1 Introduction

Titanium dioxide nanoparticles (\(\mathrm{Hereafter abbreviated as }{\mathrm{TiO}}_{2}\mathrm{ NPs}\)) have proven advantages in diverse fields such as textile, medical, food additives, cosmetic and electronic products. It is estimated that \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) production will reach 2.5 ×\({10}^{6}\) tons per year in the USA by 2025 (Robichaud et al. 2009). Intentional or accidental release of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) into aquatic environments might have detrimental impacts on flora, fauna, and human life (Gottschalk et al. 2011; Saratale et al. 2018). Studies estimate \(\mathrm{NPs}\) to be the most abundant Engineered nanomaterial in aquatic environments ranging from 21 to 10,000 ng/L in surface water (Maurer-Jones et al. 2013). Also, studies confirm that aquatic sediments are the main sink for \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) (Cross et al. 2015). For example, Fan et al (2018) reported that the level of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) in sediments was significantly higher than in water column. That's why the highest concentrations of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) are reported in aquatic sediments of the United States and Europe (Gottschalk et al. 2009).

On the other hand, \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) have a significant potential to modify bioavailability, mobility, toxicity, and accumulation of co-existing pollutants (heavy metals and toxic organics) in sediment. Fan et al (2017) revealed that \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) change concentrations of As, Pb, Cd, and Cu species in sediments. Small size, large surface area, and strong electrostatic attraction of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) are the main reasons for influencing on geochemical speciation of heavy metals (Zhang et al. 2007; Domingos et al. 2009; Luo et al. 2011). As a result, it is important to study the effect of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) on the geochemistry of heavy metals in sediment (Cai et al. 2019).

Photocatalytic activity of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) is another prominent feature, which influences heavy metals speciation, but not much research has been conducted in this field to date. Due to Photocatalytic ability, \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) have the potential to cause a series of redox reactions in the water column (Liu et al. 2017) and produce strong oxidants like hydroxyl radicals, which degrade organic and inorganic compounds and also change heavy metals distribution (Kalyanasundaram et al. 2013). Numerous studies reported the degradation of organic and inorganic compounds by \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) (Kang et al. 2019). It should be noted that photocatalytic activity of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) in an aquatic media is limited, because \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) are only active in the presence of ultraviolet light, and on the other hand, only about 4% of the sun's light spectrum contains ultraviolet ray (Zhu et al. 2014; Chakhari et al. 2017).

In this study, to assess the role of heavy metals speciation in sediments, the contaminated sediments of the Anzali wetland were applied rather than spiking heavy metal ions in sediments (which is a common method to simulate heavy metal-contaminated sediments).

To our knowledge, few studies have considered the effect of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) on sediment geochemistry. Besides, the mobility of heavy metals in sediments under photocatalytic reactions of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) has not been investigated to date. The present study is one of the first studies to investigate the potential of photocatalytic activity of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) in aquatic environments. According to the abovementioned, the aims of this study are:

  1. (1)

    To explore the impact of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) on various sedimentary phases (loosely bonded ions, Fe–Mn, organics).

  2. (2)

    To investigate the effect of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) on the interaction of heavy metals between surface sediments and overlying water.

  3. (3)

    To investigate the influence of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) on physicochemical parameters of the water column.

  4. (4)

    To evaluate the impact of Photocatalytic activity of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) on aquatic systems.

2 Methodology

  • Materials

In this study, titanium dioxide powder was purchased from Degussa (Degussa-P25). According to producer data, P25 contains 85% anatase and 15% rutile, particles size of 20 nm, and a specific surface area of 57 \({m}^{2}/g\). The chemicals applied during the experiments were analytical grade. Milli-Q water (18 MΩ-cm) was used to prepare all solutions.

  • Sample collection and preparation

The sampling station (latitude of 37°24′46″N and longitude of 49°24′24″E) was located in the southern part of the Anzali wetland (Fig. 1). It is worth mentioning that the sampling zone was exposed to pollutants transferred from four main rivers named Siahdarvishan, Masouleh Rudkhan, Palang Ver, and Khalkaee.

Fig. 1
figure 1

Map of Anzali wetland and the sampling point

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Sediment Samples (depth of 0–10 cm) were collected by a Peterson Grab sampler in July 2020, kept in pre-cleaned packages, transported to the laboratory, and dried at room temperature. Then they were homogenized using a shaker instrument from RESTCH Company. To remove gravel and other large particles, a 63-µm sieve was used. The sieved samples were used for bulk digestion and sequential extraction analysis.

Also, water samples were taken with acid-washed polyethylene plastic bottles. After transporting to the laboratory, water samples were kept in a static state for 48 h (allowing particles to settle) and then filtered by a 0.22 μm Millipore AP and HA filter.

Setup test

All experiments were performed in a series of glass aquaria containing 10 cm of pre-dried and pre-sieved sediment, and 8 cm of the filtered water column. Thereafter, \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) were added into aquaria at concentrations of 5, 10, 15, 20, 25, and 30 ug/L, respectively. These aquaria were aged outdoors for 60 days with closed mouths. Moreover, a control sample with no containing \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) was applied simultaneously under the same conditions.

  • Chemical analysis

LA-950 Laser Particle Size Analyzer was utilized to determine particle size distribution. ASTM-D7348-08 method was used to measure the loss of ignition (LOI). It must be mentioned that LOI is a common method to measure the organic contents of sediment. To analyze biological oxygen demand (BOD), the Winkler method was used. Also, the dichromate reflux method was applied to measure chemical oxygen demand (COD). Total organic carbon (TOC) was determined by oxidation and analyzed by IR gas measurement. The Brown ring test was applied to measure nitrate \({\mathrm{NO}}_{3}\). After digestion with peroxodisulfade, the molybdenum blue method was used to determine phosphate \({\mathrm{PO}}_{4}\) (APHA 1985). The detection limit for TOC, BOD, NO3, and PO4 was 0.01 mg/l. Bante900 Benchtop Multiparameter Water Quality Meter was utilized to measure pH, Oxidation Reduction Potential (ORP), and Total Dissolved Solids (TDS).

To determine the total concentration of metals, 1 g of pre-died sample was digested by Aqua regia according to the 305B method (USEPA 1996) To evaluate mobility, bioavailability, and interaction of heavy metals in an aquatic environment, it is needed to determine their geochemical species in the sediments (Saleem et al. 2018; Chakraborty et al. 2019). For this purpose, European Union’s standards, measurements, and testing scheme (SM and T, formerly BCR) was applied as a sequential extraction procedure to specify the geochemical speciation of heavy metals in sediment (Rauret et al. 1999). According to this method, metal species in sediment are divided into four fractions: exchangeable species (F1), Fe–Mn oxides species (F2), organic–metallic species (F3), and residual species (F4). Table 1 summarizes information on the extraction procedure. After each step of extraction, a centrifuge at 3000 rpm was used for 15 min to separate extraction achieving from sediment. Also, a pipette was used to remove supernatant.

Table 1 SM and T sequential extraction method
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  • Measurement of heavy metals concentration

ICP-OES was used to analyze and measure the concentration of heavy metals in the digested samples mentioned above. Hydrochloric acid and potassium iodide were added to convert As (V) to As (III), Before analyzing samples. The concentration of Hg in the digested samples was measured by a VGA-77 connected to a Varian SpectrAA.200 spectrometer. Sodium borohydride solution, as a reducing agent was added to the samples prepared for Hg analysis.

  • Risk sssessment code (RAC)

The geochemical distribution of heavy metals can give us a vision of sediment influence on an aquatic environment (Zhang et al. 2014). In general, the bioavailability order of heavy metals species in sediment is as follows: Exchangeable > Fe–Mn > Organic > Residual (Sungur et al. 2015). The risk assessment code (RAC), which is introduced by Perlin et al. (1985), is an index used to evaluate the mobility and bioavailability of heavy metals in sediment. RAC is based on sequential extraction results and is defined as follows:

$${\text{RAC}} = \frac{{F_{1} }}{{C_{t} }} \times { 1}00$$

where \({F}_{1}\) is concentration of a metal in exchangeable species, and \({C}_{t}\) is total concentration of a metal (or sum of metal concentration in four species). The RAC indices can be classified into five levels (Table 2).

Table 2 RAC (Risk Assessment Code) classification
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  • QA/QC control

To validate the analytical data:

  1. 1.

    All treatments were conducted in triplicate. The relative standard deviation was calculated (RSD < 5%).

  2. 2.

    Blank samples were applied to minimize the laboratory errors.

  3. 3.

    To assess the photocatalytic activity of \({\mathrm{TiO}}_{2}\mathrm{ NPs}\) in aquaria, a blank sample was used in dark conditions.

  4. 4.

    A referenced sediment sample (MESS-3), supplied by the National Research Council of Canada, was applied for assessing the accuracy of the bulk digestion method. Table 3 presents the analytical error between measured and published contents of metals in the referenced sediment. As shown in Table 3, the recovery ranges were between 96 and 102%.

    Table 3 The Published and measured contents of metals for MESS-3 (mg/Kg)
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  5. 5.

    The accuracy of sequential extraction was assessed by comparing the summation of 4 fractions with the total extraction as follows. The recovery ranges were between 96 and 105%.

    $${\text{Recovery}} = \frac{{100 \times \left( {F_{1} + F_{2} + F_{3} + F_{4} } \right)}}{{{\text{Total}}}}$$

  • Statistical analysis

Data were normalized and MVSP software (version 3.2) was utilized for clustering analysis. The clustering analysis was performed based on the Pearson coefficient and weighted pair group with arithmetic mean (WPGMA).

3 Results

According to Tables 4, 5, 6, TiO2 NPs addition to the aqueous media causes changes in heavy metals concentration and organic content in both overlying water and the surface sediment. Moreover, major variation in physicochemical parameters of the water column is observed. Full details are provided below:

Table 4 Water column and sediment characteristics before and after the addition of TiO2 NPs
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  • Physicochemical parameters of water column

As shown in Fig. 2, by increasing the TiO2 NPs concentration in the aqueous media, TDS, nitrate, phosphate, TOC, BOD and COD have shown an increasing trend (Fig. 2c, d, e, f, g, and h), ORP has exhibited a declining trend (Fig. 2a) and pH has shown a steady trend (Fig. 2b) after the 60-day test period.

Fig. 2
figure 2

Changes in water quality parameters after the TiO2 NPs addition; a ORP, b pH, c TDS, d NO3, e PO3, f TOC, g BOD, and h COD

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  • Heavy metals concentration in water column

According to Table 5, after the addition of TiO2 NPs, changes in heavy metals concentration in the water column are obvious. Measurements reveal that the concentration of Mn, Cr, and Ti has decreased from 44, 33, and 20 to 19, 4, and 3 ppb, respectively. Furthermore, a slighter reduction in the concentration of Zn, Co, and Hg in the water column is observed.

Table 5 Heavy metals concentration in the water column before and after the addition of TiO2 NPs
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On the other hand, whereas the initial water sample does not contain arsenic, after a 60-day test period, arsenic has been identified in the water column with a concentration of 8.7 ppb. Moreover, a slight increase in Ni concentration is observed in the water column after the addition of TiO2 NPs.

As a result, except for Pb, whose concentration remains constant after the addition of TiO2 NPs, the concentration of other heavy metals changes.

  • Texture of sediment

The used sediment with 12% LOI contains 0.9% clay, 67.7% silt, and 31.6% sand. The obtained results in this study prove that TiO2 NPs cause the decomposition of organic compounds in the sediment. Per Table 4, as the concentration of TiO2 NPs increases from 0 to 35 μg/L in the water column, the LOI decreases from 12% to 8% in the sediment.

  • Distribution of heavy metals in sediment

The data reported in Table 6 show that, except for Pb, TiO2 NPs affect the geochemistry distribution of other heavy metals in the sediment.

Table 6 Distribution of heavy metals concentration in sedimentary phases before and after the addition of TiO2 NPs
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According to bulk digestion analysis, the presence of TiO2 NPs leads to the desorption of As and also, to a lesser extent, Ni from the sediment. On the other hand, TiO2 NPs cause the adsorption of Mn, Zn, Hg, Cu, and Ti in the sediment. However, Pb and Co show conservative behavior in the aquatic environment (Table 6).

Sequential extraction analysis helps us to acquire more understanding of the interaction mechanism of heavy metals between water and sediment. So, the behavioral mechanism of each heavy metal, based on a sequential extraction procedure, is described below.

4 Titanium

The presence of TiO2 NPs in aqueous media influences Ti distribution. After adding TiO2 NPs to the test environment, Ti concentration increases in the sediment and simultaneously decreases in the water column, indicating Ti adsorption by the sediment.

As shown in Fig. 3, Ti adsorption leads to the formation of exchangeable species and Fe–Mn species in the sediment. In the case of Ti, the formation process of exchangeable species is higher than that of the Fe–Mn species. Furthermore, Ti is not adsorbed by the sediment in the form of organometallic species.

Fig. 3
figure 3

Chemical species of titanium in different levels of TiO2 NPs

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On the other hand, contrary to Fe–Mn species, the exchangeable species content increases with increasing the concentration of TiO2 NPs from 0 to 30 µg/L. Therefore, the formation process of Fe–Mn species is independent of TiO2 NPs content in the water column.

5 Zinc

Due to the addition of TiO2 NPs to the aqueous media, the concentration of Zn decreases in the water column and increases in the sediment, meaning that Zn is adsorbed by the sediment. However, increasing TiO2 NPs level has no significant effect on the zinc adsorption process.

Figure 4 illustrates that level of exchangeable species is in direct relation to the level of TiO2 NPs. In contrast, the level of Fe–Mn species is negatively related to TiO2 NPs level. However, organic–metallic species displays a relatively conservative behavior with changing TiO2 NPs level.

Fig. 4
figure 4

Chemical species of zinc in different levels of TiO2 NPs

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6 Lead

TiO2 NPs have no effects on Pb distribution in both overlying water and the sediment surface. However, according to Fig. 5, a slight change is observed in the level of exchangeable and organic–metallic species of Pb.

Fig. 5
figure 5

Chemical species of lead in different levels of TiO2 NPs

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7 Nickel

Nickel exhibits a relatively conservative behavior against TiO2 NPs content changes. To be more precise, a slight change in nickel concentration is observed in both the water column and sediment surface after adding TiO2 NPs.

Figure 6 shows that organic–metallic species of nickel are significantly reduced upon arrival of TiO2 NPs to the aqueous medium, although this reduction does not intensify with increasing TiO2 NPs levels. Furthermore, a slight reduction of exchangeable species of nickel is observed with increasing TiO2 NPs levels. However, Fe–Mn species of Ni display a completely conservative behavior in the presence of TiO2 NPs.

Fig. 6
figure 6

Chemical species of nickel in different levels of TiO2 NPs

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8 Copper

The change of copper concentration indicates its adsorption by the sediment after adding TiO2 NPs to the test environment. According to Fig. 7, copper is adsorbed mainly as Fe–Mn species and then as exchangeable species. However, organic–metallic species have no role in the copper adsorption process. The remarkable point is that the tendency for the formation of Fe–Mn species is reduced with increasing TiO2 NPs levels.

Fig. 7
figure 7

Chemical species of copper in different levels of TiO2 NPs

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9 Cobalt

Cobalt concentration in the water column decreases after adding TiO2 NPs. As shown in Fig. 8, except for a slight change in exchangeable species, no changes are observed in Fe–Mn and organic–metallic species of cobalt. Furthermore, no changes are also observed in the cobalt concentration in water and sediment with increasing species concentration from 5 to 30 µg/L. Thus, cobalt has a relatively conservative behavior upon the arrival of species in an aquatic environment.

Fig. 8
figure 8

Chemical species of cobalt in different levels of TiO2 NPs

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10 Manganese

After the addition of TiO2 NPs to aquaria, Mn concentration is reduced in the water column, so that at high levels of TiO2 NPs, its concentration is halved. Figure 9 reveals that manganese adsorption as exchangeable species and Fe–Mn species is the reason for this reduction. In contrast, manganese reduction in organic–metallic species is observed. Therefore, it can be concluded that TiO2 NPs affect manganese interactions with water and sediment through two opposite mechanisms. However, the effect of TiO2 NPs on the adsorption mechanism is stronger than the desorption one. In consequence, TiO2 NPs finally lead to the reduction of manganese concentration in the water column.

Fig. 9
figure 9

Chemical species of manganese in different levels of TiO2 NPs

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11 Arsenic

Observations confirm the unique behavior of arsenic compared to other metals. Before adding TiO2 NPs, the exchangeable level of arsenic species is extremely high, and arsenic has an anthropogenic source in the sediment. The results of the bulk digestion and the chemical extraction analysis indicate that TiO2 NPs have a substantial impact on exchangeable species of arsenic and cause their interaction with water and sediment. In comparison to other heavy metals, except for Ni, arsenic is desorbed from the sediment and enters the water column due to the presence of TiO2 NPs in the test environment.

Figure 10 proves that the addition of TiO2 NPs leads to a drastic reduction of exchangeable species of arsenic in the sediment. In addition, organic–metallic and Fe–Mn species of arsenic are slightly reduced in response to TiO2 NPs in the aqueous media. More exactly, organic–metallic and Fe–Mn species of arsenic show a relatively conservative behavior.

Fig.10
figure 10

Chemical species of arsenic in different levels of TiO2 NPs

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12 Mercury

After the arrival of TiO2 NPs to the aqueous media, Hg concentration in the water column is almost halved. The bulk digestion analysis reveals that reduced mercury is adsorbed by the sediment. According to Fig. 11, mercury adsorption occurs mainly in the form of exchangeable species and Fe–Mn ones. In addition, with increasing TiO2 NPs levels in the environment, more exchangeable species are formed, although this amount is very low. Generally, increasing TiO2 NPs raises the Hg accumulation in the sediment. Organic–metallic species do not play any role in the mercury interaction with water and the sediment in response to TiO2 NPs.

Fig. 11
figure 11

Chemical species of mercury in different levels of TiO2 NPs

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13 Discussion

As shown in Fig. 12, after the addition of TiO2 NPs, the ORP of the test medium decreases. The trend of change in ORP indicates the aquatic environment transition from aerobic to anaerobic conditions (Yavar et al. 2019). Moreover, according to the ORP classification by Bohn (1971), as TiO2 NPs concentration increases, the water column degrades from moderately reduced conditions to highly reduced conditions.

Fig. 12
figure 12

The impact of TiO2 NPs on ORP

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After the addition of TiO2 NPs, TDS increases in the water column as a result of the mineralization of the sediment organic matter. Organic compounds, which often contain carbon, nitrogen, and phosphorus structures, are converted to other compounds, such as carbonate, nitrate, and phosphate, in the mineralization process. During this process, TDS in the water column will increase (Rezaei Tavabe et al. 2018). Numerous researchers have reported that after organic matter mineralization, TDS will increase in the water column (Konstas et al. 2019). In addition, as organic content in the sediment decreases, PO4, NO3, and TDS in the water column increase. Hence, it can be inferred that a low concentration of TiO2 NPs in an aquatic environment can lead to the decomposition of organic compounds in the sediment. Increasing TOC in the overlying water provides another proof of organic compound decomposition in the sediment. Another noteworthy point is that the clustering analysis reflects a significant relationship between ORP and PO4, NO3, TOC, and TDS (Fig. 13). It implies that ORP reduction is due to organic matter decomposition. Thus, TiO2 NPs reduce the water quality by degrading organic compounds, thereby transforming the test medium into an anaerobic condition. It should be noted that TiO2 NPs can only degrade organic compounds by absorbing UV light (Nguyan et al. 2003).

Fig. 13
figure 13

The cluster analysis of physicochemical parameters of the water column

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On the other hand, the findings reveal that TiO2 NPs not only lead to the decomposition of the sediment organic matter or degradation of water qualitative parameters but also affect the geochemical distribution of heavy metals in the sediment. A very limited number of studies have considered the impact of TiO2 NPs on heavy metals distribution in the sediment (Fan et al. 2018). This study proves that TiO2 NPs are capable of affecting the distribution of heavy metals in the sediment directly and indirectly. On the one hand, the large adsorption capacity of TiO2 NPs changes heavy metal species in the sediment. On the other hand, due to their photocatalytic properties, TiO2 NPs alter water quality parameters (especially ORP as mentioned earlier), and thus, indirectly affect the distribution of the heavy metals in the sediment. It is widely believed that ORP governs the chemical species of heavy metals in the sediment (Zhang et al. 2014; Karbassi and Marefat 2017) in addition, it must be mentioned that no change in heavy metals species was observed in the blank sample used in dark condition.

According to Table 6, TiO2 NPs cause some heavy metals to be adsorbed and others to be desorbed by sediments. The change in geochemical species provides more or less detailed information concerning the adsorption and desorption process of heavy metals in sediments (Marefat et al. 2019). The data analysis in Table 6 reveals that heavy metals might be classified into 5 general groups based on their adsorption and desorption mechanisms in sediments, as follows:

  • Group 1: Heavy metals, desorbed from exchangeable and organic–metallic bounds, enter the aquatic environment (As and Ni).

  • Group 2: Heavy metals, adsorbed as exchangeable species and iron-manganese ones in the sediment (Zn, Cu, and Ti).

  • Group 3: Heavy metals, adsorbed only as exchangeable species in the sediment (Hg).

  • Group 4: Heavy metals, adsorbed as iron-manganese species and exchangeable species while desorbed from organic–metallic species (Mn).

  • Group 5: Heavy metals, having no interaction with sediment and water and showing a conservative behavior in an aquatic system (Pb and Co).

According to the cluster analysis (Fig. 14), there is a direct relationship between the concentration of Mn, Ti, Zn, and Hg in the sediment and the level of TiO2 NPs added to the water column. It is concluded that after the accumulation of TiO2 NPs in the sediment, they cause the adsorption of some heavy metals by the sediment surface. In confirmation of this conclusion, several investigators have reported that TiO2 NPs have a high adsorption capacity for heavy metals (Zhang et al. 2007; Sun et al. 2009; Fan et al. 2017, 2018).

Fig. 14
figure 14

The cluster analysis of heavy metals in the aqueous solution

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Fan et al. (2017) Proved that TiO2 NPs in sediments play a substantial role in reducing water pollution. They demonstrated that heavy metals released from the sediment toward overlying water were re-adsorbed by TiO2 NPs in the sediment surface. However, the findings of this study are in some respects inconsistent with the results of the study done by Fan et al. (2018). They claimed that the reduction of heavy metals concentration in exchangeable species and Fe–Mn ones stemmed from the fact that these species, desorbed from sediments, were re-adsorbed by TiO2 NPs. On the contrary, the present study shows that neither the concentration of heavy metals decreases in exchangeable species and Fe–Mn ones nor they are adsorbed by TiO2 NPs in the sediment. The potential reason for inconsistent results is that TiO2 NPs do not necessarily adsorb heavy metals on their surface and may cause heavy metals adsorption on the sediment surface.

Among heavy metals adsorbed to the sediment, Cu showed different behavior. According to the cluster analysis (Fig. 12), unlike Zn, Hg, Mn, and Ti, no relation existed between the adsorption rate of Cu and the change in the level of TiO2 NPs. As observed in the chemical fractionation experiment (Table 6), although adding TiO2 NPs to the aqueous media leads Cu to be adsorbed by the sediment, the greater level of TiO2 NPs does not cause the higher adsorption of Cu. Yin et al. (2020) showed that TiO2 NPs in the sediment had a significant role in adsorbing Cu from the water column. However, unlike the present study, where Cu is adsorbed as Fe–Mn species and exchangeable species, in the study reported by Yin et al., Cu adsorption was mainly as residual species. The synthesis of TiO2 NPs in activated carbon granules by Yin et al. may be the reason for conflicting results.

Figure 14 illustrates a strong correlation coefficient between concentrations of As and Ni in the sediment with a change in OPR and sediment organic content. This relationship is mainly attributed to the photocatalytic properties of TiO2 NPs, enabling them to cause a series of redox reactions in the aqueous media (Kang et al. 2019). Hence, organic compound degradation occurs, and Ni and As enter the aqueous phase. This hypothesis could be supported by the reduction of Ni and As organic species in the sediment. Therefore, it can be concluded that the photocatalytic properties of TiO2 NPs affect the geochemical distribution of some heavy metals, and this mechanism should not be ignored in future studies. In comparison to other heavy metals, TiO2 NPs have a much greater effect on arsenic. According to Fig. 15, before the addition of TiO2 NPs to the aquatic environment, arsenic proportion in exchangeable species was extremely high in the sediment. In other words, the external environment was at high risk of As contamination. Furthermore, with increasing the TiO2 NPs content in the water column, the quantity of arsenic in exchangeable species gradually decreases until at very high concentrations of TiO2 NPs, the arsenic exchangeable species approach zero in the sediment. In other words, at high concentrations of TiO2 NPs, the sediment potential for further contamination of aqueous media with arsenic disappears. It is worth noting that more than 230 million people worldwide are affected by arsenic-contaminated water. Alluvial sediments are the main source of arsenic contamination (Shaji et al. 2021). Heavy metal-contaminated sediments could be remediated using nanomaterials, including TiO2 NPs. However, just a few studies have examined the potential of TiO2 NPs for contaminated-sediment remediation (Duncan et al. 2019; Cai et al. 2019; Debnath et al. 2021). For example, Yin et al. (2020) demonstrated that TiO2 NPs could be applied to remediate sediments contaminated by copper. High reactivity and strong adsorption capacity make TiO2 NPs appropriate for remediating contaminated soil and sediments (Akcil et al. 2015; Peng et al. 2018).

Fig. 15
figure 15

The impact of TiO2 NPs on RAC value for arsenic

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However, unlike other studies that recognize the large adsorption capacity of TiO2 NPs as an effective factor in the remediation process, the present study knows the photocatalytic capability of TiO2 NPs as the main factor in remediating arsenic-contaminated sediment. To be more precise, TiO2 NPs remediate the arsenic-contaminated sediment indirectly. Redox reactions, balanced by TiO2 NPs cause the water column parameters to be changed, followed by releasing arsenic from its exchangeable and weak bounds in the sediment (Peng et al. 2009; Tandon and Singh 2016). Thus, TiO2 NPs lead to sediment remediation from exchangeable and hazardous species of arsenic.

As mentioned, manganese exhibits two distinct adsorption–desorption behavior in the presence of TiO2 NPs. Its exchangeable species and Fe–Mn ones decrease while its organic–metallic species increase. The results illustrate that manganese desorption is in direct relation with the sediment organic content reduction. Therefore, TiO2 NPs affect the geochemical distribution of manganese in two different ways: (1) The adsorption properties of TiO2 NPs, resulting in the increase of exchangeable species and Fe–Mn ones; and (2) Photocatalytic properties of TiO2 NPs, which causes reduction of organic–metallic species.

No changes in the Pb and Co species content are observed before and after the addition of TiO2 NPs. It is while Pb adsorption by TiO2 NPs has been reported by fan et al. (2018). It might be because, contrary to the present study, in which TiO2 NPs are added into the water column, in the Fan et al. (2018) study, both Pb and TiO2 NPs were spiked into the sediment.

Concerning the above-mentioned results, however, the study outcomes confirm that TiO2 NPs have a major impact on heavy metals distribution in the sediment, considering the geochemical species of heavy metals, are still not comprehensive. Other factors such as ultraviolet intensity, TiO2 NPs distribution in the sediment, TiO2 NPs size and surface, external environment conditions, initial concentration of heavy metals in the sediment and water column, and aquatic organisms might also have a crucial role. So, further studies are required in the future.

14 Conclusions

In the present study, the influence of TiO2 nanoparticles (NPs) on heavy metals speciation is investigated experimentally. According to our observation, strong adsorption capacity and photocatalytic capability are two prominent features of TiO2 NPs in aquatic systems, affecting the distribution of heavy metals in sediments.

The results show that TiO2 NPs make Cu, Hg, Ti and Zn be adsorbed by sediments as exchangeable and Fe–Mn species, while As and Mn be desorbed as exchangeable and organometallic species.

The photocatalytic degradation of sediments' organic content leads to significant changes in the water quality parameter. According to measurements, ORP sharply decreases after the introduction of TiO2 NPs to the water environment. On the other hand, TOC, BOD, \({\mathrm{NO}}_{3}\) and \({\mathrm{PO}}_{4}\) increase due to the introduction of TiO2 NPs.