Reservoir bottom sediments as heterogeneous catalysts for effective degradation of a selected endocrine-disrupting chemical via a Fenton-like process

Journal of Water Process Engineering 32 (2019) 100950

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Reservoir bottom sediments as heterogeneous catalysts for effective degradation of a selected endocrine-disrupting chemical via a Fenton-like process

T

Sabina Ziembowicz , Małgorzata Kida, Piotr Koszelnik ⁎

Department of Chemistry and Environmental Engineering, Faculty of Civil and Environmental Engineering and Architecture, Rzeszów University of Technology, 35-959, Rzeszów, al. Powstańców Warszawy 6, Poland

ARTICLE INFO

ABSTRACT

Keywords: Bottom sediment Endogenous catalysts Heterogeneous Fenton processes Di-n-butyl phthalate Green environmental remediation technology

The work detailed here investigated the efficiency of Fenton-like processes in degrading endocrine-disrupting chemicals, as exemplified by di-n-butyl phthalate (DBP). Specifically, a heterogeneous Fenton process was applied, as catalyzed by natural or modified bottom sediments collected from Rzeszów and Solina Reservoirs (Poland). A number of sediments with different physicochemical properties and different contents of Fe, Mn and Cu were in fact obtained and used, with “modification” entailing ion-exchange reaction and impregnation. It thus proved possible to demonstrate the possibility and legitimacy of Fenton's reagent being modified by natural bottom sediments that may therefore be regarded as heterogeneous catalysts, given a status as endogenous sources of iron, manganese and copper ions. While the effectiveness of DBP degradation was indeed influenced by sediment type and composition (and modification thereof), it was also affected by the factors of time, pH, DBP concentration, sediment dose and H2O2. The use of natural bottom sediments in the role of solid catalysts in heterogeneous Fenton processes represents a promising alternative to environmental remediation technology, given their abundance, low cost and environment-friendly nature.

1. Introduction In recent years, increasing attention has been paid to heterogeneous Fenton-like reactions. While the classic Fenton process sees soluble iron (II) ions deployed in an acidic environment, in such a way that oxidation takes place with part of the catalyst remaining in the treated wastewater and part precipitating out as Fe(OH)3 that then constitutes additional sediment, heterogeneous catalysis sees the place of the soluble catalyst taken by a solid alternative that can be recovered and reused, with only small amounts of Fe passing through to the wastewater. A further benefit of the heterogeneous catalyst is the way it extends the pH range across which the Fenton reaction remains effective. However, as a heterogeneous catalyst is selected, account needs to be taken of such characteristics as the content and degree of leaching of the ions present in the material, as well as abrasion resistance, ease of particle sedimentation to achieve separation from wastewater, chemical stability and capacity for rapid regeneration, and catalytic activity in the Fenton reaction [1–8]. In these contexts, catalysts used in versions of the Fenton reaction can either be classed as natural (in the form of minerals, soils or clays),

⁎

synthetic, or obtained by means of modification of various materials, Fe0 or industrial waste. Catalysts in the first group have in fact proved to be excellent alternatives, given their low cost, high level of availability and catalytic activity, and long-term stability. Examples have included metal oxides and iron-containing zeolites, with particular success demonstrated in the cases of goethite, pyrite, hematite, broadleaf and magnetite. The most important zeolites making up deposits include clinoptilolite, filipse, chabasite and mordenite [9–11]. Silva et al. [12] purified soil contaminated with polycyclic aromatic hydrocarbons using a Fenton reaction whose only iron was that occurring naturally in the soils studied (where the easily extractable iron content in soils 1 and 2 respectively were of 9.1 and 0.6 g/kg). In both cases, a high (> 90%) degree of removal of impurities was obtained, where process parameters were optimal. These studies confirmed the possibility of natural materials being used in the Fenton process, even with no modification necessary. Nevertheless, a desire to improve on physicochemical properties and increase catalytic activity has encouraged effort to modify natural materials or engage in the synthesis of new ones of specific composition. Zeolites can be variously modified, e.g. by co-precipitation, ion

Corresponding author. E-mail address: [email protected] (S. Ziembowicz).

https://doi.org/10.1016/j.jwpe.2019.100950 Received 3 July 2019; Received in revised form 3 September 2019; Accepted 8 September 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 32 (2019) 100950

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exchange, impregnation and/or encapsulation [3,4]. Further heterogeneous Fenton-process catalysts that have been made use of include: ion-exchange resins immobilised with Fe(III) ions [13], Raschig glass rings [14], active alumina coated with an Fe2O3 layer [15], granulated activated carbon impregnated with iron [16,17] and zeolites with Fe(III) ions or other transition metal ions immobilised on their surface [18]. The composites and artificially-produced/modified materials studied in turn include: Fe2+/AC, α-Fe2O3/S, CuO/Al, FeOOH-C, iron(III) on algae fibers, iron oxide/SiO2 and magnetitebased catalysts [19]. Described in this paper is a preliminary study on the use of natural and modified bottom sediments as heterogeneous catalysts in a Fentonlike process seeking to achieve effective degradation of DBP, a known endocrine-disrupting compound. The basis for this approach lay in the status of bottom sediments as natural sources of iron, manganese and copper, and in the assumption that ions of these metals might be released sufficiently to ensure reactions with hydrogen peroxide mirroring those involving the classical Fenton reagent. Di-n-butyl phthalate is the most frequently occurring pollutant in the natural environment, and because it is soluble in water and stable in environmental conditions, it is often detected in aquatic ecosystems, landfill leachates and wastewater, and even in drinking water. Typically, the amounts of DBP in surface waters are in the range of 0.013 to 122 μg/L, there is often confirmation in the literature of much higher phthalates in surface waters, e.g. the concentration of DEHP in Germany was recorded at 2700 μg/L, in Italy 4600 μg/L. In landfill leachates there are much larger amounts of this compound, up to 18 mg/L. On the other hand, poor solubility of phthalates in water results in adsorption on suspended solid particles and accumulation in bottom sediments (from 0.5 to 30 mg/kg dry weight) [20–22]. Due to the common occurrence of din-butyl phthalate in the aquatic environment and its negative impact on the environment and living organisms, research has been undertaken to remove this compound from aqueous solutions.

procedure then resembling that with ion exchange. Contents of Fe, Mn and Cu in the catalysts obtained were determined, as was the degree of leaching of metals and other components into an aqueous medium. 2.3. Physicochemical analyses of sediments and catalysts

Samples of bottom sediment were collected once from Rzeszów and Solina Reservoirs. These sites differ in that the first of the bodies of water is under strong anthropopressure associated with local agriculture that causes severe land erosion, with the result that bot the depositing of rubble and diffuse pollution take place [23]. Sampling made use of a gravity sediment corer (KC Kajak), and samples obtained were pre-cleaned and dried to an air-dry state. The bottom sediments were then ground in a mortar, sieved through a 1 mm sieve and dried to constant mass at 105 °C. Sediment pH, content of organic matter and CaCO3, and content of Fe, Mn and Cu were then all determined, prior to investigation of how substances leached from the sediment into aqueous solution.

To determine the pH of bottom sediments, 2 g of air-dried samples were weighed into centrifuge tubes and 25 mL of distilled water or 1 M KCl solution added, prior to shaking for 5 min on a shaker. Suspensions were then left for 2 h and centrifuged, before pH values were measured by immersing an electrode in the supernatant. The content of organic matter was determined directly, in terms of mass loss from sludge following roasting at high temperature. For this purpose, weighed (2 g) portions of pre-prepared bottom sediments were placed in crucibles transferred to a muffle furnace and roasted for 4 h at 550 °C. Roasted samples were then cooled in an exicator and weighed on an analytical balance. The percentage of organic matter (%OM) was calculated from the difference in mass between samples before and after ignition. Fe, Mn and Cu concentrations in collected bottom sediments and obtained catalysts were determined following mineralisation of the samples. The latter process used microwave energy in a multi-station Mars-6 mineraliser. For this purpose, 0.5 g portions of sediment were weighed out, before being transferred to Teflon vessels (X-press), to which 10 mL of concentrated HNO3 were added. Prior to being placed in the mineraliser, sediment samples were left for two hours at room temperature to ensure that material was pre-dissolved. After cooling, the solutions were filtered through a soft filter paper into volumetric flasks, and diluted with distilled water to a volume of 50 mL. The resulting solutions were then filtered through a 0.2 μm syringe filter, prior to determinations for concentrations of metals being carried out using an ICP-OES spectrometer. The leachability of substances (metals and ions) from the examined bottom sediments was investigated using PN-Z-15009 [24]. This entailed 1 g of sediment being placed in a conical flask along with 10 mL of distilled water. To obtain the required solution pH values (i.e. 3 or 7), H2SO4 or NaOH were dosed into the solutions. The flask was then sealed with a stopper and shaken for 60 min. Determinations for the metals were then carried out by filtering the contents of the first flask into a 50 mL flask, with 10 mL of HNO3 added to the solution and the distillate made up to the mark with distilled water. In aqueous extracts obtained in this wat, determinations of metal concentrations were again made using an ICP-OES spectrometer. To determine concentrations of selected ions in the extracts, a leaching test similar to that for the metals was carried out. Determinations of inorganic anions and cations (Cl−, Mg2+, Ca2+, K+, Na+) eluted from precipitates into solution were made using an ICS-5000 ion chromatograph from Dionex. Analysis was preceded by filtering of solutions through a 0.2 μm syringe filter and a membrane filter. The CaCO3 content was determined by Scheibler's method.

2.2. Methodology of modification of bottom sediments

2.4. Removal of DBP from water using a modified Fenton process

Ion-exchange and impregnation methods were applied to enrich natural bottom sediments further with Fe, Mn, Cu, i.e. to obtain heterogeneous catalysts containing various amounts of catalytically active substances, the. For this purpose, 500 mL of aqueous solutions containing specific concentrations of Fe, Mn, Cu ions prepared by dissolving salts of these metals in distilled water. 50 g portions of dried sediment were then added to the solutions and stirred on magnetic stirrers for 12 h. The suspension obtained was then filtered, before precipitates with repeat-washed with distilled water and then dried to constant mass at 105 °C for 12 h, to obtain portions of the catalyst ready for use. The impregnation method in turn involved mixing sediment with salt solution until the water had evaporated completely, with further

The setup for the removal of DBP using Fenton-type processes consisted of a 4-position mixer and reactors (beakers) of capacity 50–500 mL. These were filled periodically with a 5 mg/L DBP solution, with pH correction using a solution of H2SO4 or NaOH. The heterogeneous Fenton process used natural or modified bottom sediments, with appropriate amounts of catalyst introduced into the DBP solution and the whole stirred for 0.5 h. Further pH correction followed, with the H2O2 solution then introduced. After an appropriate reaction time, reaction solutions were neutralised to pH 8–9 using sodium hydroxide. 20 mL of solution samples were then taken and the residual concentration of DBP assessed. The effectiveness of this modified Fenton process in degrading DBP in model aqueous solutions was investigated more thoroughly, in

2. Experimental section 2.1. Bottom sediments collection and preliminary preparation

2

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relation to hydrogen peroxide dose, catalyst dose, initial solution pH value and concentration of the studied chemical compound, and reaction time. Also analysed were change in reaction solution parameters and the leaching of selected chemical compounds from the applied sediments and catalysts into the reaction mixture. 2.5. Determination of DBP using gas chromatography DBP isolation involved solid-phase extraction and determination using a gas chromatograph. SPE extraction procedure were as reported above [25]. Analysis of the DBP was performed using a GCeMS system coupling Trace GC ULTRA with TriPlus to an ITQ 1100 (Thermo Scientific). DBP elimination in the processes studied was evaluated by reference to changes in concentration. Quantification of DBP was performed using internal calibration. Benzyl benzoate served as the internal standard by which DBP was quantified. The linearity of the calibration curve has been tested over a range of DBP concentrations from 0 to 7.5 mg/L, with data obtained showing that, across this range, detector readings as a function of DBP concentrations remain linear (R2 coefficient > 0.99). Average levels of DBP recovery from distilled water are in the 97–109%. The limit of quantification (LOQ) was found at 53 μg/L and the limit of detection (LOD) at 20 μg/L.

Fig. 1. The effect of mixture components and time on DBP degradation (CDBP: 5 mg/L, pH: 3, sediment dose: 5 g/L, H2O2 dose: 100 mmol/L).

for the concentration of selected ions was carried out. Most likely, the change in pH is the result of alkaline compounds leaching (e.g. calcium and magnesium compounds) from the bottom sediments/catalysts into the reaction medium. The effectiveness of Fenton-process DBP degradation was then considered in relation to the bottom sediments used, from either Rzeszów Reservoir or Solina Reservoir. As differences between removal efficiency after 120 and 180 min of the Fenton heterogeneous process were found to be non-significant, sampling after 180 min was discontinued, and the reaction time extended to 420 min. The results obtained at this stage are as shown in Fig. 2. Collected data show that type of sediment had an impact on the effectiveness of the Fenton process, with fuller degradation of DBP obtained with sediments originating from Solina. 60 min into the reaction, the 60% yield was twice as high as that obtained using sediments from Rzeszów Reservoir. In turn, after 240 min, complete degradation of DBP has been achieved where the process used Solina sediments, while even after 7 h, the maximum rate of removal in the process using Rzeszów sediments was 83%. Such differences presumably relate to physicochemical properties of sediments (as detailed in Table 2). While Rzeszów Reservoir sediments are slightly alkaline (pH 7.88–7.95), those from Solina are near neutral (pH 6.54–6.67). However, the main differences between the two lies in the OM content, as well as those of calcium carbonate and different metals. Specifically, the Rzeszów sediments contain twice as much organic matter, ten times as much CaCO3 and levels of iron, copper and manganese of 16,884; 45.32 and 457.8 mg/kg DM respectively (22,332; 27.42 and 335.9 mg/ kg DM in Solina sediments). All the parameters described are capable of exerting a significant

3. Results and discussion 3.1. Use of natural bottom sediments Detailed study on the catalytic activity of bottom sediments in degrading DBP was preceded by preliminary experiments inter alia seeking to exclude sorption of the test substance on sediments and reaction vessels. As Table 1 shows, these were carried out on systems of differing composition. Processes were carried out for 240 min, with sampling for analysis every 60 min. The results obtained at this stage are shown in Fig. 1. Our work precluded the sorption of DBP on either glass or sediments, as well as showing that hydrogen peroxide alone could not remove the test compound completely, given an oxidation reaction yield not exceeding 30%. In contrast, the result of using hydrogen peroxide in the presence of bottom sediments extracted from Rzeszów Reservoir was a significant increase in DBP degradation efficiency (a 71% yield after 4 h of the process). The classic four-phase procedure for using a Fenton reagent entails acidification of the reaction medium, catalyst dosage, dosage of hydrogen peroxide and final neutralisation. However, the use of bottom sediments following prior acidification of the reaction environment is found to increase pH value (and hence reduce DBP removal efficiency) significantly. Our studies thus had sediments dosed first, with the solution only being acidified 30 min later, with H2O2 then dosed. During the processes it was also observed that the pH value of the solution changed, which systematically increased and after about 2 h of the Fenton process the reaction mixture was slightly acidic, and after 4 h in some cases close to neutral. This is an important advantage in terms of technology, since after the process it is no longer necessary to neutralize the reaction solution. In order to determine the possible cause of the pH value change, an analysis of selected reaction solutions Table 1 Reaction systems used to exclude DBP sorption on bottom sediments and reaction vessels. Number

Components of the reaction mixture

1 2 3 4

DBP DBP DBP DBP

solution solution + bottom sediments (Rzeszów Reservoir) solution + H2O2 solution + H2O2 + bottom sediments (Rzeszów Reservoir)

Fig. 2. The effect of natural bottom sediments in Fenton-process degradation of DBP (CDBP: 5 mg/L, pH: 3, sediment dose: 5 g/L, H2O2 dose: 100 mmol/L). 3

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Table 2 Selected physicochemical properties of dredged bottom sediments. Parameters pH [-] Organic matter [%] CaCO3 content [%] Metal content [mg/kg DM] Fe Cu Mn

H2O KCl

Rzeszów

Solina

7.95 7.88 6.56 3.22

6.54 6.76 3.18 0.32

16,884 45.32 458

22,332 27.42 336

impact on the process by which DBP is degraded. First, a higher content of organic compounds requires higher doses of reagents, and secondly, the presence of both organic and inorganic contaminants (e.g. carbonates) scavenges hydroxyl radicals responsible for oxidation. The strong effect of radical scavenging of ·OH by HCO3− and CO32- is shown in Eqs. (1.1) and (1.2). It is also worth mentioning the higher iron content in the Solina sediments, which could also account for more complete degradation of the tested pollutant. ·

OH + HCO3−→CO3−·+ H2O

·

CO3−→CO3−·+

OH +

Fig. 3. Effect on DBP degradation of the heterogeneous catalyst used, and of time (CDBP: 5 mg/L, pH: 3, sediments/catalysts dose: 5 g/L, H2O2 dose: 100 mmol/L). Table 4 Leaching of inorganic ions from sediments into solution.

(1.1)

−

OH

Parameters

(1.2)

Sediments/catalyst Natural sediments (Rzeszów Reservoir)

Fe5%

starting pH [-] 3 Parameters of the solution after shaking − Cl [mg/L] 0.59 1.32 Mg2+ [mg/L] Ca2+ [mg/L] 71.58 K+ [mg/L] 0.72 78.10 Na+ [mg/L]

3.2. Use of modified bottom sediments The stage of the research involving modification of the naturallyoccurring Rzeszów-Reservoir sediment (catalyst 1) revolved around the iron, manganese and copper contents. Eight catalysts (numbers 2–9) were obtained via ion exchange, a further 2 (numbers 10 and 11) by impregnation. The catalysts are presented in terms of their physicochemical properties in Table 3. Thus, the Fe1% catalyst was obtained by adding Fe2+ salt during modification so that the proportion of added iron ions reached 1% in relation to the weight of the modified sediments. An analogous situation applied to the Fe5% catalyst (Fe2+ contribution = 5%). In turn, to obtain the FeMnCu1% catalyst, salts of iron, copper and manganese were used, the proportion of each being 1% in relation to the entire sediment sample. The Fe5%i and FeMnCu5%i catalysts were obtained by way of impregnation, given that this method entails the filling of pores in the carrier with a solution of a metal salt at the correct concentration for specific surface coverage. In contrast, the essence of ion exchange is the exchange of ions between the support (sediments) and the ions present in the surrounding solution. The limitation of the ion exchange method is the possibility of obtaining catalysts of low metal content only [26]. Fenton-reaction effectiveness was then compared between sediments modified using the ion-exchange method and natural bottom

3 0.62 0.07 71.01 0.50 155

sediments dredged from Rzeszów Reservoir. Three replicates were run in the testing of each catalyst. The results are as presented in Fig. 3, in the form of the arithmetic mean for these replicates. The error bars are not shown due to the obscuration of the drawing. The process was seen to be associated with a pH change in solutions, in the sense of a steady increasing, such that after 2 h of the Fenton process, the reaction mixture is only slightly acidic, while after 4 h it is close to neutral in some cases. This is an important advantage from the technological point of view, as no longer necessitates neutralisation of the post-reaction solution. To determine the possible cause of pH change, post-reaction solutions were analysed for concentrations of different ions (see Table 4). It was found to be most likely that the change in reaction reflected a leaching of alkaline (for example calcium and magnesium) compounds, from sediments into the reaction medium. Evaluation of the impact of basic parameters on the efficiency of DBP removal via the heterogeneous Fenton process was then possible. The effect of pH on effectiveness was tested for first, by reference to 2

Table 3 Selected physicochemical properties of natural and modified bottom sediments (catalysts). Number

Catalyst

pH

Organic matter [%]

Fe [mg/kg DM]

Cu [mg/kg DM]

Mn [mg/kg DM]

1 2 3 4 5 6 7 8 9 10 11

Sediments Fe1% Cu1% Mn1% FeMnCu1% Fe5% Cu5% Mn5% FeMnCu5% Fe5%i FeMnCu5%i

7.92 7.20 6.81 6.73 6.70 6.58 6.57 6.81 5.29 3.07 4.91

6.56 7.05 6.25 5.49 7.46 7.22 6.02 6.20 8.53 9.5 11.46

16,884 20,648 12,176 12,468 20,708 32,664 13,236 13,684 31,680 49,680 35,328

45.32 28.92 1260 26.42 1188 30.56 6952 29.00 10184 29.15 29380

457 257 302 2868 690 147 270 4256 1049 457 1284

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Fig. 5. Effect of reaction time and iron content in sediments on DBP degradation (CDBP: 5 mg/L, sediment/catalyst dose: 5 g/L, H2O2 dose: 100 mmol/L, pH: 3).

Fig. 4. Effect of pH on DBP degradation (CDBP: 5 mg/L, sediments/catalysts dose: 5 g/L, H2O2 dose: 100 mmol/L, time: 60 min).

exemplary catalysts differing markedly in metal content (i.e. FeMnCu5% sediments versus natural bottom sediments from Rzeszów Reservoir). Fenton processes were then run in solutions of pH = 3, 5 or 7. The results are presented in Fig. 4. pH was found to have a clear effect on the course of the reaction, with the course of the process best favoured by acidic environment, whether sediments are natural or modified (FeMnCu5%). A higher yield of the reaction in an acidic environment reflects intensified leaching of metals from sediments to the medium as compared to in a neutral solution. To confirm the effect of pH, water extracts were prepared, with the elution liquid being deionised water at either pH = 3 or pH = 7. Table 5 presents the results of the Fe, Mn and Cu leaching tests for 1 g portions of sediment, with the weight ratio of sediment to water being 1:10. The results confirm much greater leaching in an acidic environment, hence easier access to the ions of these metals and a more effective Fenton reaction. Three catalysts differentiated in terms of iron content were then selected and compared These were natural sediment from Rzeszów Reservoir or else sediment modified into Fe1% and Fe5% catalysts. These sediments were characterised by respective levels of iron equal to 16,884, 20,648 and 32,664 mg/kg DM Results concerning the influence of iron content in sediments on the effectiveness of DBP removal from water are as shown in Fig. 5. An undisputed assumption regarding the Fenton reaction is that the higher the iron content, the greater the efficiency of removal of organic impurities. This dependence reflects the leaching of iron from the catalysts into the acidic solution. A straightforward relationship between a higher iron content in sediments and higher concentration in aqueous extract is demonstrable.

Fig. 6. Effect of H2O2 dose and catalyst on DBP degradation (CDBP: 5 mg/L, sediment/catalyst dose: 5 g/L, pH: 3, time: 60 min).

Differences in the efficiency of removal of DBP were most evident in the first 2 h of reaction, when the solution was still acidic (with a pH around 3). In the following hours, the differences in DBP oxidation efficiency in the processes used were negligible, due to the increase in pH noted in all reaction solutions. It was the effect on DBP removal of increasing the dose of hydrogen peroxide that was checked subsequently, with the H2O2 concentration tested over a 2-fold range from 100 to 200 mmol/L. The results are as shown in Fig. 6.

Table 5 Metals leaching from the sediment/catalyst into reaction media of differing pH (shaking time: 60 min). Sediments/catalysts

Amount of eluted element from 1 g of sediment to 10 ml of water [mg] Fe

Sediments (Rzeszów) Sediments (Solina) Fe1% Cu1% Mn1% FeMnCu1% Fe5% Cu5% Mn5% FeMnCu5% Fe5%i FeMnCu5%i

Cu

Mn

pH = 3

pH = 7

pH = 3

pH = 7

pH = 3

pH = 7

0.104 0.17 1.78 0.01 0.009 0.16 2.09 0.09 0.05 4.38 0.44 0.42

0.017 0.005 0.02 0.015 0.009 0.006 0.009 0.012 0.020 0.013 0.713 0.120

0.0004 0.002 0.13 4.27 0.003 5.55 0.056 5.56 0.007 6.42 0.03 5.51

0.002 0.003 0.0018 0.015 0.0009 0.004 0.0003 0.030 0.0012 0.464 0.002 3.82

0.213 0.016 0.129 0.146 0.405 0.465 0.040 0.068 2.82 0.795 0.112 4.99

0.0016 0.0013 0.0022 0.001 0.044 0.124 0.0006 0.0008 0.102 0.798 0.061 7.19

5

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Fig. 8. Effect of DBP concentration and of catalyst/sediment on DBP degradation (pH: 3, catalyst/sediments dose: 5 g/L, H2O2 dose: 100 mmol/L, time: 60 min).

Fig. 7. Effect of catalyst/sediments and catalyst/sediments dose on DBP degradation (CDBP: 5 mg/L, pH: 3, H2O2 dose: 100 mmol/L, time: 60 min).

Increasing the dose of oxidant increased DBP degradation slightly where natural sediments were used, while impairing the removal process by 20% in the presence of the FeMnCu5% catalyst. As the natural bottom sediments have lower contents of the transition metals Fe, Mn and Cu, an increased dose of H2O2 allowed for the formation of more of the hydroxyl radicals responsible for degrading DBP. While an increased dose of H2O2 up to a certain threshold is known to increase the rate of degradation of organic contaminants, via strengthened hydroxyl radical formation, a further increase in H2O2 beyond the threshold reduces reaction rate. This effect was to be observed in the process involving FeMnCu5%, and is explicable in terms of radical scavenging by excess H2O2, with numbers of hydroxyl radicals in solution actually reduced in this way [27]. At higher H2O2 concentrations, the formation of radicals is followed by their exhaustion thanks to a hydrogen peroxide scavenging mechanism that gives rise to hydroperoxyl radicals characterised by lower oxidation strength (reaction 1.3), with these radicals also able to trap hydroxyl in line with Eq. (1.4) [28]. H2O2 + ·OH→H2O + HOO· ·

·

HOO + OH→H2O+O2

concentration of impurities is an important parameter in wastewater treatment. The results are shown in Fig. 8. As Fig. 8 shows, a higher DBP concentration was associated with a level of degradation efficiency in relation to sediments from Rzeszów Reservoir that was lower by about 8%. However, in the case of the process using the FeMnCu catalyst, the efficiency was higher (by about 15%). One conclusion would hold that, the lower the initial DBP concentration, the shorter the reaction time needed to achieve complete degradation. Similar results were obtained by Xu et al. [30] in studies into the removal of dye (acid fuchsine), with natural kaolinite modified with iron ions used as a catalyst for the Fenton reaction. The effectiveness of this process in decolouring solutions with different (50, 100, 150 and 200 mg/L) concentrations as tested in these studies. The results make it clear that a higher pigment concentration is associated with lower efficiency, but the reduction in efficiency is not proportional to the increase in concentration. For example, for a 50 mg/L solution 100% degradation was achieved after 20 min of reaction, while for a 100 mg/L solution the efficiency after the same time was 90%. On the other hand, however, higher concentrations increase the number of DBP molecules per unit volume and increase the likelihood of reaction between DBP and oxidising molecules, leading to increased degradation efficiency. This effect was observed in the reaction where the FeMnCu5% catalyst was present. Compared after that were processes involving catalysts obtained by way of ion exchange or through impregnation. The reaction was allowed to continue for either 60 or 240 min. The results (as mean values for 3 repetitions) are as presented in Fig. 9. Greater efficiency of the catalysts obtained by impregnation is revealed. In the process with the

(1.3) (1.4)

The effect of the dose of bottom sediment (as catalyst) on the efficiency of oxidation of DBP in water was then checked for, with results as presented in Fig. 7, in the form of an average from the results obtained. The dose of bottom sediment was varied across the 2.5–10 g/L range, while remaining operating parameters were left unchanged. On the one hand, the degree of DBP degradation was greater where the sediment dose was 5, as opposed to 2.5 g/L. On the other hand, a quantity of sediment as great as 10 g/L was associated with lower removal efficiency – a phenomenon that may reflect an increase in the amount of metal ions involved in the process, with a resultant significant increase in the number of OH radicals, followed by their recombination with an increased capture effect [29]. Similar conclusions were drawn in research carried out by other scientists. For example, Idrissi et al. [28] used a clay modified by impregnation with iron ions in a Fenton process removing dye from water. A 75 mg dose of catalyst as compared with 25 mg was associated with a greater yield, whereas a quantity of catalyst equal to 100 mg was again associated with a lower rate of degradation of the test substance. Another example confirming that too high a dose of heterogeneous catalyst is not always associated with better reaction efficiency came with studies on the removal of dye (methyl orange) in a Fenton process using clay modified with copper ions. A dose of over 40 mg was not merely associated with no higher level of pollutant degradation, but was actually linked with a lower level of effectiveness. The effect of initial DBP concentration on the course of a heterogeneous Fenton process was also investigated, because the

Fig. 9. Effect of sediment modification method and time on DBP degradation (CDBP: 5 mg/L, pH: 3, catalyst dose: 5 g/L, H2O2 dose: 100 mmol/L). 6

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different contents of catalytically active substances. All tested catalysts showed catalytic activity in the context of the Fenton process. However, the most effective were those containing large amounts of iron. The effectiveness of the Fenton-like process in DBP degradation is shown to depend on many parameters, i.e. type and dose of sediment, pH, dose of H2O2, catalyst dose, DBP concentration and reaction time. A Fenton process using a natural heterogeneous catalyst based on bottom sediments containing iron, manganese and copper ions is an effective and environmentally friendly method of oxidising selected phthalic acid esters in aqueous solution. The use of bottom sediments in chemical reactions represents an alternative way of managing dredged bottom sediments containing high concentrations of heavy metals. The approach to the use of natural materials in catalytic processes is not only consistent with chemistry of a "sustainable" (i.e. ecological and environmentally friendly) nature, but also cost-effective, given that processes managed in this way are less expensive. The main limitation on the use of natural bottom sediments is the high content of organic matter and other impurities in the matrix.

Fig. 10. Effect of pH and catalyst on DBP degradation (CDBP: 5 mg/L, pH: 3, catalyst dose: 5 g/L, H2O2 dose: 100 mmol/L, time: 60 min).

FeMnCu5%i catalyst, almost complete removal of DBP was obtained after 60 min of reaction time. The higher reaction rate is due to the greater number of ions involved in the reaction with hydrogen peroxide, resulting in a higher number of hydroxyl radicals responsible for the reaction, i.e. the degradation of DBP. Due to the high efficiency of the Fe5%i and FeMnCu5% catalysts, it was decided to carry out processes with these catalysts without correcting the reaction mixture pH. A comparison of the results obtained in these processes as opposed to their counterparts run in acidic solutions is as shown in Fig. 10. The graph reveals how these catalysts prove highly active whether the solution is at pH = 3 or else at a pH of about 7. However, greater efficiency of degradation of DBP was obtained in the solution with a neutral reaction. Regardless of the catalyst used, 100% DBP removal was achieved 60 min into the reaction. A similar relationship was noted in the study by Idrissi and others [28]. The highest degree of dye degradation was obtained in a solution at pH = 7, as followed by pH = 9, and then 5. The lowest efficiency was achieved where the solution was at pH = 3. In this case, this distribution was due to the zero point of catalyst charge, which was about 8. When the pH was close to 8, the catalyst was charged negatively, while crystal violet (the removed substance) was positively charged, with therefore being a strong electrostatic attraction between the catalyst surface and the substance being removed. As a result, crystalline violet had easier contact with the active sites and was therefore degraded more easily. Where the pH of the solution was above 8, both the crystal violet and the catalyst were positively charged, encouraging repulsion between molecules. Crystalline violet had limited contact with active areas, therefore the effectiveness of wastewater decolouration was significantly more limited. No such tests were carried out in this study, so it is not possible to assess whether such an effect was also present in our case. Nevertheless, the comparison of the catalysts obtained by impregnation with natural sediments and those modified using the ion-exchange method reveals the higher metal content and lower pH of the impregnated versions, with this perhaps explaining the similar results obtained irrespective of the initial pH of the reaction mixture.

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4. Conclusions Bottom sediment-based catalysts could be prepared and used successfully in the role of heterogeneous catalyst in a Fenton-like system used in the degradation of di-n-butyl phthalate. Iron, manganese and copper ions deposited in the bottom sediments serve as the active catalytic substances in the Fenton–reaction context. The composition of bottom sediments makes it possible to use them in modification. By way of ion-exchange and impregnation methodologies, ten modified catalysts based on bottom sediments were developed, and found to have 7

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