# Role of manganese oxides and oxyhydroxides in phosphate sequestration in natural aquatic environments

Pierre Anschutz, Florian Benony, Céline Charbonnier

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Pierre Anschutz, Florian Benony, Céline Charbonnier. Role of manganese oxides and oxyhydroxides in phosphate sequestration in natural aquatic environments. Chemical Geology, 2024, 662, pp.122231. 10.1016/j.chemgeo.2024.122231. hal-05045039

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# Role of manganese oxides and oxyhydroxides in phosphate sequestration in natural aquatic environments

Pierre Anschutz *, Florian Benony, Céline Charbonnier

Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 585, F-33600 Pessac, France

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

Editor: Karen Johannesson

**Keywords:**

Eutrophication

Phosphorus

Watershed

River

Coastal zone

iron oxides

Manganese oxides

## ABSTRACT

Excessive inputs of phosphorus (P) to aquatic environments can lead to eutrophication. Adsorption of phosphate on Fe oxides is one of the main process that can limit P availability. Oxidized forms of Mn have also been suggested to play a role in P trapping. However, although a considerable number of studies have shown the relationships between the geochemistry of Fe and that of phosphates, few studies have attempted to show the direct links between Mn and P. In the present study, we studied the adsorption of phosphate on synthetized Mn (III) and Mn(IV) oxides placed under natural conditions. The aim was to compare the role of Fe and Mn oxides in phosphate adsorption. Two muddy sediments were collected in a river bed at the edge of a large agricultural area. A sandy sediment was collected downstream. A muddy and a sandy sample were taken in a coastal environment. The experiments on phosphate adsorption by sediment and Mn oxides were carried out in slurries containing in situ waters spiked with 10 µM phosphate. Control experiments without Mn-oxide addition showed that the natural sediments tested still had the capacity to adsorb phosphate, in particular due to the presence of reactive Fe(III) oxides, extractable by an ascorbate solution. The addition of Mn(III) and Mn(IV) oxides in much larger quantities than the initial quantity of Fe oxides had little impact on the rate of phosphate adsorption. For both Mn(III) and Mn(IV) oxides, the Mn/P ratio between added particulate Mn and adsorbed P was very high, with values between 260 and 1000. The Fe/P ratio of the iron oxides contained in the slurries was between 6 and 20. On average, the P adsorption capacity of the Fe oxides was 50 times greater than that of the Mn oxides. Manganese oxides are generally less abundant than iron oxides in natural environments. Mn oxides therefore play a minor role in P sequestration compared with Fe oxides. Even if there are environments where Mn oxides can concentrate, the reduction of Mn oxides and subsequent liberation of adsorbed P does not represent a major risk for eutrophication of aquatic ecosystems.

## 1. Introduction

One of the critical nutrients limiting the growth of phytoplankton and aquatic plants in lakes and seas is phosphorus (P). Oppositely, excessive inputs of phosphorus to the environment can lead to eutrophication (Schindler, 1977; Hecky and Kilham, 1988; Correll, 1998; Reddy et al., 1999), especially if the environment is already enriched in nitrogen (Paerl, 2009; Conley et al., 2009; Wetzel, 2001; Smith, 2003). The bioavailable form of P is phosphate (PO₄³⁻). Phosphate concentrations in natural waters are generally quite low, as this compound has a strong affinity for the particulate phase. Iron and aluminium oxides are the main phases that trap phosphate. Aluminium oxides are mainly found in certain soils. Iron oxides are found in almost all natural environments such as soils, suspended particles and sediments (Ruttenberg,

2014). Indeed, coupling between Fe and P geochemistry is one of the main processes that control P availability in aquatic system (e.g. Lij-klema, 1980; Baken et al., 2014). Manganese (Mn) is the eleventh most abundant element on Earth. In the environment it occurs in three redox states: the reduced form Mn(II) and the oxidized forms Mn(III) and Mn (IV), giving rise to a variety of multivalent phases (Hem and Lind, 1983; Burdige, 1993; Post, 1999). Oxidized forms of Mn have also been suggested to play a role in phosphorus trapping in marine environments, for example in the Black Sea, where a mixed Fe—Mn (oxyhydr)oxide has been reported (Dellwig et al., 2010; Kraal et al., 2017). The role of Mn oxides in P adsorption has been suggested for the sediments of an oligotrophic lake on the basis of a correlation between P and Mn concentrations (Brannon et al., 2023). Phosphorus adsorption is indistinguishably associated with Fe and Mn oxides in the sediment column of

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: pierre.anschutz@u-bordeaux.fr (P. Anschutz).</footnote>
an eutrophic lake in Switzerland (Tu et al., 2020).

However, although a considerable number of studies have shown the relationships between the geochemistry of Fe and that of phosphates, few studies have attempted to show the direct links between Mn and P (e.g., Hermans et al., 2021). Thus, many studies rightly mention the uptake of PO₄³⁻ on Fe oxides, while the uptake of PO₄³⁻ on Mn oxides is mentioned by analogy, but remains poorly documented in reality. Iron and aluminium (oxyhydr)oxides have positive surface charges at pH values close to neutral, which favours the adsorption of negatively charged species such as the forms of phosphate present at these pH values (HPO₄²⁻, H₂PO₄). Mn oxides have negative surface charges at neutral or slightly acidic pH (Murray, 1974), which is not favourable for anion adsorption due to electrostatic repulsion. However, MnO₂ and nano-sized hydrous Mn oxides can adsorb phosphate through the formation of both outer- and inner-sphere complexes (Yao and Millero, 1996; Mustafa et al., 2008; Pan et al., 2014).

The few studies on phosphorus adsorption on manganese oxides and (oxyhydr)oxides (Mn-ox) have often been carried out under experimental conditions where pure Mn-ox phases were brought into contact with phosphate-enriched solutions. These experimental conditions were sometimes very different from those found in natural environments, in order to highlight the effects of the different parameters influencing the phosphorus adsorption on Mn-ox. In particular, those carried out with variable pH levels show that phosphate is more effectively adsorbed at acidic pH levels (Yao and Millero, 1996).

Phosphorus concentration affects adsorption on solid phase surfaces. The lower the phosphorus concentration, the greater the proportion of adsorption sites available, favouring proportionally higher adsorption. At high concentrations, the adsorption sites of Fe and Mn oxides become saturated: a large proportion of the phosphate remains in solution (Zhang et al., 2009). The surface properties of Mn-ox can be modified by the presence of aluminium, iron and impurities, which affects its phosphorus adsorption capacity (Pan et al., 2014). An important parameter is the contact time between the manganese oxides and the phosphorus containing solution. Initial adsorption can be rapid and equilibrium is reached in the long term. Maximum rate of phosphorus adsorption is reached in the first few hours and can then gradually decrease (Yao and Millero, 1996). This study showed that Mn oxides could act as important adsorbents of phosphate when they are present at higher concentration than iron oxides.

Thus, previous works dedicated to the study of phosphate adsorption on Mn oxides, of which there are not many, have either attempted to show the role played by Mn oxides naturally present in the environment studied, or have provided information on the efficiency and rate of adsorption based on purely experimental situations. In the study presented here, we studied the adsorption of phosphorus on artificial Mn oxides placed under natural conditions. The aim was to compare the role of Fe and Mn oxides in phosphate adsorption. To achieve this, particulate forms of Mn(III) and Mn(IV) were synthesized and mixed with sediments collected from coastal environments and rivers. The sediments were selected to be naturally rich in iron oxides but low in Mn oxide. Water in contact with these sediments was enriched with phosphate and changes in concentrations were monitored over time. This paper describes and discusses analyses of the solid fraction, changes in phosphate concentrations and comparisons with controls, and the use of these measurements to determine the effectiveness of Mn-ox in sequencing phosphate and to compare it with that of iron oxides.

# 2. Material and methods

## 2.1. Sampling sites

In order to experimentally observe the effect of Mn oxides on P adsorption in sediments, the aim was to add Mn oxides to different types of Mn-oxide poor sediments. We chose sediments from freshwater or marine environments, fine or sandy, and rich in iron oxides. The

sampling sites are located in the watershed and the intertidal zone of the Arcachon lagoon, whose surface geological formations are essentially composed of sandy plio-quaternary deposits forming acidic podzols rich in iron oxide (Jolivet et al., 2007) and poor in Mn oxide (Buquet et al., 2017). Three sediments were collected from river beds (Fig. 1). Two were muddy (Jolles and Ditch 2) and the third was sandy (station H). Sediments from Ditch 2 and Jolles were collected at the edge of a large agricultural area. Station H is located about 9 km downstream from the two other sites (Fig. 1). The catchment along this continuum consists of maritime pine and oak forests. The sediments at Ditch 2 are very muddy and black in colour. The sediments at Jolles are sandy-muddy with visible plant debris. The sediments at station H are medium sands with a rusty reddish shade due to high iron oxide content. These sediments are in contact with water with a conductivity between 200 and 600 µS cm⁻¹. Dissolved phosphate concentrations range from 0 to 2 µM (Buquet et al., 2017 and unpublished data). Two other samples were taken in a coastal environment in the Arcachon lagoon, more precisely sand in the upper part and mud in the lower part of the intertidal zone of the Bay of Aiguillon (Fig. 1). The mud is greyish and homogeneous and the sandy sediment is medium sand. At the Aiguillon site, the salinity of the water at high tide was 26 and the phosphate concentration was below 1 µM. These sediments were collected on the foreshore at low tide. For each site, we collected the top 1 cm of sediment by manually filling a flask under a thin water layer. Water from the sites was collected and filtered with GF/F filters with a pore size of 0.7 µm.

## 2.2. Characterization of the sediments

The sediments were dried in an oven at 40 °C for 72 h. The content of Mn oxides, reactive iron oxides and phosphate associated with these phases was determined using ascorbate leaching. Ascorbate leaching is a selective extraction that dissolves amorphous iron oxides and oxyhydroxides (Kostka and Luther, 1994; Anschutz and Deborde, 2016) and manganese oxyhydroxides (Anschutz et al., 2005) by reduction. Reduction of iron oxides also results in the release of phosphorus adsorbed on them (Anschutz et al., 1998). The ascorbate solution is buffered to pH 8 with sodium bicarbonate to avoid acid dissolution of the minerals. The solution contains 50 g sodium bicarbonate, 50 g sodium citrate and 20 g ascorbic acid per 1 L deionised water. One hundred mg of dry sediment was leached with 10 mL of the solution for 24 h. After centrifugation, 1 mL of the leaching solution was recovered and diluted with 9 mL of 0.2 M HCl. The concentrations of Fe, Mn and P were determined from this diluted solution using spectrophotometric

<figure><img src="image_3.png" /><figcaption>Fig. 1. Location of sampling sites.</figcaption></figure>
methods according to Stookey (1970), Charbonnier and Anschutz (2019) and Anschutz and Deborde (2016). The percentage error of this method is about 10% for the final concentrations obtained for the sediments.

## 2.3. Synthesis of manganese oxides

In nature there are two forms of oxidized manganese, Mn(III) and Mn (IV), which essentially form minerals of the MnO(OH), Mn₂O₃ and MnO₂ types. The Mn(III) and Mn(IV) oxides were prepared using two separate protocols:

The Mn(III) forms were synthesized using a method inspired by the Winkler method for measuring dissolved oxygen (Anschutz et al., 2005), which involves the quantitative oxidation of dissolved Mn²⁺ in a basic medium to form Mn hydroxides (reactions 1 and 2).

$$Mn^{2+} + 2OH^{-} \rightarrow Mn(OH)_{2} \quad (1)$$

$$2Mn(OH)_{2} + \frac{1}{2}O_{2} + H_{2}O \rightarrow 2Mn(OH)_{3} \quad (= Mn_{2}O_{3}, 3H_{2}O) \quad (2)$$

These Mn(III) phases were prepared from 3.95 g (0.031 mol) MnCl₂, 6H₂O and 1.659 g (0.041 mol) NaOH in 50 mL deionised water, with O₂ added by bubbling air for at least 24 h.

Mn(IV) oxides were prepared according to the method of Murray (1974) by oxidation of Mn²⁺ with permanganate (MnO₄⁻):

$$3Mn^{2+} + 2MnO_{4}^{-} + 2H_{2}O = 5MnO_{2} + 4H^{+} \quad (3)$$

To this end, 0.008 mol NaMnO₄ and 0.016 mol NaOH were dissolved in 200 mL of deionised water and 0.012 mol MnCl₂·6H₂O was added. The solution was stirred for 24 h.

To recover the manganese oxides, the solutions were placed in 50 mL centrifuge tubes and centrifuged a first time to remove the supernatant. The centrifuged particles were rinsed three times with deionised water to bring the pH close to neutral. The particles were stored as a suspension in a 50 mL flask.

To check the redox state of the synthesized Mn-ox, a Winkler-type titration was carried out. In this procedure, the Mn(III) and Mn(IV) forms react with iodide in an acidic medium to form iodine I₂ (reactions 4, 5). The amount of iodine formed is determined by the average oxidation state of the Mn in the sample (Murray et al., 1984; Anschutz et al., 2005). Iodine was determined by titration with thiosulphate (reactions 6, 7).

$$\text{For the Mn(IV) form: } MnO_{2} + 2I^{-} + 4H^{+} \rightarrow Mn^{2+} + I_{2} + 2H_{2}O \quad (4)$$

$$\text{For the Mn (III) form: } 2Mn(OH)_{3} + 2I^{-} + 6H^{+} \rightarrow 2Mn^{2+} + I_{2} + 6H_{2}O \quad (5)$$

$$I_{2} + I^{-} \leftrightarrow I_{3}^{-} \quad (6)$$

$$I_{3}^{-} + 2S_{2}O_{3}^{2-} \rightarrow 3I^{-} + S_{4}O_{6}^{2-} \quad (7)$$

In practice, these reactions show that 2 mol of thiosulphate (S₂O₃²⁻) are used for 1 mol of MnO₂ and for 2 mol of Mn(OH)₃. Five hundred microlitres of 4 M NaI solution and 1 mL of 4.5 M H₂SO₄ were added to a beaker containing a little water, a magnetic stirrer and 250 µL of homogenized Mn oxide suspension. The yellow iodine formed was titrated with 0.01 M thiosulphate. The titration was carried out stepwise with a micropipette to the colourless endpoint (after adding starch to enhance the detection of the colour change).

## 2.4. Adsorption kinetics

The experiments on phosphate adsorption by sediment and Mn oxides were carried out in experimental units consisting of 50 mL polypropylene centrifuge tubes. 7 experimental units were prepared for each type of sediment. In each of them, the dried sediment was weighed

exactly (200 mg for the fine sediments of Jolles, Ditch 2 and Aiguillon-mud and 1 g for the sandy sediments of station H and Aiguillon-sand). In order to ensure the equivalence of initial iron oxide levels in the two types of sediment, a greater quantity of sediment was used for the sandy sediment than for the muddy sediment. The sediments were brought into contact with 50 mL of water with a salinity similar to that of the sampling sites and under oxic conditions. For the Aiguillon coastal sediments, in situ seawater with a salinity of 26 was used. For the freshwater streambed sites (Ditch 2, Jolles, H), the in situ water had a low conductivity, but this varied from site to site, and contained 0 to 2 µM concentrations of phosphate. Therefore, to maintain the low conductivity and to control the phosphate concentration in the experiments, we used deionised water rather than the water from each site. Phosphate was added from a stock solution of 5 mM KH₂PO₄ to reach an initial concentration of 10 or 20 µM. Ten µM is a much higher concentration than the particles used in our experiments are exposed to. This allowed us to observe additional adsorption of phosphate. This concentration was also used in previous experiments of P adsorption on Mn-oxides (Yao and Millero, 1996). We also carried out some experiments with 20 µM to briefly examine the effect of phosphate concentration on adsorption by Mn oxides. In 3 units we added 250, 500 and 1000 µL of Mn(III) oxide suspension and in 3 others we added 250, 500 and 1000 µL of Mn(IV) oxide suspension. In the 7th unit no Mn oxide was added. A series of units with Aiguillon-mud was made in duplicate. A series of units with Aiguillon-sand solution was carried out with 5 g of sediment. Control units containing only 200 µL of Mn oxide suspensions in contact with water spiked with 10 µM and 20 µM phosphate were also prepared. Blanks were prepared using tubes containing only the phosphate solution. All experiments were performed in a room at a constant temperature of 20 °C. This matrix represents 58 experimental units whose concentrations were monitored from time 0 over a period of 2 to 6 days. After the addition of the 10 or 20 µM phosphate solution (time 0), the tubes were placed on a shaker. After 30 min, 60 min, 120 min and the following days, shaking was briefly stopped and the tubes were centrifuged for 5 min and 500 µL of the supernatant was collected to measure the phosphate concentration of the solution according to the protocol of Murphy and Riley (1962). All concentrations of dissolved phosphate and particulate Fe, Mn and P are expressed in µmol per kg of water (µM).

Finally, the samples were centrifuged one last time for 1.5 h to separate the solid phase from the liquid. After this step, 30 mL of solution was recovered to measure the pH of each sample using a calibrated WTW probe.

# 3. Results

## 3.1. Nature of sediments

Samples composed of muddy sediments had high iron concentrations, with values between 86 and 153 µmol/g (Table 1). A concentration of 35 µmol/g was found in the red sands of station H. The Aiguillon sand, on the other hand, had a lower concentration of 6.8 µmol/g (Table 1). The concentrations of P extracted with the ascorbate leaching solution were more or less proportional to those of iron. They were

**Table 1**

Concentrations of Fe, P and Mn extracted by an ascorbate solution in the sediments used for the experiments. Concentrations are expressed in micromoles per gram of dry sediment (µmol/g) for sediments and in µmol/mL suspension for synthesized Mn oxides.

<table><thead><tr><th>Sample</th><th>Type</th><th>Fe-asc<br>(µmol/g)</th><th>Mn-asc<br>(µmol/g)</th><th>P-asc<br>(µmol/g)</th><th>Fe/P (mol/mol)</th></tr></thead><tbody><tr><td>Ditch 2</td><td>mud</td><td>153</td><td>5.3</td><td>23.3</td><td>6.6</td></tr><tr><td>Jolles</td><td>mud</td><td>86</td><td>3.7</td><td>14.1</td><td>6.1</td></tr><tr><td>Station H</td><td>sand</td><td>35</td><td>1.6</td><td>1.2</td><td>29.2</td></tr><tr><td>Aiguillon</td><td>mud</td><td>154</td><td>2.5</td><td>10</td><td>15.4</td></tr><tr><td>Aiguillon</td><td>sand</td><td>6.8</td><td>0.5</td><td>0.9</td><td>7.6</td></tr></tbody></table>
highest in the mud of Ditch 2, with 23.3 µmol/g (Table 1). The lowest concentration was 0.9 µmol/g for the Aiguillon sand. The Fe/P ratio ranged from 29 for station H to 6.1 for Jolles. The concentrations of Mn extracted by ascorbate were low, with a maximum in the Ditch 2 mud (5.3 µmol/g) and a minimum of 0.9 µmol/g for the Aiguillon sand.

The synthesized Mn oxides and oxyhydroxides were in the form of suspensions in water. The Mn concentrations of these suspensions were 184 ± 7 µmol/mL and 117 ± 1 µmol/mL for Mn(III) oxide and Mn(IV) oxide, respectively. The exact redox state of Mn in the suspensions was determined from 250 µL of suspension, i.e. 46 µmol in the suspension theoretically containing Mn(III) and 29.3 µmol in the suspension theoretically containing Mn(IV). In the first suspension, the average amount of thiosulphate used was 40.2 ± 0.6 µmol (Table 2), and in the second, 58.3 ± 0.3 µmol. These values are very close to the theoretical value. In fact, for the Mn(IV) suspension, 29.3 µmol should produce a titration with 58.6 µmol of thiosulphate, and for Mn(III), 46 µmol should produce a titration with 46 µmol of thiosulphate. Our results show that the synthesized Mn(IV) particles have a redox state very close to that expected. The stoichiometry of the manganese oxide is MnO₁.₉₉, very close to that obtained by Murray (1974), which was similar structurally to naturally occurring mineral birnessite. The average Mn(III) particles stoichiometry was MnO₁.₄₄ (instead of MnO₁.₅) suggesting that the synthesized particles still contain a small amount of Mn(II).

## 3.2. Phosphate adsorption

### 3.2.1. Characterization of the adsorption kinetics of manganese oxide

The experimental units initially contained a phosphate concentration of 10 or 20 µmol/L. The concentrations measured were indeed those initially expected. The phosphate concentration of the control unit remained stable during the 6 days of incubation (Fig. 2). Units containing 200 µL of a suspension of Mn(IV), i.e. 468 µM of Mn(IV), showed no difference from the control, suggesting that these oxides alone had no effect on phosphate sequestration. Units containing 200 µL of Mn(III) suspension or 736 µM of Mn(III) phases showed a slight decrease in phosphate concentration of 1 µM during the first hour, then a slower decrease until stabilizing at a concentration of 1.8 µM below the initial value. Thus, 736 µmol of Mn(III) adsorbed 1.8 µmol of phosphate with an initial phosphate concentration of 10 or 20 µM.

### 3.2.2. Control units with natural sediment

Phosphate was partially adsorbed by the natural sediments tested. Most of the phosphate was adsorbed within 24 h and then the levels reached a plateau, i.e. the last value was similar (±0.2 µM) as the previous one (Fig. 3). In particular, the Ditch 2 and Jolles sediments reached levels of <2 µM. The Aiguillon muddy sediments adsorbed less phosphate as the plateau was at 4.0 ± 0.1 µM. The sands of Station H had a plateau at 4.8 µM and those of Aiguillon adsorbed little phosphate, since in units of 1 g of sediment (20 g/L), the concentration changed from 10 µM initially to 9.3 µM after 3 days and from 10 µM to 6.7 µM with 5 g of sediment (100 g/L) (Fig. 3).

<caption>**Table 2**</caption>

Quantity of thiosulphate used to titrate the iodine resulting from the oxidation reaction of iodide with Mn oxi(hydroxy)de.

<table><thead><tr><th>Replica</th><th>Thiosulphate (µmol) for 0.25 mL « Mn(III) » suspension</th><th>Thiosulphate (µmol) for 0.25 mL « Mn(IV) » suspension</th></tr></thead><tbody><tr><td>1</td><td>40.2</td><td>58.5</td></tr><tr><td>2</td><td>39.4</td><td>58.6</td></tr><tr><td>3</td><td>40.4</td><td>58.3</td></tr><tr><td>4</td><td>40.1</td><td>58.0</td></tr><tr><td>5</td><td>41.0</td><td>58.1</td></tr><tr><td>Mean ± SD</td><td>40.2 ± 0.6</td><td>58.3 ± 0.3</td></tr><tr><td>Expected for Mn(III)</td><td>46</td><td>-</td></tr><tr><td>Expected for Mn(IV)</td><td>-</td><td>58.6</td></tr></tbody></table>

### 3.2.3. Sediment and Mn-oxides

Samples from Ditch 2 spiked with Mn(IV) or Mn(III) showed changes in phosphate content close to those of the control (Fig. 4). Phosphate concentrations decreased rapidly in the first few hours and then stabilized at low levels. In the presence of Mn(III), the levels decreased more rapidly than in the control during the first two hours. However, the addition of Mn oxides does not increase the amount of phosphate adsorbed after 4 days. On the contrary, the final phosphate concentration was 3.4 ± 0.4 µM, whereas it had reached 1.8 µM without Mn oxides. In the presence of large amounts of Mn(III), the final value was the same as the control, with 1.8 µM. In this 50-mL unit, 1000 µL of Mn(III) suspension was added, i.e. 3680 µM.

The experimental units carried out under the same conditions with the Jolles sediment show a similar behaviour. Units spiked with Mn(IV) adsorbed less phosphate than the control, and those spiked with Mn(III) adsorbed phosphate faster than the control at the beginning, but just as much at the end. It should also be noted that phosphate levels increased slightly in the presence of large amounts of Mn(III), suggesting phosphate desorption over time.

The units containing sandy sediments from station H showed a very similar evolution of phosphate concentration with or without addition of Mn(IV). The addition of 250 µL of Mn(III) suspension (920 µM) did not change anything with respect to the blank. On the other hand, when the addition of Mn(III) was 2 or 4 times higher, the phosphate levels decreased more rapidly and reached lower values than the control, with phosphate concentration around 2 µM after 3 days instead of 5.4 µM for the control. The addition of a sufficient amount of Mn(III) (> 920 µM) therefore resulted in a greater adsorption of phosphate for these sediments.

The units in the presence of Aiguillon muds and seawater from the site with a salinity of 26 all showed a stronger adsorption of phosphate when Mn(III) or Mn(IV) oxides were added, since after 48 h the phosphate contents in the presence of Mn-ox were systematically lower than the control (Fig. 4). The control slurries reached a phosphate concentration of 4 µM. The phosphate content decreased more rapidly at higher Mn-ox concentrations, but after 2 or 3 days the plateau value was the same, around 2 µM, i.e. half that of the control. For the units containing Aiguillon sand (Fig. 4) and Mn-ox, the phosphate concentration decreased significantly, contrary to the control. The reduction was proportional to the amount of Mn-ox added. Mn(IV) phases adsorbed slightly more phosphate than Mn(III) phases. The 50-mL units with initially 1 g of sediment and with 5 g of sediment showed the same trends.

## 3.3. pH

The phosphate-enriched freshwater solutions had a pH of 6.0 ± 0.1. These values were those found in the natural waters of the watercourses of the Carcans-Hourtin basin. The units to which Mn(IV) suspension was added had more acidic values (5.3 ± 0.1) and those to which Mn(III) was added had slightly less acidic pH than the blank (6.15 ± 0.05). In the presence of Jolles, Ditch 2 or H sediment, the pH was slightly higher, ranging from 6.2 to 6.5. The addition of suspensions of 250 µL, 500 µL and 1000 µL caused the pH values to change towards lower values for Mn(IV) and higher values for Mn(III). However, the values remained between 6 and 6.8 for all freshwater units. For the saltwater units of the Arcachon lagoon, the pH without addition of the Mn-ox suspension was 7.6 ± 0.2. The pH remained between 7.5 and 7.9 for all the experimental units.
<figure><img src="image_4.png" /><figcaption>Fig. 2. Evolution of phosphate concentrations over time (in hours) in the experimental units containing phosphate at concentrations of 10 µM and 20 µM with or without 200 µL of Mn(III) and Mn(IV) oxide suspension.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Evolution of phosphate concentrations over time (in hours) in the experimental units containing 10 µM phosphate with sediments and without addition of Mn (III) and Mn(IV). For Jolles and Ditch 2 a test with 20 µM phosphate was also carried out. The 50 mL units contained 200 mg dry sediment for Jolles, Ditch 2 and the Aiguillon muds, 1 g for Station H and 1 g or 5 g for the Aiguillon sands.</figcaption></figure>

# 4. Discussion

## 4.1. Sediment characteristics

The sediments in the Ditch 2, Jolles and station H samples, taken from a stream in the Arcachon Lagoon catchment area, contain high concentrations of reactive iron oxide extracted with the ascorbate solution. These characteristics can be explained by the geological nature of the area. The soils are underlain by Quaternary siliceous sands from the Landes region, which are rich in iron oxides (Legigan, 1979; Jolivet et al., 2007). The water table is no deeper than 3 m below the surface, and the total thickness of the phreatic aquifer is 10 to 25 m. Streambed Fe-oxides are formed by precipitation-oxidation of reduced Fe(II) dissolved in forested soil groundwater that percolates into aerated stream water (Baken et al., 2013; Buquet et al., 2017; Deirmendjian et al., 2019). Newly formed iron oxides are colloidal forms that are easily extracted with the ascorbate solution. These iron oxides have a high sequestration capacity for phosphate (Lijklema, 1980; Ruttenberg, 2014; Anschutz et al., 1998). Phosphate dissolved in stream water can be adsorbed on newly precipitated Fe-oxides (Anschutz et al., 2015), as observed in other lowland hydrosystems (Van der Grift et al., 2014; Baken et al., 2013, 2015a, 2015b). As a result, dissolved phosphate concentrations are very low in the rivers of the area (Canton et al., 2012; Buquet et al., 2017).

The Ditch 2 and Jolles sites are located immediately downstream of a large agricultural area whose soils are amended with phosphate of

mineral and organic origin. During rainfall, some of this phosphate is probably loosed and adsorbed onto the newly formed iron oxides (Baken et al., 2016). This would explain the high proportions of P extracted by the ascorbate and the relatively low Fe-asc/P-asc ratio, close to 6. However, experiments carried out with sediments from the Ditch 2 and Jolles stations, without the addition of Mn(III) or Mn(IV) phases, trapped additional dissolved phosphate. The control unit with sediment from Ditch 2 contained 612 µM of Fe-asc and 93.2 µM of associated P, to which 8.2 and 18 µM of P were added to the particles from water initially spiked with 10 and 20 µM of phosphate, respectively. The initial Fe-asc/P-asc ratio of 6.6 was thus reduced to 5.5. The ratios are similar for the Jolles sediments. Fe/P ratios below 5 for poorly crystallised Fe(III)-phases have been reported in many aquatic environments (Lijklema, 1980; Gunnars et al., 2002; Katsev and Dittrich, 2013; Anschutz et al., 2019). This suggests that the iron oxide adsorption sites present in the sediments studied were not saturated and had the capacity to trap additional phosphate. Most of the phosphate was adsorbed in the first 2 h of the experiments, suggesting that in the natural environment dissolved phosphate concentrations rapidly dropped and become low downstream of the system.

Sediments from downstream station H, far from agricultural areas, had a much higher Fe-asc/P-asc ratio, with a value of 29. The groundwater of the forest area that feeds the stream is indeed rich in dissolved iron and poor in phosphate (Deirmendjian et al., 2019; Buquet et al., 2017). The iron oxides that precipitate on contact with oxygenated water explain the relatively high Fe-asc concentrations in the sands at
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Evolution of phosphate concentrations over time (in hours) in the experimental units containing 10 µM phosphate with sediments and 0, 250, 500 or 1000 µL of Mn (III) or Mn(IV) oxide suspension. The 50-mL slurries contained 200 mg of dry sediment for Ditch 2 (A), Jolles (B), Aiguillon mud duplicates (D and E), 1 g for station H (C) and 1 g and 5 g for Aiguillon sand (F and G).</figcaption></figure>
this station (35 µmol/g; Table 1). On the other hand, P-asc concentrations are low and the Fe-asc/P-asc ratio is high (29). These Fe-oxides therefore still had a capacity to adsorb some phosphate. The experiment carried out in the control unit with 20 g/L of sediment from station H showed that 4.2 µM of phosphate out of the 10 µM added was adsorbed. The adsorption capacity remained lower than that obtained for muddy sediments, as the Fe-asc/P-asc ratio was still 25 for sands after the experiment compared to a ratio close to 6 for muds. The difference in adsorption capacity is probably related to the size and the surface area of iron oxide particles (Waychunas et al., 1994). The muddy sediments of the Aiguillon show high levels of iron and phosphate extracted by ascorbate, with an Fe-asc/P-asc ratio close to 15. These values are similar to those already measured for the Arcachon lagoon muds (Deborde et al., 2008; Delgard et al., 2013). In units containing 4 g/L of these muddy sediments (616 µM Fe-asc; 40 µM P-asc), an additional 6 µmol of phosphate was adsorbed, bringing the Fe-asc/P-asc ratio to 13. This suggests that in the natural environment the sediment has a phosphate adsorption capacity that is not saturated. The Aiguillon sand is low in iron extracted by ascorbate. The Fe-asc/P-asc ratio was 7.6. The phosphate adsorption capacity of this sediment was almost reached, as the incubations with the control units showed very little additional P trapping.

## 4.2. Artificial Mn-ox characteristics

The sediments have been spiked with synthesized Mn oxides. The mineralogy of these oxides has not been accurately characterized by physical methods such as X-ray diffraction or electron microscopy (e.g. Chen et al., 2022). The most common Mn(III,IV) phases found in the aquatic and soil environments are birnessite (δ-MnO₂), pyrolusite (β-MnO₂), hausmannite (Mn₃O₄), and manganite (γ-MnOOH) (Post, 1999). The method used here to synthesize Mn(IV) phases is known to produce low crystalline birnessite (Murray, 1974; Yao and Millero, 1996). The oxidation state obtained for the phase is exactly 4, which suggests that the Mn phase prepared with permanganate and Mn²⁺ was actually of MnO₂ type. The phase obtained from the oxidation of Mn²⁺ in an alkaline medium has an average redox state of 2.87 and not quite 3, suggesting that a Mn(III) phase has been synthesized, but that a fraction of Mn²⁺ is present, either in the form of Mn²⁺ ions adsorbed on these particles, or as hausmannite (Mn₃O₄), an oxide of Mn(II) and Mn(III).

The Mn oxides present in aquatic environments are generally derived from the oxidation of Mn(II). Abiotic manganese oxidation is kinetically limited at near-neutral pH. Under most environmental conditions, Mn oxidation is a biologically catalysed process (Tebo et al., 2007). The synthetic phases used here for the phosphate adsorption experiments do not represent all the Mn oxide phases found in aquatic environments. However, they can be considered as representative of Mn(IV) and Mn (III) phases, and they are comparable to those used in previous experiments (e.g., Yao and Millero, 1996; Kawashima et al., 1986; Chitrakar et al., 2005).

## 4.3. Effect of Mn-ox phases on P sequestration

### 4.3.1. Mn(IV)

Phosphate concentration trends in experimental units containing only suspended Mn(IV) oxides show that the addition of 472 µM Mn(IV) does not significantly reduce phosphate concentration when starting from 10 or 20 µM dissolved phosphate. In our experiments with freshwater sediments, the addition of Mn(IV) oxides had no significant effect on phosphate adsorption compared to control units containing sediments not spiked with Mn(IV) oxides. In fact, sediments from the Ditch 2 and Jolles control units sequestered >80% of the added phosphate, and the addition of Mn(IV) oxide at 1 to 7 times the Fe-asc concentration had no effect on the sequestration kinetics and efficiency. This is because Mn (IV) oxide has a negative surface charge at near neutral pH, as its pH of zero charge is about 2.25 (Murray, 1974), and is not expected to adsorb

anions. This has already been shown in similar experiments in deionised water (Kawashima et al., 1986). However, Mn(IV) oxide has a high capacity to adsorb cations (Murray, 1975). Alkaline earth and divalent transition metal cations cause Mn(IV) oxide to adsorb phosphate between pH 6 and 9 because the surface charge is reversed by exchange of H⁺ on the Mn(IV) oxide surface for divalent cation from solution (Murray, 1975; McKenzie, 1979). The positive charged surface leads to the adsorption of phosphate.

In experiments carried out in the presence of cation-loaded natural solutions, such as seawater, or in cation-doped solutions, phosphate is partially sequestered by Mn(IV) oxides (Yao and Millero, 1996; Kawashima et al., 1986). The experiments presented here carried out with seawater (S = 26; Ca²⁺ = 7.6 mM, Mg²⁺ = 19.2 mM) in contact with Aiguillon sediments show that the addition of Mn(IV) oxides increases the phosphate sequestration efficiency and that this increase is proportional to the amount of oxides added. For example, the fine-grained sediments of the Aiguillon, without spiking with Mn oxide, adsorbed 62% of the 10 µM added phosphate: In these test units, the 616 µmol of Fe-asc present in the sediment adsorbed 6.2 µmol of phosphate. Ascorbate extraction shows that the Fe-oxide particles already had 40 µmol of adsorbed phosphate before experiments. So in the end of the experiment, the iron oxides contained 46.2 µmol of phosphate, giving a Fe-asc/P-asc ratio of 13.3. The addition of 585, 1170 and 2340 µM Mn(IV) increased the adsorption efficiency to 75%, 80% and 86% respectively. Mn(IV) oxides, which adsorbed between 1.3 and 2.4 µM additional phosphate therefore have a Mn/P ratio of between 450 and 975, which means that the role of these oxides in phosphate sequestration is somewhat limited in comparison with ascorbate extractible Fe-oxides. This result shows that Mn oxides alone (Fig. 2) or mixed in a sediment do not have a high phosphate adsorption capacity.

The calculation of the mass balance for Aiguillon's sandy sediments yields a similar result. The slurry, containing 100 g of sand per litre, shows an initial concentration of 680 µM Fe-asc and 90 µM P-asc. Subsequently, 36% of the 10 µM phosphate added was absorbed after 48 h, resulting in a final Fe-asc/P-asc ratio of 7.3. Upon adding 585, 1170, and 2340 µM Mn(IV), an additional 2.1, 3.1, and 4.2 µM phosphate were adsorbed, respectively. This leads to the indication that the Mn/P ratio of Mn(IV) oxides lies between 280 (585/2.1) and 560 (2340/4.2).

The sediments from Station H were analysed using slurries containing particles at a concentration of 20 g per litre. This quantity likely yielded an adequate amount of cations from the pore water for the phosphate adsorption by the Mn(IV) oxides introduced. In the presence of these oxides, the sequestration is marginally higher than in their absence. With respect to the amount of oxides added, the Mn/P ratio exceeds 1000.

### 4.3.2. Mn(III)

The experiments carried out with the Mn(III) oxides alone showed that the 736 µM of Mn(III) adsorbed 1.8 µM of phosphate, whether with 10 µM at the start or 20 µM, giving a particulate Mn/P molar ratio of 400. In the experiments with sediment, the additions of Mn(III) was five times higher, i.e. 3680 µM Mn(III). Theoretically, they could sequester five times as much phosphate, or 9 µM, i.e. 90% of the amount added in the experimental units. However, for the Jolles and Ditch 2 units, this proportion of phosphate was already adsorbed by the Fe-oxide bearing sediment without the addition of Mn-ox, although in these units the iron oxide content was only 344 µM for Ditch 2 and 612 µM for Jolles. In addition, the iron oxides were already loaded with adsorbed phosphate, with P-asc concentrations of 56 µM and 93 µM respectively. This clearly shows that the small amount of iron oxide has a much greater phosphate adsorption capacity than a large amount of Mn-ox.

For the sediments of station H and those of Aiguillon, the control experimental units without Mn addition showed that the 10 µM of phosphate added was not completely adsorbed. The significant additions of Mn(III) contributed to trapping a further part of the phosphate. However, this contribution was small. For example, in the experimental
units containing the fine-grained sediments of Aiguillon, we had 616 µM of iron oxide which originally contained 40 µM of adsorbed phosphate. Doping the solution with 10 µM phosphate resulted in an additional adsorption of more than the half the phosphate (6.2 µmol) by these oxides. The addition of 920, 1840 and 3680 µM phosphate-free Mn(III) resulted in the adsorption of 1.4, 1.9 and 2.2 µM more phosphate than the control. Thus, 616 iron oxides were able to adsorb 46.2 phosphates (Fe/P = 13.3), whereas the Mn(III) oxides were only able to adsorb up to an Mn/P ratio >660 (920/1.4). Similar calculations can be made with the results from the other experimental units.

### 4.3.3. Particulate Mn/P molar ratio

The Mn/P molar ratio of oxides that can be deduced from experiments with Mn(III) or Mn(IV) oxides is very high in our experiments. Kawashima et al. (1986) show that 360 µM MnO₂ can adsorb 1.5 µM phosphate (Mn/P = 240) in the presence of 200 µM Ca²⁺ or Mg²⁺. Yao and Millero (1996) show that 1000 µM MnO₂ could adsorb 6 µM phosphate (Mn/P = 166) at salinities between 5 and 35 and pH 7. These experimental values carried out with pure phases and in the presence of dissolved cations show a slightly higher proportion of P adsorbed than in our experiments, but the values remain very low compared to the Fe/P molar ratio found in sediment iron oxides in contact with high concentrations (> 1 µM) of dissolved phosphate. These ratios are generally between 5 and 30 in sediments from continental margins (Jensen and Thamdrup, 1993; Charbonnier et al., 2019; Anschutz et al., 1998, 2019), lakes (Jensen et al., 1992; Kleeberg et al., 2013; Alam et al., 2020), oceanic hydrothermal springs (Feely et al., 1991) and the experiments in our study, although the maximum molar Fe/P ratio for phosphate incorporation within ferric oxides has been shown to be 2 (Thibault et al., 2009). Thus, sequestering a given quantity of phosphate would require on average 50 times more Mn oxides than Fe oxides, which can be extracted with ascorbate.

### 4.3.4. Role of Mn oxides on P sequestration

The objective of our study was to examine the impact of a substantial addition of Mn oxide to natural sediments rich in iron oxides on the adsorption of phosphorus. This experimental design enabled us to specifically investigate the role of Mn oxides and compare it to the role of Fe oxides, as our results were not influenced by the initial presence of Mn oxides. Consequently, although we have employed sediments naturally deficient in Mn to elucidate the function of Mn oxides in P adsorption, our findings furnish crucial quantitative data for elucidating the respective roles of Fe and Mn oxides in P mobility in contexts where Mn oxides are present in substantial quantities.

Manganese oxides are generally less abundant than iron oxides in natural environments. This is primarily due to the relative abundance of these two elements in the upper continental crust, with iron representing 3.9% and Mn 0.077%, giving an Mn/Fe ratio of 0.02 (Rudnick and Gao, 2003). Manganese is commonly found substituted in small amounts in iron minerals. Mn oxides therefore generally play a minor role in P sequestration compared with Fe oxides. However, there are environments where Mn oxides can concentrate. During early diagenesis processes, Fe and Mn oxides present in sedimentary particles below the water-sediment interface can be reduced in an anaerobic environment. The Fe²⁺ and Mn²⁺ ions thus produced can either diffuse towards the oxidized zone of the sediments and re-precipitate by oxidation, or be trapped in the form of authigenic minerals such as iron sulphides or solid solutions of Mn carbonate (Mucci, 1988, Middelburg et al., 1987). These processes do not all take place with the same intensity or in the presence of the same oxidising or reducing compounds. This is because the redox potentials of the Fe(III)/Fe(II) and Mn(IV,III)/Mn(II) pairs are different, and the oxidation kinetics of Fe(II) are generally faster than those of Mn (II) (Stumm and Morgan, 1996). Dissolved Mn(II) can diffuse upwards in pore waters across the oxic-anoxic boundary and sometimes across the sediment-water interface where it precipitates as Mn(III,IV) (Sundby and Silverberg, 1985). In general, Mn oxides concentrate in the aerobic

zone and iron is more effectively trapped in an authigenic reduced form, particularly in marine sediments (Calvert and Pedersen, 1996; Froelich et al., 1979; Anschutz et al., 1998). There may therefore be a chromatographic separation of Fe oxides and Mn oxides (Maynard, 2003), which can produce an oxidized Mn enriched layer at the sediment surface (Katsev et al., 2006; Mouret et al., 2009; Hermans et al., 2021). The ratio of Mn oxides to Fe oxides can be close to 1. However, our results show that this ratio is still insufficient for Mn oxides to be the main carrier phase for adsorbed P, as Mn-ox have a sequestration capacity 50 times lower than that of Fe-ox.

## 5. Conclusion

The aim of this study was to determine the role of manganese oxyhydroxides in phosphate mobility in aquatic environments. Although manganese oxide is generally negatively charged at near-neutral pH and cannot effectively remove anionic pollutants such as phosphate, phosphate concentrations decrease slightly in a slurry containing Mn oxides. However, the adsorption of phosphate by these oxides is very low compared to the adsorption capacity of iron oxides. Furthermore, Mn is a much less abundant element than Fe in aquatic environments. Its role in phosphorus bioavailability is therefore even more limited. The results suggest that manganese oxides can act as accessory adsorbents of phosphate in natural waters and surface sediments when present in much higher concentrations than ascorbate-extractable iron oxides.

Fe-ox and Mn-ox are subject to reductive dissolution and consequently redox conditions play an important role in P bioavailability (e.g. Krom and Berner, 1980; Sundby et al., 1992; Slomp and Van Raaphorst, 1993; Jensen et al., 1995; Slomp et al., 1996a, 1996b; Anschutz et al., 1998; Smolders et al., 2017). Although it may play a role in the P cycle in environments particularly enriched in Mn (e.g. Yakushev et al., 2009), the reduction of Mn oxides does not represent a major risk for eutrophication of aquatic ecosystems.

### CRediT authorship contribution statement

Pierre Anschutz: Writing – original draft, Supervision, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. Florian Benony: Methodology, Investigation, Formal analysis.

Céline Charbonnier: Writing – review & editing, Methodology, Investigation, Formal analysis.

### 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

This study is a part of the project REZIN funded by the SIAEBVELG (Syndicat Intercommunal Aménagement des Eaux du Bassin Versant des Etangs du Littoral Girondin), the New Aquitaine Regional Council, the Adour-Garonne Water Agency, and the Gironde Department. We are grateful to Vincent Hanquiez for technical support.

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