and As (42, 24 and 11 % addition, respectively), is rather small (5–10 %) for Co, Cd, Be, Cr, V, Cu, Ga, W and LREE, is minor (1–5 %) for Ni, Zr, HREE, Y, K, Al, Sb, Rb, Ba, Pb and Ti, and is negligible (< 1 %) for other major and trace solutes. At the same time, in local depressions along the Ob River floodplain, where much higher amount of PSM can be accumulated, the effect of solute release from suspended matter into snowmelt waters may become strongly pronounced. However, at the scale of the water column of the Ob River main stem, the release of major and trace elements from the PSM is not capable of significantly modifying the hydrochemistry of river water during spring flood.

## 3.2. Adsorption of solutes from tributary, fen and grass leachate onto PSM of the Ob River

Experiments on adsorption of floodplain water solutes onto PSM of the Ob River demonstrated that only some elements decreased their concentrations after several days of interaction; for other elements, desorption/release from PSM led to an increase in aqueous concentration over the course of experimentation. For the same element, the adsorption yield vs. release pattern was different among waters of tributary, fen and grass leachate and partially depended on the PSM concentration (Table S3 of the Supplement). Note that the experimental Milli-Q blanks (controls) never exceeded 10 % of measured change in solute concentration during adsorption.

A summary of the percentage of adsorption (relative to initial concentration) in the floodplain waters and fixed PSM concentration of 1 g·PSM·L⁻¹ (Table 1) demonstrated the following general characteristics. Ob River PSM was capable of removing 20–90 % of DOC, major nutrients (Si and P) and trace elements (Al, Ti, V, Zn, As, Cd, and Rb) from the waters of the tributary and the floodplain fen. The DOC adsorption onto PSM amounted to 23 % in the tributary and 74 % in the fen. A number of elements could be entirely (> 90 %) removed from solution after contact with PSM: Al and V (tributary and fen), P and As (tributary), and Ti (fen). We did not reveal any systematic patterns in the degree of dissolved element adsorption dependent on the general chemical properties of solutes (i.e., cationic versus anions forms, degree of complexation with organic ligands, element affinity to mineral surfaces). The removal of Carex leachate products (DOC, major and trace elements) by PSM of the Ob River was rather negligible (< 10 %) for most solutes. Exceptions for this water type were Si, V, As, Rb, Sb, W and Pb, which were adsorbed at 63, 100, 22, 19, 64, 100 and 31 %, respectively.

To estimate the capacity of PSM to adsorb riverine solutes, we calculated the number of surface sites capable to bind dissolved components. Regardless of the nature and identity of the surface water, these calculations demonstrate that adsorption is a dominant mechanism for major and trace solute removal onto PSM. Indeed, for example of the Andreva River, major adsorbed elements included Fe, Al, P and K, with typical adsorption around 100 µg·L⁻¹; (Table S3; Fig. 4); this is <10 µmol·L⁻¹ for the sum of all 4 elements. At minimal PSM concentrations of 1 g·L⁻¹, given its measured specific surface area of 37.5 m²/g, and typical range of surface site density on clays and oxides (10⁻⁵ to 10⁻⁶ mol·m⁻², Stumm, 1992; Pokrovsky et al., 2012), the total adsorptive site
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Percentage of solute release from PSM of the Ob River (at a concentration of 100 $mg_{PSM} L^{-1}$, assessed in this study) relative to solute concentration in the Ob River during spring flood (Vorobyev et al., 2019). PSM concentration of 100 $mg L^{-1}$ is maximal that was observed during spring flood period (Krickov et al., 2023). There is a sizable contribution of PSM to P, Mn, As, Co and Cd dissolved concentration, whereas other elements are affected by the PSM presence by <10 %.</figcaption></figure>

<caption>**Table 1**</caption>

Percentage of solute adsorption by PSM of the Ob River during its interaction with a tributary, grass leachate, and floodplain fen water. Extreme case of 1 $g_{PSM} L^{-1}$ is considered. “Release” means that there was no adsorption because the concentration of a solute increased during PSM – water interaction.

<table><thead><tr><th>Elements</th><th>Tributary (Andreva)</th><th>Carex leachate (Carex)</th><th>Fen water (Ishtan)</th></tr></thead><tbody><tr><td>DOC</td><td>23</td><td>Release</td><td>74</td></tr><tr><td>DIC</td><td>2</td><td>88</td><td>24</td></tr><tr><td>Li</td><td>15</td><td>2</td><td>0</td></tr><tr><td>Be</td><td>Release</td><td>Release</td><td>100</td></tr><tr><td>B</td><td>10</td><td>15</td><td>3</td></tr><tr><td>Na</td><td>1</td><td>17</td><td>3</td></tr><tr><td>Mg</td><td>8</td><td>4</td><td>3</td></tr><tr><td>Al</td><td>94</td><td>Release</td><td>100</td></tr><tr><td>Si</td><td>26</td><td>63</td><td>92</td></tr><tr><td>P</td><td>100</td><td>Release</td><td>66</td></tr><tr><td>K</td><td>28</td><td>2</td><td>7</td></tr><tr><td>Ca</td><td>Release</td><td>Release</td><td>20</td></tr><tr><td>Ti</td><td>55</td><td>Release</td><td>100</td></tr><tr><td>V</td><td>100</td><td>100</td><td>100</td></tr><tr><td>Cr</td><td>45</td><td>7</td><td>28</td></tr><tr><td>Fe</td><td>30</td><td>Release</td><td>Release</td></tr><tr><td>Cu</td><td>Release</td><td>Release</td><td>67</td></tr><tr><td>Zn</td><td>80</td><td>1</td><td>80</td></tr><tr><td>Ga</td><td>Release</td><td>Release</td><td>18</td></tr><tr><td>As</td><td>100</td><td>22</td><td>48</td></tr><tr><td>Rb</td><td>70</td><td>19</td><td>40</td></tr><tr><td>Sr</td><td>27</td><td>Release</td><td>27</td></tr><tr><td>Mo</td><td>28</td><td>Release</td><td>Release</td></tr><tr><td>Cd</td><td>55</td><td>Release</td><td>100</td></tr><tr><td>Sb</td><td>12</td><td>64</td><td>69</td></tr><tr><td>Ba</td><td>10</td><td>Release</td><td>Release</td></tr><tr><td>Hf</td><td>2</td><td>Release</td><td>Release</td></tr><tr><td>W</td><td>Release</td><td>100</td><td>Release</td></tr><tr><td>Pb</td><td>Release</td><td>31</td><td>Release</td></tr></tbody></table>

concentration ranges from 37.5 µM to 375 µM. This is large enough to accommodate all adsorbed major and trace elements (from $1 \times 10^{-5} M$ to $2 \times 10^{-5} M$). With addition of 700–900 µg g$^{-1}$ (at $1 g L^{-1}$) of adsorbed Si, the total adsorbed solute concentration increases by 50 µM which is still below the maximal surface site density of 375 µM. Therefore, single surface layer adsorption reaction allows to account for the entire amount of solute retained by the PSM, and no multiple layer adsorption or coprecipitation process need to be considered.

In addition to these general features, the adsorption pattern was found to be different between different concentrations of PSM because the maximal adsorption (per mass of PSM) was observed at 1 $g_{PSM} L^{-1}$ and the minimal at $10 g_{PSM} L^{-1}$, as illustrated in Fig. 4 for DOC, macro- and micro-nutrients (P, K, Zn, Rb) and some toxicants (Al, Cr, As, Sb, Cd). Tentative explanations for such a decrease in adsorption yield with PSM concentration include (i) decrease of availability of surface sites

with high concentrations of solid particles, (ii) agglomeration of grains and a decrease of effective surface area, (iii) decrease in effectiveness of diffusion from solution to surface of PSM particles, and (iv) local redox control due to creation of bacterial microniches with partially anoxic conditions, leading to mobilization of redox sensitive elements (Fe, Mn hydroxides and coprecipitated trivalent and tetravalent hydrolysates, oxyanions) from PSM to the aqueous solution, thereby decreasing net adsorption of these solutes.

Overall, partial non-additivity (non-proportionality) between the concentration of added PSM and mass-normalized element adsorption (Fig. 4) and release (Fig. 2) suggests a non-conservative behavior of solutes during PSM interaction with aqueous solution. For example, at high PSM concentration, suboxic conditions can provoke Fe(III) hydroxide reduction into $Fe^{2+}$ (aq) and/or Mn(III, IV) oxyhydroxides into $Mn^{2+}$ (aq). This can mobilize into the solution insoluble elements that were associated with particulate Fe and Mn hydroxides in the form of coprecipitates (such as As, Ge, REE, Y and Ga). Note that in PSM, the most likely carriers of trivalent and tetravalent trace elements are amorphous/semi-amorphous Fe and Mn hydroxides, as is demonstrated for small tributaries of the Ob River during base flow period (Krickov et al., 2020, 2023) and generally consistent with other boreal rivers (Pontér et al., 1992). The high affinity of trivalent hydrolysates, such as REEs, to these solid phases is also fairly well documented (i.e., Liu et al., 2017).

### 3.3. Impact of adsorption on elemental size fractionation and properties of DOM

Colloidal (3 kDa – 0.22 µm) status of organic matter, major and trace elements was assessed via centrifugal ultrafiltration in initial waters of the floodplain and after adsorption onto $10 g L^{-1}$ PSM of the Ob River. Following general features of the colloidal pattern in organic rich waters of the boreal zone, initial water (< 0.22 µm) of the Andreva River exhibited a sizable proportion of colloidalily bound elements. Only alkalis and some anion/neutral molecules were bound <20 % to colloids: Li, Na, K, Rb; B, Si, As, Mo and Sb. The majority (> 80 %) of trivalent and tetravalent hydrolysates (Al, Fe, Ga, Y, REEs, Ti, Zr, Hf and Th), Be, Nb, Bi, Pb and U were in colloidal forms, whereas divalent transition metals were typically 40–60 % associated with colloids. Similar features of elemental colloidal status were observed in the fen water, with a somewhat lower proportion of colloids for divalent metals. The grass leachate displayed >70 % colloidal fraction for $TE^{3+}$, $TE^{4+}$, Be, Nb, Pb and U.

The presence of PSM measurably impacted the molecular size distribution of major and trace element solutes in the tributary via decreasing (by 20–30 %) the Low Molecular Weight (< 3 kDa or $LMW<3 kDa$) fraction of Be, Al, Cr, Cd, Cs and REEs and increasing (by 20–70 %) the $LMW<3 kDa$ proportion of alkaline earth metals, P and trace metals
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** PSM mass-normalized adsorption of DOC, Al, P, K, Cr, Zn, As, Rb, Cd and Sb from the tributary and fen waters, at different concentrations of PSM in the reactors. Error bars represent s.d. of duplicate experiments.</figcaption></figure>

(V, Cr, Ni, Cu, Tl, Bi and W) as illustrated in Fig. 5. A decrease of $LMW< 3$ kDa forms of trace elements upon adsorption onto PSM was also observed in *Carex* leachates and floodplain fen waters. The changes in element size fractionation, however, did not occur uniformly and some elements

(V and U in the leachate; Fe, Se, Sb and U in the fen water) sizably (> 30 %) increased the proportion of $LMW< 3$ kDa forms. Therefore, the adsorption of solutes from water bodies of the floodplain on the PSM of the Ob River occurred via both ionic, neutral molecules or LMW form,

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Histogram illustrating a decrease in the $LMW< 3$ kDa fraction of Al (A), Ni (B) and Cu (C), and an increase in this fraction of Ca (D), Zn (E), and P (F) upon adsorption of floodplain water constituents onto PSM of the Ob River.</figcaption></figure>
and large size high molecular weight colloids. Note that aqueous humate complexes of REEs are readily adsorbed on montmorillonite surfaces (i. e., Takahashi et al., 2004), but virtually nothing is known for adsorption of colloidal forms of other trace elements on either clays or Fe/Mn hydroxides.

The finding that organic and organo-Fe/Al colloids (3 kDa – 0.22 µm) are subjected to adsorption onto PSM of the Ob River was further supported by changes in optical properties of DOM during adsorption (Table S5). Among various optical parameters of DOM, the most pronounced trends were observed for SUVA<sub>254</sub>, molar absorptivity and WAMW, whose values in the tributary, grass leachate and floodplain fen decreased by a factor of 1.5 to 3.0 between the initial waters and after the adsorption onto PSM of the Ob River (Fig. 6). Such a strong decrease of SUVA<sub>254</sub> and molecular weight during the interaction with PSM for all three types of waters suggest a preferential adsorption of DOM aromatic compounds onto mineral surfaces. Note that there was no sizable release of non-chromophoric organic matter from the suspended matter into the distilled water; for example, SUVA progressively increased, by a factor of 1.5 to 1.8 during PSM interaction with distilled water (Table S6).

The finding of considerable adsorption of aromatic compounds whose WAMW was higher than the average of the initial floodplain waters has a fundamental significance for understanding of DOM interaction with mineral surfaces. Currently, there are certain controversies about the molecular nature of natural organic compounds

susceptible to adsorb onto mineral surfaces. From the one hand, it is known that low molecular weight (less than a few nm, or several kDa) fractions of organic carbon and ionic forms of metals are most active in adsorption processes onto solid surfaces (Stumm, 1992) and that carbohydrates and aliphatic compounds of natural DOM adsorb preferentially to aromatic compounds on ferrihydrite colloids (Xu et al., 2019). In particular, the carboxylic groups of soil DOM preferentially adsorb on the ferrihydrite surface through ligand exchange, hydrogen bonding and electrostatic attraction (i.e., Kleber et al., 2015; Liu et al., 2022). From the other hand, recent works demonstrated that HMW humic-like substances of high aromaticity also exhibit strong binding affinity to ferrihydrite (Li et al., 2022; Guo et al., 2021).

In the case of DOM-rich waters of the Ob River floodplain, aromatic compounds of higher molecular weight were capable of adsorbing onto river suspended matter, composed of aluminosilicates likely coated by Fe(III) oxy(hydr)oxides. These aromatic compounds, however, were equally distributed among low (< 3 kDa) and high molecular weight (3 kDa – 0.22 µm) colloids, because the proportion of LMW< 3 kDa forms of organic carbon did not sizably change during adsorption (Table S4). It should be noted that there is no apparent inconsistency of organic carbon size fractionation between <3 kDa and 3 kDa – 0.2 µm colloids assessed via centrifugal ultrafiltration. This is because the calculated WAMW was based on optical parameters (Fig. 6; Table S5) that ranged between 800 and 2000 Da, which is below the cutoff of 3000 Da of physical separation by ultrafiltration. Therefore, despite the fact that the average molecular weight of organic ligands and DOM – trace metal complexes and colloids initially present in natural waters has decreased after adsorption onto riverine PSM, overall, these ligands remained within the low molecular weight range that corresponds to the most labile fraction of natural DOM.

## 4. Conclusions

Towards better understanding the natural and physico-chemical factors controlling the interaction between river suspended matter and surface waters of floodplains in pristine boreal taiga/wetland regions, we performed laboratory experiments on 1) the release of DOC, major and trace elements from PSM of a large Siberian river into distilled water, imitating snowmelt, and 2) the adsorption of dissolved (< 0.22 µm) elements from organic-rich waters of the floodplain onto PSM surfaces.

The majority of solutes (DOC, major nutrients [Si, P] and insoluble trace metals) were released from PSM into distilled water during the first hour of reaction, suggesting fast desorption of organic and organo-mineral colloids as a main mechanism for major and trace element mobilization from riverine particles. Some soluble, labile elements (DIC, Si, alkalis and alkaline-earth metals) demonstrated gradual increase in concentration, likely reflecting the dissolution of soluble carbonate minerals or reactive amorphous silicate phases present in PSM.

The removal of 20–30 major and trace solutes from the waters of a tributary and floodplain fen, representing typical surface waters of the floodplain, on the Ob River PSM was significant and ranged from 10 to 90 % of their initial concentration. We also demonstrated that aromatic compounds and organic/organo-Fe, Al colloids of higher molecular weight, which accommodate various trace elements including micro-nutrients, exhibited fast adsorption onto and release from particulate pool. Overall, the PSM can play important role as regulator of hydro-chemical composition for the water bodies of the floodplain. Further systematic assessment of colloidal adsorption parameters (fluid – solid partitioning coefficients) in the presence of natural organic and mineral surfaces is necessary for biogeochemical modeling of element mobility in such fluvial systems.

### CRediT authorship contribution statement

Artem G. Lim: Writing – original draft, Methodology, Investigation,

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** The change of DOM optical properties for the tributary, Carex leachate and fen waters after adsorption onto PSM of the Ob River water. Error bars are s.d. of duplicate experiments. Initial conditions (at 0 g<sub>PSM</sub> L<sup>-1</sup>) are slightly shifted to the right (X axis) for better visibility.</figcaption></figure>
Formal analysis, Data curation. **Ivan V. Krickov:** Methodology, Investigation, Formal analysis. **Oleg S. Pokrovsky:** Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Acknowledgements

Support from TSU Development Program “Priority-2030” and the RSF grant No. 23-17-00281 is acknowledged. OP was partially supported by project PEACE of PEPR FairCarboN ANR-22-PEXF-0011. We are grateful to anonymous reviewers for their very constructive and thorough remarks.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2024.174735.

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