Methodologies for the Analytical Determination of Ferrate(Vi) a Review

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Potassium Ferrate (Half dozen) as the Multifunctional Amanuensis in the Treatment of Landfill Leachate

¹

Chemiqua Water&Wastewater Visitor, Skawińska 25/1, 31-066 Kraków, Poland

ⁱⁱ

Institute of Chemistry, Academy of Silesia, Szkolna ix, 40-007 Katowice, Poland

³

Department of Water and Wastewater Engineering, Silesian University of Technology, Konarskiego 18, 44-100 Gliwice, Poland

⁴

Center for Translational Research and Molecular Biological science of Cancer, Maria Skłodowska-Curie Memorial Cancer Center and Institute of Oncology, Wybrzeże AK 15, 44-101 Gliwice, Poland

^v

Section of Analytical Chemistry, Kinesthesia of Natural Sciences, Comenius University, Ilkovicova half dozen, 84215 Bratislava, Slovakia

^*

Authors to whom correspondence should exist addressed.

Received: v October 2020 / Revised: 23 Oct 2020 / Accepted: 4 November 2020 / Published: vi November 2020

Abstruse

Possible apply of potassium ferrate (Six) (K₂FeO₄) for the handling of landfill leachate (pH = 8.9, Chemic Oxygen Demand (COD) 770 mg O_ii/L, Total Organic Carbon (TOC) 230 mg/L, Total Nitrogen (Total Due north) 120 mg/L, Total Phosphorus (Total P) 12 mg/L, Total Coli Count (TCC) 6.8 log CFU/mL (Colony-Forming Unit/mL), Most Probable Number (MPN) of fecal enterococci 4.0 log/100 mL, Total Proteolytic Count (TPC) 4.iv log CFU/mL) to remove COD was investigated. Central Composite Pattern (CCD) and Response Surface Methodology (RSM) were applied for modelling and optimizing the purification process. Conformity of experimental and predicted information (R ² = 0.8477, R _adj ² = 0.7462) were verified using Analysis of Variance (ANOVA). Application of Thou_iiFeO₄ using CCD/RSM allowed to decrease COD, TOC, Full North, Total P, TCC, MPN of fecal enterococci and TPC by 76.2%, 82.half dozen%, 68.iii%, 91.6%, 99.0%, 95.8% and 99.3%, respectively, by using K₂FeO₄ 0.390 1000/L, at pH = 2.3 within 25 min. Application of equivalent amount of iron (as FeSO₄ × 7H_iiO and FeCl₃ × 6H₂O) under the aforementioned conditions allowed to diminish COD, TOC, Full N, Total P, TCC, MPN of fecal enterococci and TPC merely by 38.1%, 37.0%, 20.eight%, 95.8%, 94.4%, 58.ii%, xc.8% and 41.6%, 45.seven%, 29.ii%, 95.8%, 92.1%, 58.2%, ninety.0%, respectively. Thus, Grand_twoFeO_four could be applied as an environmentally friendly reagent for landfill leachate treatment.

i. Introduction

Several aspects of human agricultural and industrial activeness are related to adverse changes in the quality of water resources worldwide. Undoubtedly, such activeness has a direct impact on the waste production. In fact, some waste matter undergoes various disposal processes while other is deposited in municipal landfills. As a result of the municipal waste landfills exploitation, leachate is generated. In case of insufficient protection, it may get into the soil or groundwater and, due to its physicochemical and microbiological limerick, this may significantly contribute to the groundwater contamination. The amount of leachate and their characteristics depend on a number of factors, including: type of waste, caste of fragmentation, compaction and storage method, landform, corporeality of precipitation, method of sealing the bottom of the landfill, type of vegetation covering the landfill, soil weather, etc. [1]. It is widely known that the values of parameters (e.yard., COD, TOC etc.) for the landfill leachate were higher in the dry season than in the rainy flavour for the fresh leachate samples. Recent enquiry points to a higher content of heavy metals in the suspensions. Moreover, in that location were no significant seasonal changes in the concentration of heavy metal ions in suspended solids and sediment samples [2].

However, depending on the age of the landfill (young < 5 years; medium v–x years; erstwhile > 10 years), the physicochemical composition of the leachate may vary [3,iv]. According to the available data [iv,five,6,7,8,9,10,11,12], the pH-value of the leachate varies in the range of 4–7.half-dozen, 6.9–9 and 8.1–9.5 for the leachate from immature, medium and erstwhile landfills, respectively. In the case of COD and TOC, a decrease in the values of these indicators was observed co-ordinate to the age of the landfills, and for COD they amounted to approx. ane.87–84.30, 0.56–9.50 and 0.ten–3.46 g O₂/L [4,13,14,15,xvi], and for TOC approx. 1.60–13.61 m/L, 0.19–2.05 g/L and 0.04–1.90 g/L [sixteen,17,18,nineteen,20,21,22,23]. Similar dependencies were observed in the case of full nitrogen (approx. 1.75–4.37, 0.35–iii.00 and 0.42–ii.64 k/L) and full phosphorus (approx. ii–655, 3–18 and 1–seven mg/L) [18,24,25,26,27,28,29,30,31]. Municipal landfills effluents may besides contain pocket-sized amounts of heavy metals such as: Cd (<0.02–six.five mg/L), Cu (0.005–6 mg/50), Lead (0.01–3.l mg/L), and even Cr⁶⁺ (0.04–viii.4 mg/L) [4,31,32,33,34].

Due to the fact that the leachate from municipal landfills comprises a significant amount of organic compounds and shows meaning physicochemical parameters variation depending on their age, they create many technical and technological problems during their handling. For this reason, various physicochemical and biological methods are used to treat the leachate. The landfill leachate could exist treated with: ozone after coagulation treatment [vii], ozonation [21], hydrodynamic cavitation [11], catalytic oxidation (by using Ni/Al₂O_iii as the catalyst) by supercritical water oxidation [12], coagulation-flocculation, chemic coagulation and reverse osmosis organisation [13,30], hybrid coagulation-nanofiltration procedure [20], electrocoagulation [15], electrochemical oxidation [17], photoelectrochemical treatment in a continuous period reactor [16], coagulation and Fenton reagent, UV, H₂O_ii and UV/H₂O₂ process [35,36,37].

The presented physicochemical methods crave the utilize of physical factors (e.g., pressure, electric current etc.), diverse chemical compounds (coagulants, catalysts, etc.) devices and conditions in order to obtain high purification efficiency. Not all of these methods have found practical application due to, sophisticated technical solutions or complicated cleaning procedures implemented.

The results reported in literature indicate that the treatment of leachate requires biological procedures with activated sludge [38,39] and even phytoremediation methods [31]. Biological methods are characterized by varying effectiveness, which is related to higher biodegradability of leachate from young landfills compared to the old ones (reduction in the BOD_v/COD ratio where BOD₅ is v-day Biochemical Oxygen Need) [twoscore]. Therefore, landfill leachate introduced into biological wastewater treatment plants may have a negative influence on the microorganisms of the activated sludge and reduce the treatment efficiency. In particular, some chemical compounds present in landfill leachate, such as chlorobenzene, dichlorobenzene, chlorophenols, chloroaniline, toluene, ethylbenzene, xylene, phthalates and polycyclic effluvious hydrocarbons (PAHs) [twoscore,41] have a negative influence on the activeness of activated sludge. For this reason, directly application of biological methods for leachate treatment before prior implementation of physicochemical methods is not always possible. Biological methods seem to be the near environmentally friendly, unfortunately, may evidence variable effectiveness due to changing concentrations of pollutants in the treated leachates.

Currently, more and more attention is being paid to such methods of leachate treatment which are environmentally friendly and do not cause additional negative effects. This concept is ideally suited to the method using potassium ferrate (VI) (K₂FeO₄), which is an eco-friendly powerful oxidant with a dual mechanism of action. On the one hand, it acts in the oxidation of organic and partially inorganic pollutants (simultaneous reduction of Iron⁶⁺ to Fe^three+), and in the coagulation of pollutants or oxidation products and their adsorption on the flocs of hydrated Atomic number 26(OH)₃. Due to the fact that the decomposition products of K₂FeO_four are atomic number 26 oxides and oxygen, it is defined every bit a light-green oxidant and can exist a promising alternative to the conventional coagulants [42]. Among others, potassium ferrate (Half dozen) has been engaged for degradation of endocrine-disrupting compounds (EDCs), decomposition of surfactants (SPCs), personal intendance products (PCPs), pharmaceuticals [43], and also for oxidation of cyanides (CN⁻) originating from the mining and processing of gilt ore, degradation of natural organic matter (NOM), oxidation of North,N–diethyl–three–toluamide (DEET), many dyes (Methylene Blue, Orangish Ii, Vivid Cherry X-3B, Acid Green 16), removal of algae [44] and for wastewater handling [45].

The master objective of the presented study was to assess the possibility of using M₂FeO₄ for the treatment of leachate from a municipal waste landfill and to select the most favorable conditions (pH, K_iiFeO_iv conc., reaction time) for the handling of leachate ensuring the maximum reduction of the COD value. Comparative studies were also carried out with the use of conventional coagulants (FeSO₄ × 7H₂O, FeCl₃ × 6H₂O) containing an equivalent corporeality of iron (in relation to the amount independent in the near favorable dose of K₂FeO_iv) and the effect of the iron salts used on the concentration of Total Coli Count (TCC), Most Probable Number of fecal enterococci (MPN) and Total Proteolytic Count (TPC) in the treated leachate.

two. Materials and Methods

2.1. Chemicals

Envifer^® (Nano Atomic number 26, Zidlochovice, Czech republic) was engaged every bit the Yard₂FeO₄ source. Due to its chemical instability the content of M₂FeO₄ in Envifer^® was specified direct before the procedures provided in the Analytical Procedures section. Envifer^® was entirely characterized (UV-VIS spectrum, free energy-dispersive X-ray spectroscopy (EDXS) analysis, scanning electron microscopy (SEM) analysis) previously [46]. To adjust the leachate sample pH, 5% and 20% solutions of H_twoSO₄ (Avantor^TM, Gliwice, Poland) was used. Solid FeSO₄ × 7H₂O and FeCl_iii × 6H₂O (Chempur, Piekary Śląskie, Poland) were applied as conventional coagulants. For sludge flocculation a 0.05% solution of anionic flocculant Furoflock CW277 (Chemische Fabrik Wocklum Gebr. Hertin GmbH&Co. KG, Balve, Germany) was employed. All chemicals (except K₂FeO₄) were of analytical grade. Additionally, deionized h2o (<2 µS/cm) was used for preparation and dilution of the solutions.

two.2. Origin and Physicochemical Parameters of the Landfill Leachate

Leachate from the quondam (>10 years) municipal waste landfill located in southern Poland was investigated. The effluents were collected in summer during the rainy season (air temperature 25 ± i °C, precipitation viii mm of water column) from the effluent reservoir, where it flowed through a system of drainage pipes. Fresh leachate (arrival) was collected inside 24 h, every 60 minutes, into sterile i 50 bottles, which were stored at the temperature of four ± 1 °C without fixing the leachate earlier farther investigation. The average sample of the leachate used for the test was obtained by mixing 1 L of unit samples in a sterile 25 L canister. The raw landfill leachate was analyzed as described in the Belittling Procedures department.

two.3. Appliance and Experiment Conditions

All experiments were conducted at a constant temperature (19 ± 1 °C), in beakers containing 250 ± ane mL of tested leachates. The samples were mixed using a magnetic stirrer (MS11, Wigo, Pruszków, Poland) at a constant speed of 250 rpm at the oxidation/coagulation stage and fifty rpm at the flocculation stage. The experiments with M_twoFeO₄ were carried out in such a manner that K₂FeO_iv was added to the measured volume of wastewater, the pH was corrected with 20% H₂SO₄ and the reaction was carried out for the causeless time span. The quantity of K_iiFeO₄, pH and reaction time were prepare as predetermined at the stage of planning the experiments. After the oxidation/coagulation procedure was completed, the pH was adjusted to 9.0 ± 0.one in each experiment using 20% NaOH in order to precipitate the Fe³⁺ ions as Fe(OH)_three. Subsequently, 0.25 mL of 0.05% Furoflock CW277 solution (anionic flocculant) was added, and the stirring speed was decreased to 50 rpm. Later on thirty sec, stirring was halted in society to sediment the formed precipitate. A sample of the liquid above the precipitate was nerveless and filtered using a 0.45 µm PTFE syringe filter before COD decision. The filtrate was analysed co-ordinate to the process provided in the Analytical Procedures department. Nether the virtually favorable conditions of reducing the COD value (pH, G₂FeO₄ conc., reaction time), a verification experiment was carried out using sterile glass and laboratory equipment. In this instance, a sample of treated leachate above the sediment was collected without filtering it through a 0.45 µm PTFE syringe filter and microbiological tests were performed. Under the same weather (pH, reaction time), comparative tests were performed using an equivalent iron dose (G_twoFeO₄ vs. FeSO₄ × 7H₂O and FeCl₃ × 6H₂O). In each case the treated leachate was analyzed equally described in the Belittling Procedures section.

two.4. Analytical Procedures

Before performing the tests the chromite titration method was engaged to specify the content of K_twoFeO₄ in Envifer^®. The in a higher place method is composed of oxidizing Cr(OH)₄ ⁻ ions using FeO₄ ²⁻ under extremely alkaline conditions, which results in the generation of Iron(OH)₃, CrO₄ ²⁻, OH⁻. The G₂FeO_iv content in technical grade production (%) was calculated using the post-obit formula:

$$\%of{K}_{\mathrm{two}}Fe{O}_4=\frac{c_{F\mathrm{eastward}\left(II\right)}\times V_{Fe\left(II\right)}\times K_{K_{\mathrm{ii}}Fe{O}_4}\times 100\%}{3000\times m_{sample}}$$

(1)

where

$c_{Fe\left(II\right)}$

and

${\mathrm{Five}}_{Fe\left(II\right)}$

are the concentration (0.0850 mol/L) and the volume (mL) of the standard Mohr'southward common salt (ammonium fe(2) sulphate, (NH_iv)₂Fe(Then₄)₂) solution,

$M_{K_{2}Fe{O}_{\mathrm{iv}}}$

is 198.04 g/mol, and

$m_{sampl\mathrm{eastward}}$

represents the sample weight (g) [47]. The determination of the K₂FeO_iv content in Envifer^® was also carried out spectrophotometrically (Cary^® 50 UV-VIS, Varian Inc., Melbourne, Australia) [48]. In this case, an Envifer^® sample (with accuracy ± 0.001 1000) was dissolved in deionized water, and the book was adapted to 100 mL in a volumetric flask. Subsequently, the sample was filtered (0.45 µm) into a quartz cuvette (light path = 10 mm) and the absorbance values at λ = 505 nm was measured instantaneously. The K₂FeO₄ content in Envifer^® (%) was specified according to the following formula:

$$\%of{K}_{\mathrm{two}}F\mathrm{eastward}{O}_4=\frac{A\times 0.1\times M_{K_{\mathrm{ii}}F\mathrm{east}{O}_{\mathrm{four}}}\times 100\%}{1070\times {\mathrm{one\; thousand}}_{samp\mathrm{fifty}e}}$$

(2)

where A is the absorbance at 505 nm,

$M_{{\mathrm{Thou}}_{\mathrm{two}}Fe{O}_4}$

is 198.04 grand/mol, 1070 is the tooth absorbance coefficient, G^−one cm⁻¹, and

${\mathrm{chiliad}}_{\mathrm{south}a\mathrm{chiliad}ple}$

represents the sample weight (one thousand).

The pH-values and temperature were measured using an Inolab^® pH/Ion/Cond/Temp 750 chiliad and SenTix^® 81 electrodes (WTW, Weilheim in Oberbayern, Germany) [49]. The landfill leachate COD values were evaluated employing a dichromate method and the PF-eleven spectrophotometer [l]. TOC was assayed using the tube test kit Nanocolor^® TOC 60, while the end-point was specified using the PF-eleven spectrophotometer. TOC assessment was conducted in two stages. In the offset 1, inorganic carbon was eradicated from the samples by calculation NaHSO_iv and stirring the sample (500 rpm, 10 min). In the second stage, organic compounds were degraded by awarding of Na₂S₂O_eight at 120 °C for 120 min, and thymol blue absorbance variations of sodium table salt solution were measured spectrophotometrically at λ = 585 nm [51]. Determination of Total Nitrogen (TN) was performed by two-stride spectrophotometric method using Nanocolor^® Total Nitrogen 220 test tube kit (Macherey-Nagel, Düren, Germany). In the first stage, the wastewater sample was mineralized (Na₂S_twoO₈, H₂So₄, 120 °C, 30 min), and in the second stage, spectrophotometric decision of nitrogen compounds after their reaction with 2,six–dimethylphenol (DMP, also usually known equally 2,6–xylenol), in a mixture of H₂So₄ and H₃PO₄ were carried out [52]. Determination of Total Phosphorus (TP) was performed after effluent sample mineralization (Na₂S_twoO₈, H_twoSo_four, 120 °C, thirty min) by using a test tube kit Nanocolor^® ortho– and full Phosphate fifteen, with spectrophotometric endpoint detection using a PF-eleven apparatus (Macherey-Nagel, Düren, Germany) [53]. The dilution of leachate samples before microbiological enumeration was performed according to ISO 6887-i:2017 [54]. The enumeration of Full Coli Count (TCC, CFU/mL), Total Proteolytic Count (TPC, CFU/mL) and the Nearly Probable Number of faecal enterococci (MPN/100 mL) were determined co-ordinate to ISO 4832:2006 [55], PN-75/C-04615/17:1975 [56] and PN-C-04615-25:2008 [57], respectively. For the precipitation of gelatin in the Frazier's medium, Frazier'southward reagent (the mixture of HgCl₂, HCl and H₂O) was used.

two.5. Response Surface Methodology

Central Composite Design (CCD) and Response Surface Methodology (RSM) were engaged to specify the most favorable atmospheric condition for lowering the COD landfill leachate value. The optimization of the COD removal procedure consisted of determining the numerical values of three independent variables (pH, K₂FeO₄ dose and reaction time) for which the value of the dependent parameter (COD) was the everyman. Based on the literature data on the implementation of Grand₂FeO_four for the treatment of wastewater from various sources and taking into account the value of the redox potential for the FeO_four ²⁻ ion (East° = +two.20 V in acidic and East° = +0.72 V in neutral media) and own experience, several preliminary experiments were carried out. The results of these experiments made it possible to judge the pH, K₂FeO₄ and reaction fourth dimension adopted at the stage of planning experiments with the use of CCD. Therefore, the following values of input parameters were investigated: pH in the range 2–six, M₂FeO₄ dose 0.2–0.4 one thousand/50 and reaction time x–20 min. The values of the remaining variables, including i.e., temperature, stirring speed, and volume of the treated wastewater sample were set as abiding in each experiment, respectively. Table 1 reports the set-up of the sixteen experiments designated by using CCD.

The obtained empirical findings (the arithmetics hateful of 3 runs was adopted) were investigated statistically; and the bear on of the independent (input) variables (pH, concentration of K₂FeO_iv (g/Fifty), and reaction fourth dimension (min)) on the dependent (output) parameter (COD, 1000 O_two/L) was illustrated every bit a response surface graph. For the most favorable values of the iii input parameters, an experimental verification of the model was carried out (additionally, the COD changes later on 25 min, 30 min, 35 min and 40 min reaction time were investigated).

iii. Results and Discussion

iii.one. Physicochemical Parameters of the Landfill Leachate and K₂FeO_four

The physicochemical analysis of technical grade potassium ferrate (Six) showed that it independent 40% of pure Chiliad₂FeO₄. Additionally, previous research revealed that it was composed of 47.31% ± 1.50% Yard, fifteen.00% ± 0.45% Iron, and 37.69% ± 5.20% O, along with impurities such as Yard₂O and ferrous compounds other than K₂FeO_four (i.east., K_threeFeO_iv and KFeO₂). Information technology was probably related to the method used at the stage of its synthesis [46].

Table 2 presents chosen physicochemical and microbiological variables of the landfill leachate.

Initiatory specification of the chosen physicochemical and microbiological parameters of the landfill leachate unveiled that they were slightly alkali metal (pH = 8.9) and contained a certain amount of organic compounds expressed as chemical oxygen demand and full organic carbon (COD 770 mg O_ii/L and TOC 230 mg/L, respectively). Additionally, the content of organic (and probably inorganic) nitrogen compounds in the tested leachates was specified by the content of full nitrogen and phosphorus (TN 120 mg/L and TP 12 mg/Fifty, respectively).

On the other hand, the conducted microbiological tests showed significant contamination of the investigated leachates with coliforms, fecal bacteria and proteolytic bacteria (TCC half dozen.eight log CFU/mL, MPN iv.0 log/100 mL and 4.iv log CFU/mL, respectively). The obtained test results are comparable with the previous findings, especially for leachate from quondam landfills, for which a decrease in the value of pollution indicators was observed. Generally, in the instance of pH-value values were 4–9.five [4,five,vi,seven,8,9,10,eleven,12], for COD 100–84 300 mg O₂/L [4,13,xiv,xv,sixteen] and for TOC 40–13 610 mg/L. In addition, for TN and TP, the values were 350–4.370 mg/L and 1–655 mg/Fifty, respectively [xviii,24,25,26,27,28,29,thirty,31]. The parameter values presented in Table 2 betoken that the tested leachate did come from the old landfill and was collected during the rainy flavour, every bit presented in the section concerning origin and physicochemical parameters of the landfill leachate. Other studies indicated the presence of pathogenic bacteria, not but in the leachate, merely also in groundwater as a outcome of leachate infiltration into the ground (coliform bacteria, Escherichia coli, Enterococci, Pseudomonas aeruginosa). In groundwater, high concentrations of coliform leaner (20,000 CFU/100 mL), Escherichia coli (15,199 CFU/100 mL) and Enterococci (3290 CFU/100 mL) were specified [58]. The conducted studies of leachate revealed that in the consequence of their uncontrolled release, they may have a negative bear upon on the natural environment.

3.ii. CCD/RSM Findings

The employment of CCD and RSM in investigation planning enabled 16 experiments to exist performed (see Table 2). The findings of COD values (g O₂/L) linked to each experiment are reported in Table 2 (come across column 5). The lowest COD values (<0.25 chiliad O₂/L) were recorded in experiments 9, 12, and xiv (0.245, 0.205, 0.240 g O_two/50), respectively. In the experiment number 12, the highest dose of K₂FeO₄ (0.468 yard/Fifty) was used at pH iii.five during 15 min, and the lowest COD value was obtained for the purified effluents (0.205 g O₂/L). This indicates a significant influence of the M₂FeO₄ dose on the COD value of the effluents, along with other parameters (pH-value and reaction time).

Table iii presents the evaluation of the parameters and their influence of the COD of the landfill leachate.

The constant value, pH (L), 1000₂FeO₄ (L) and One thousand₂FeO₄ (Q), concentration were specified to be statistically valid (p < 0.05), while the pH (Q), Time (L) and Time (Q) were not statistically pregnant (p > 0.05). Moreover, the values of the calculated determination coefficient R^two and the adjusted determination coefficient R² _adj (0.8477 vs. 0.7462) depicted the ratio of the variance in the dependent variable (COD) that was foreseen based on the contained variables (pH -value, K_iiFeO₄ conc. and fourth dimension).

In the example of the existent sewage from the cloth manufacture, R² and R² _adj reached values of 0.8799 and 0.7999 [45]. In the case of using K_twoFeO₄ for the treatment of wastewater from the tanning industry other studies accept reported R^two and R² _adj values of 0.77 and 0.59 [59] versus 0.95 and 0.74 in the case of employing Grand₂FeO₄ for the treatment of constructed sewage containing azo dye Anilan Bluish GRL 250% [60]. A good fit between the empirical and approximated information was observed in the latter study.

Table 4 reports the effect of verifying the adequacy of the model coefficients using ANOVA, which confirmed the statistical significance (p < 0.05) of the main input parameters i.e., pH (L), One thousand_iiFeO₄ (L) and Chiliad₂FeO₄ (Q). These findings are also presented graphically in a class of bar chart (come across Figure one).

The estimators of the standardized effects were prioritized according to their accented value; the vertical line pinpoints the minimum absolute value for statistical significance. In the investigated wastewater samples, Yard₂FeO₄ (L), Thousand₂FeO_four (Q), and pH (L), revealed the largest impact on decreasing the COD value under the empirical conditions. The other parameters i.due east., pH (Q), Time (L), fourth dimension (Q) exerted the smallest impact on the COD value. Effigy 2 presents the relationship between the predicted COD value and observed COD value.

The data presented a linear correlation between the empirical and approximated data in the range of verified COD values. Figure three illustrates the response surface plots for COD with respect to K_twoFeO_four conc. and pH, Time and pH and Time and to K₂FeO₄ conc. (see Figure 3A–C).

The CCD/RSM study showed (see Effigy 3A) that the lowest COD value (<0.225 g O_two/L) was obtained for 1000₂FeO_four approx. 0.34–0.43 g/Fifty and a pH between ane.four–iii.3 with the time parameter set at 15 min. It can be seen in Figure 3B that for a constant dose of One thousand₂FeO₄ 0.300 chiliad/Fifty the lowest COD values (<0.225 g O_two/50) were specified for pH approx. one.7–2.nine in more than 25 min. In turn, the data presented in Figure 3C indicate that the adoption of a constant pH value of iii.five allows to obtain the everyman COD values of purified effluents (<0.200 chiliad/50) for K₂FeO₄ conc. approx. 0.35–0.43 g/Fifty over a time greater than 25 min. The presented results of model tests show that the lowest COD values for purified leachates were determined for the highest doses of Thou₂FeO₄ used in the experiments, in the acidic environment (1.7 < pH < 3.3) for more than 25 min.

The generated examination results stand for to the literature data, which point that the value of the redox potential for the FeO₄ ²⁻ ion is greater in an acidic surround than in a neutral environment (East° = +ii.20 V in acidic and E° = +0.72 5 in neutral media). More often than not, greater efficiency of the oxidation of organic compounds can be observed, while conducting the oxidation procedure in an acidic surroundings, rather than in a neutral one [43,44]. Some other study indicated that utilise of K₂FeO_four for the purification of highly polluted tannery wastewater from leather dyeing processes resulted in the discoloration (98.four% removal), chemic oxygen demand (77.two% removal), total organic carbon (75.vii% removal), and suspended solids (96.nine% removal); the reported values were the smallest when 1.200 g/L K_iiFeO_iv at pH three within ix min was used [59]. On the other mitt, the awarding of K_iiFeO_four for the degradation of trichloroacetic acid and turbidity removal in synthetic water revealed that the highest efficiency achieved for trichloroacetic acid was 24%, while for turbidity the maximum removal efficiency was in the range of 85%–95%. Additionally, the optimum atmospheric condition for initial turbidity, pH, and ferrate (6) dosage were 8.89 NTU, iii, and 4.26 mg/L equally Fe, respectively [61]. Other study indicates that the leachate treatment is besides possible in an element of group i status. In this example the pH value was 10, the dosage of Grand₂FeO_iv was vi yard/L and the reaction time was 30 min. Unfortunately, the experiment required an boosted use of a stabilizer at a dose iv g/L (sodium silicate, Na_iiSiO₃). Under those conditions, the COD removal efficiency was only 36% [62] compared to 76.ii% in this written report. An application of Yard₂FeO_four in the leachate treatment at the college temperature (thirty °C) past the initial ferrate (VI) to COD mass concentration ratio of 0.fifty, pH 4.00 and reaction fourth dimension 40 min was suggested every bit well. Information technology was stated that the leachate from hazardous waste landfill which was pretreated by K₂FeO₄ could be directly discharged into the biological handling system. However, COD value of leachates from the refuse incineration plant which was pretreated by K₂FeO_iv was as much every bit 2861 mg O₂/L. These leachates required re-handling before the introduction into the subsequent biochemical handling organization [63].

Moreover, information technology should be taken into account that the total efficiency of removing organic (and partially inorganic) compounds expressed as COD, TOC, TN and TP results not but from their oxidation by Fe^+vi, only also to some extent from their adsorption on freshly precipitated Fe(OH)_three flocs with a large active surface. In the case of phosphorus compounds (present equally PO₄ ⁱⁱⁱ⁻), it is possible to remove them past co-atmospheric precipitation with Fe³⁺ ions, which results in the formation of hardly soluble ferric phosphate. To sum up, it should exist stated that nether the experimental conditions, the total efficiency of removing contaminants expressed every bit COD resulted from their oxidation and coagulation and, probably, to some extent from adsorption and co-precipitation. Table v presents the calculated coefficients of the fitted model.

Consequently, the changes in the COD value can exist calculated according to the following formula:

COD (g O₂/L) = 1.206468−0.059897(pH) + 0.012988(pH)² − 4.423640(K₂FeO_iv) + 5.750741 (K₂FeO₄)² − 0.006018(Time) + 0.000073(Time)²

(3)

For the about favorable values of the iii input parameters (pH = 2.31, Yard₂FeO_four 0.38 g/L and Time 41 min) calculated from the model, the estimated COD value was 162 mg O₂/50. In the conducted verification experiment, the COD value was 178 mg O₂/L. Bold that the uncertainty of COD determination is ± xv%, the actual COD value of sewage treated under the most favorable conditions is in the range from 151 to 205 mg O_ii/L (180 ± 27 mg O₂/L), which also includes the estimated value from model for the most favorable pH values, Chiliad₂FeO₄ and time.

For a constant pH value three.v (see Figure 3B) the lowest COD values (<200 mg O₂/L) were obtained after 25 min of reaction time, therefore an additional verification experiment was carried out for the pH value and 1000₂FeO₄ concentration estimated from the model for the most favorable conditions (i.eastward., 2.31 thousand/Fifty and 0.38 g/L, respectively), and the COD was determined after 25 min, xxx min, 35 min and 40 min reaction fourth dimension. The subsequent COD values of the treated effluents were 180 ± 27 mg O₂/L, 172 ± 26 mg O_ii/L, 170 ± 26 mg O₂/50, 168 ± 25 mg O₂/L, respectively. Considering the uncertainty of the COD determination (± 15%), it was institute that the COD of the treated leachate was non significantly reduced. Therefore, the most favorable values for the independent parameters i.e., pH = two.3 ± 0.one, K₂FeO_four 0.390 ± 0.001 g/Fifty and Time 25 ± 1 min were adopted. Under these conditions, a reduction in TOC, TN and TP was also observed (82.half dozen%, 68.3%, 91.half dozen%, respectively) every bit shown in Table half-dozen (column 3).

3.3. Coagulation/Flocculation Findings and Grand_twoFeO_four Biocidal Properties

Table 6 reports the findings of tests of purified leachates after the awarding of Thousand₂FeO_iv (under optimal conditions) and FeSO_iv × 7H₂O and FeCl₃ × 6H_iiO in an amount equivalent to the dose of iron independent in 0.390 k of K_iiFeO₄.

The test results revealed that the use of an equivalent dose of fe in the form of FeSO_iv × 7H_twoO and FeCl_three × 6H₂O fabricated information technology possible to reduce the COD, TOC, TN values only past 38.i%, 37.0%, 20.8% (in the example of FeSO₄ × 7H₂O) and 41.half-dozen%, 45.7%, and 29.2% (in the case of (FeCl₃ × 6H₂O), compared to K₂FeO_iv, the application of which was much more than effective (see Table 6, column 3). Additionally, in all cases a reduction of the TP value > 90% was achieved. It is articulate that the removal of impurities from the tested leachates was not only due to coagulation, co-atmospheric precipitation and adsorption (as in the instance with conventional coagulants), but likewise as a result of oxidation procedure using M₂FeO₄. Additionally, information technology should exist noted that in the example of using conventional coagulants, the efficiency of removing microorganisms was comparable and amounted to 92.1%, 58.2%, 90.0% (in the case FeCl_three × 6H₂O for TCC, MPN, TPC) and 94.four%, 58.2%, xc.8% (in the case FeSO_four × 7H_iiO for TCC, MPN, TPC), respectively. When M₂FeO₄ was used the microorganism removal efficiency was 99.nine%, 95.eight% and 99.3% for TCC, MPN and TPC, respectively.

The obtained results bespeak an boosted biocidal effect related to the oxidizing backdrop of the FeO₄ ²⁻ ion in an acidic environs. Other studies revealed that K_iiFeO_iv can accomplish the disinfection targets (>6 log inactivation of Escherichia coli) at a very low dose (6 mg/L every bit Fe) and over wide working pH range compared to chlorination (10 mg/L as Cl_two) and coagulation (Fe_ii(SO_four)₃ iii.iv mg/L as Fe). In wastewater treatment, K_twoFeO_iv impale iii log more bacteria in comparison with Al₂(SO_four)₃ and Fe₂(And then_iv)_three at a similar or even smaller dose [64]. Reports have shown that ferrate (Vi) has excellent disinfectant properties and can inactivate a wide variety of microorganisms at low ferrate (Vi) dosages. Additionally, ferrate (Half-dozen) can disable many chlorine-resistant organisms, such as aerobic spore-formers and sulphite-reducing Clostridia. The ferrate (Half dozen) can deactivate not only Escherichia coli at lower dosages or shorter contact time than hypochlorite, but also Bacillus cereus, Streptococcus bovis, Staphylococcus aureus, Shigella flexneri, Streptococcus faecalis and Salmonella typhimurium, respectively. In plough, ferrate (V) has been proven to exist highly reactive and most 10³–ten⁵ times more reactive to impurities than ferrate (VI), suggesting that the eradication of toxins past ferrate (VI) may be enhanced in the presence of appropriate one-electron-reducing agents. The ferrate (V) has the capability of inactivating biological species and toxins, which cannot exist reached by ferrate (VI) [65]. Since the high reactivity of ferrate (V) allows to inactivate biological species and toxins which cannot be eliminated past ferrate (Vi), information technology seems that this property may likewise exist responsible for inactivation of bacteria.

Moreover, contempo investigations suggested that iron sludge containing iron (Iii) salts and hydroxides that left afterwards the treatment of leachate may be reused for manufacturing of ferrate (Vi) [66]. This possibility of reusing sludge after treatment fits very well to the concept of a circular economy. From the practical and technological betoken of view, it is important to be able to generate ferrate (VI) in situ [67], which reduces the costs of synthesis, transport, storage and handling.

iv. Conclusions

The use of potassium ferrate (6) for the treatment of leachate from a municipal landfill site made it possible to obtain clean leachate characterized by low values of physicochemical (COD, TOC, TN, TP) and microbiological (TCC, MPN, TPC) parameters. Under optimal weather, potassium ferrate (VI) finer decomposed organic compounds present in the leachate and inactivated microorganisms, which was related to its disinfecting consequence. The apply of conventional coagulants in the form of iron (II) and (3) salts allowed for only fractional removal of impurities from the tested leachate. Both in the case of potassium ferrate (VI) and conventional coagulants, iron (2) and (III) hydroxides are formed, which can adsorb impurities or lead to their co-atmospheric precipitation. The maximum efficiency of pollutant removal was obtained with the use of K_twoFeO₄ in the process of their oxidation, then coagulation, adsorption and co-precipitation. Moreover, atomic number 26 sludge left after the treatment of leachate may be reused for generation of ferrate. Thus, One thousand₂FeO₄ can be treated as an effective, multi-functional and environmentally-friendly coagulant for the handling of leachate from municipal landfills.

Author Contributions

Conceptualization, Yard.T.; methodology, M.T., A.B., Grand.B., V.Chiliad.; statistical assay, M.T.; formal analysis, One thousand.T.; data curation, M.T.; writing—original typhoon training, M.T.; writing—review and editing, One thousand.T., Five.K., A.Southward., K.B., A.B.; visualization, M.T.; supervision, K.B. and J.J.; projection administration, Five.K. and A.B.; funding conquering, V.K., A.B. and J.J. All authors take read and agreed to the published version of the manuscript.

Funding

This work was supported by Ministry of Scientific discipline and Higher Educational activity Republic of Poland within statutory funds. This work was besides supported past the Slovak Inquiry and Development Agency (APVV-17-0373, APVV-17-0318) and the Slovak Grant Agency for Science (VEGA 1/0787/xviii).

Acknowledgments

The authors would like to give thanks the anonymous reviewers for their helpful comments.

Conflicts of Involvement

The authors declare no conflict of interest.

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Figure 1. Bar chart of standardized effects (COD, grand O₂/L, 3 value, 1 cake, sixteen experiments, MS = 0.0040, L–linear effect, Q–quadratic effect, p–the absolute value of the standardized consequence evaluation).

Figure 1. Bar nautical chart of standardized effects (COD, g O_two/50, iii value, 1 block, 16 experiments, MS = 0.0040, L–linear result, Q–quadratic effect, p–the absolute value of the standardized consequence evaluation).

Figure 2. Predicted vs. observed values plots for COD (g O₂/L).

Figure 2. Predicted vs. observed values plots for COD (grand O_ii/L).

Figure 3. Response surface plots for COD (g O_ii/L) with respect to K_twoFeO_four (g/Fifty) and pH (A), fourth dimension (min), and pH (g/L) (B) and fourth dimension (min) and Chiliad_iiFeO_iv (g/L) (C).

Figure iii. Response surface plots for COD (g O₂/L) with respect to Thou₂FeO_four (grand/L) and pH (A), time (min), and pH (g/L) (B) and fourth dimension (min) and G₂FeO_four (grand/Fifty) (C).

Tabular array 1. Empirical atmospheric condition for the CCD/RSM and result (COD) for landfill leachate (pH 0.98–half dozen.02, Grand₂FeO₄ 0.132–0.468 m/Fifty, Fourth dimension vi.59–23.41 min); (C)-middle of the plan.

Tabular array 1. Empirical weather condition for the CCD/RSM and outcome (COD) for landfill leachate (pH 0.98–6.02, K₂FeO_four 0.132–0.468 chiliad/Fifty, Fourth dimension six.59–23.41 min); (C)-center of the plan.

Run	Experimental Conditions			Experimental Results *
	pH	1000twoFeOfour (g/L)	Time (min)	COD (g O2/L)
1	2.00	0.200	ten.00	0.385 ± 0.058
ii	2.00	0.200	xx.00	0.355 ± 0.053
three	2.00	0.400	ten.00	0.295 ± 0.044
iv	2.00	0.400	20.00	0.260 ± 0.039
5	5.00	0.200	10.00	0.465 ± 0.070
half dozen	5.00	0.200	20.00	0.450 ± 0.068
7	5.00	0.400	10.00	0.300 ± 0.045
eight	v.00	0.400	20.00	0.295 ± 0.044
9	0.98	0.300	fifteen.00	0.245 ± 0.037
10	6.02	0.300	fifteen.00	0.495 ± 0.074
11	3.fifty	0.132	15.00	0.695 ± 0.104
12	3.50	0.468	15.00	0.205 ± 0.031
13	iii.50	0.300	6.59	0.345 ± 0.052
14	3.50	0.300	23.41	0.240 ± 0.036
15 (C)	iii.50	0.300	15.00	0.265 ± 0.040
16 (C)	3.l	0.300	15.00	0.275 ± 0.041

Table 2. The specified physicochemical and microbiological parameters of the landfill leachate.

Table two. The specified physicochemical and microbiological parameters of the landfill leachate.

Parameter	Unit	Upshot *
pH	–	8.ix ± 0.ane
Chemical Oxygen Demand, COD	mg O2/L	770 ± 116
Total Organic Carbon, TOC	mg/50	230 ± 35
Total Nitrogen, TN	mg/50	120 ± 18
Total Phosphorus, TP	mg/Fifty	12 ± two
Full Coli Count, TCC	CFU/mg/Fifty	6.ii × 106 (6.eight log)
Most Probable Number of fecal enterococci, MPNfe	MPN/100 mL	i.1 × 104 (4.0 log)
Full Proteolytic Count, TPC	CFU/mL	ii.half-dozen × ten4 (iv.four log)

Table 3. Statistical parameters of the experiments using CCD/RSM with Statistica 13–evaluation of the furnishings.

Tabular array 3. Statistical parameters of the experiments using CCD/RSM with Statistica 13–evaluation of the effects.

Parameter	Evaluation of the Effects, COD, 1000 Oii/L, R 2 = 0.8477, R two adj = 0.7462, 3 Parameter, one Block, 16 Experiments, MS = 0.0040
	Upshot	Standard Error	p-Value *	−95% Conviction Interval	+95% Conviction Interval	Factor	Standard Error of Factor	Lower Confidence Interval	Upper Confidence Interval
Constant Value	0.2725	0.0448	0.0002	0.1713	0.3738	0.2725	0.0448	0.1713	0.3738
pH (L)	0.0931	0.0344	0.0240	0.0153	0.1708	0.0465	0.0172	0.0077	0.1708
pH (Q)	0.0584	0.0417	0.1947	−0.0359	0.1528	0.0292	0.0209	−0.0180	0.1528
K2FeO4 (Fifty)	0.1946	0.0344	0.0003	−0.2724	−0.1169	−0.0973	0.0172	−0.1362	−0.1169
KiiFeO4 (Q)	0.1150	0.0417	0.0222	0.0207	0.2094	0.0575	0.0209	0.0103	0.2094
Time (L)	0.0383	0.0344	0.2937	−0.1160	0.0394	−0.0192	0.0172	−0.0580	0.0394
Fourth dimension (Q)	0.0036	0.0417	0.9323	−0.0907	0.0980	0.0018	0.0209	−0.0454	0.0980

Table 4. Analysis of the CCD/RSM experiment using Statistica 13—verification of the capability of the model using ANOVA.

Table 4. Analysis of the CCD/RSM experiment using Statistica 13—verification of the adequacy of the model using ANOVA.

Parameter	Assessment of Effects, COD, g O2/50, R 2 = 0.8477, R 2 adj = 0.7462, 3 Parameter, one Cake, sixteen Experiments, MS = 0.0040
	SS	MS	F	p *
pH (50)	0.029567	0.029567	7.33757	0.024045
pH (Q)	0.007911	0.007911	ane.96337	0.194681
KtwoFeOiv (L)	0.129345	0.129345	32.09914	0.000307
ThousandiiFeO4 (Q)	0.030637	0.030637	vii.60318	0.022207
Time (L)	0.005011	0.005011	1.24345	0.293696
Time (Q)	0.000031	0.000031	0.00764	0.932269
Error	0.036266	0.004030	–	–

Tabular array v. Coefficients of the fitted model.

Table 5. Coefficients of the fitted model.

Predictor	Regression Coefficient	Standard Error	t-Value, df = 9	p-Value	−95% Confidence Interval	+95% Conviction Interval
Intercept	1.206468	0.354637	3.401978	0.007849	0.404223	2.008712
pH (L)	−0.059897	0.065887	−0.909076	0.387006	−0.208944	0.089151
pH (Q)	0.012988	0.009269	1.401202	0.194681	−0.007980	0.033957
KtwoFeO4 (Fifty)	−4.423640	1.263080	−3.502263	0.006700	−seven.280926	−i.566353
M2FeOfour (Q)	v.750741	2.085576	2.757387	0.022207	1.032839	x.468642
Time (50)	−0.006018	0.025262	−0.238234	0.817035	−0.063164	0.051128
Time (Q)	0.000073	0.000834	0.087398	0.932269	−0.001814	0.001960

Table 6. Selected physicochemical parameters of treated landfill leachate after Grand_iiFeO₄, FeSO₄ × 7H₂O and FeCl₃ × 6H₂O application.

Table half-dozen. Selected physicochemical parameters of treated landfill leachate after K₂FeO₄, FeSO₄ × 7H₂O and FeCl₃ × 6H₂O application.

Parameter *	Unit	After ThousandtwoFeOiv Awarding in Optimal Conditions **	After FeSO4 × 7H2O Application ***	After FeCl × 6H2O Application ****
		Removal, % (in Brackets) *****
pH	–	ix.0 ± 0.1	9.0 ± 0.1	ix.0 ± 0.i
Chemic Oxygen Demand	mg Otwo/L	180 ± 27 (↓76.2)	475 ± 71 (↓38.1)	450 ± 68 (↓41.6)
Total Organic Carbon	mg/L	40 ± vi (↓82.half dozen)	145 ± 22 (↓37.0)	125 ± 19 (↓45.7)
Total Nitrogen, TN	mg/L	38 ± half-dozen (↓68.3)	95 ± 14 (↓20.8)	85 ± thirteen (↓29.2)
Total Phosphorus, TP	mg/L	1.0 ± 0.2 (↓91.vi)	0.fifty ± 0.08 (↓95.8)	0.5 ± 0.08 (↓95.8)
Total Coli Count, TCC	CFU/mL	5.9 × ten2; 2.viii log (↓99.9)	3.five × 105; v.5 log (↓94.iv)	4.9 × 10v; 5.7 log (↓92.1)
Most Likely Number of fecal enterococci, MPN	MPN/100 mL	4.6 × ten2; 2.seven log (↓95.8)	4.6 × 10three; 3.7 log (↓58.2)	iv.6 × 103; 3.seven log (↓58.2)
Total Proteolytic Count, TPC	CFU/mL	1.nine × 102; two.three log (↓99.iii)	2.iv × 103; 3.4 log (↓90.viii)	ii.6 × ten3; three.iv log (↓90.0)

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