The Efficiency of Chemical and Electrochemical Coagulation Methods for Pretreatment of Wastewater from Underground Coal Gasification

Abstract

This article compares chemical coagulation with electrocoagulation, two popular methods for the primary treatment of wastewater generated in the process of underground coal gasification (UCG). The primary aim was to determine which method is more effective in the removal of cyanide and sulphide ions, metals and metalloids, as well as organic compounds. In both cases, experiments were conducted in batch 1 dm³ reactors and using iron ions. Four types of coagulants were tested during the chemical coagulation study: FeCl₂, FeSO₄, Fe₂(SO₄)₃, and FeCl₃. In the electrocoagulation experiments, pure iron Armco steel was used to manufacture the sacrificial iron anode. Both processes were tested under a wide range of operating conditions (pH, time, Fe dose) to determine their maximum efficiency for treating UCG wastewater. It was found that, through electrocoagulation, a dose as low as 60 mg Fe/dm³ leads to >60% cyanide reduction and >98% sulphide removal efficiency, while for chemical coagulation, even a dose of 307 mg Fe/dm³ did not achieve more than 24% cyanide ion removal. Moreover, industrial chemical coagulants, especially when used in very high doses, can be a substantial source of cross-contamination with trace elements.

1. Introduction

Underground coal gasification (UCG) is a technology that involves converting coal directly within its seam into a useful gas. This gas can be utilised for producing fuels and commodities such as hydrogen, synthetic natural gas (SNG), or Fischer–Tropsch liquids, as well as serving as an energy source for heat and power generation. While UCG is considered an economically viable and technically feasible clean coal technology, several concerns remain. One major issue involves environmental safety, particularly the generation of wastewater. This liquid effluent, produced during the cleaning and cooling of producer gas generated by UCG.

The characteristics of wastewater produced from UCG are similar to those of wastewater from coking plants. The formation and composition of this wastewater are influenced by several factors, including the type of coal used, process conditions (such as pressure, temperature, composition, and distribution of the gasification agent), and the configuration of the gas cleaning system [1]. The contaminants that eventually migrate into the wastewater reflect the components present in the gasification system. As a result, metals, cyanides (CN), sulphur compounds (S), and various organic pollutants such as phenols, benzene, toluene, xylene (BTX), and polycyclic aromatic hydrocarbons (PAHs) are commonly detected [2,3].

The removal of these contaminants is essential due to well-established ecological concerns. Organic compounds, for instance, are known for their biotoxicity, while phenols, BTX, and PAH are regulated because of their carcinogenic and mutagenic properties. Hydrogen sulphide imparts a characteristic ‘rotten egg’ odour to liquid effluents and is both toxic and a potent corrosive agent. Upon oxidation, sulfuric compounds can form precipitates that clog pipes and damage equipment. Cyanides are primarily removed due to their high toxicity and potential environmental impact.

In general, if not properly treated, the effluent from the UCG process can cause environmental contamination in surrounding areas, both underground and on the surface. However, due to the complex nature of this effluent, its treatment poses significant technical and financial challenges. The UCGWATERplus project explored the feasibility of using constructed wetlands as a novel and cost-effective phytoremediation-based solution to this issue. Given the biological nature of this treatment, the UCG effluent required pre-treatment to mitigate its potentially biotoxic characteristic. The focus of the research presented here is on evaluating the efficiency of chemical and electrochemical coagulation methods for this pre-treatment.

Chemical coagulation is an essential process in industrial wastewater treatment, consisting of two main stages: coagulation and flocculation. During coagulation, coagulants such as Fe²⁺ and Fe³⁺ ions are added to neutralise the negative surface charge of colloidal particles, causing them to aggregate into larger microflocs. This step involves high-intensity mixing to ensure rapid dispersion of the coagulating agent and its subsequent interaction with contaminants. Subsequent flocculation is a step where gentle mixing allows microflocs to grow into larger, stable macroflocs. Subsequently, macroflocs are separated from the water via sedimentation, centrifugation, flotation, or barrier filtration [4].

In wastewater treatment, ferrous sulphate (iron (II) sulphate) is one of the most commonly used coagulants, as it typically allows for the formation of denser flocs compared to alum (aluminium sulphate). Additionally, ferric chloride is often considered an alternative because, in many cases, it promotes even faster sedimentation [5]. However, a drawback of using iron chlorides is the potential exacerbation of chloride-induced corrosion [6].

Chemical coagulation is effective at removing a variety of contaminants, including suspended solids, organic compounds, colour, and some dissolved metals [7,8,9,10]. Iron-based coagulants are particularly effective at removing phosphates and heavy metals. This method is highly valued for its simplicity and effectiveness, especially in treating large volumes of water. It is commonly used in municipal water treatment, industrial wastewater management, mining operations, and chemical manufacturing. However, the process generates a significant amount of sludge, which necessitates proper handling and disposal [5]. Understanding and optimising chemical coagulation is crucial for improving wastewater treatment processes and effectively addressing various contaminants. However, the use of coagulating and flocculating agents, along with chemicals for pH and alkalinity adjustment, can lead to secondary contamination of the treated stream. In extreme cases, this may compromise the overall effectiveness and environmental soundness of the purification strategy.

An alternative to conventional chemical coagulation is electrocoagulation (EC) systems. EC is a versatile method for treating liquid effluents through five main mechanisms: coagulation and flocculation, chemical oxidation, pH adjustment, electroflotation, and microbial activity. The effectiveness of these mechanisms depends on process conditions and electrode materials. Iron and aluminium are commonly used for coagulation and flocculation, with iron also being effective in removing heavy metals and phosphate ions [11,12,13]. Other electrodes like stainless steel, zinc, titanium, or graphite are used in harsh conditions or for chemical oxidation and flotation [12,14].

The adaptability of EC allows it to address a wide range of contaminants, including heavy metals, organic compounds, oils, dyes, suspended solids, emulsified oils, phosphorus, nitrogen, pesticides, pathogens, and, by extension, Chemical Oxygen Demand (COD) [15,16,17,18,19]. Recent studies have highlighted EC’s economic efficiency compared to other waste treatment methods. EC can effectively remove various contaminants using different electrode configurations and power input designs, making it suitable for municipal and residential wastewater treatment. Its applications extend to drinking water treatment, industrial effluents, metal machining, textiles, mining, oil and gas, petrochemicals, pharmaceuticals, semiconductors, electronics, food processing, dairy, agriculture, landfilling, and gemstone washing [20,21,22,23,24,25,26,27,28,29,30,31,32].

Interestingly, even though the subject of chemical coagulation and electrocoagulation finds great attention, we still see a limited number of papers presenting results of its integration with effluents generated in coal processing or in particular UCG technologies [33,34,35].

This research gap led to the conclusion that investigating the characteristics and limitations of chemical and electrocoagulation methods for the pretreatment of UCG wastewater will complement future studies in environmental and power engineering. Therefore, this work aims to compare the effectiveness of batch chemical coagulation and electrocoagulation processes in purifying raw wastewater produced by a pilot UCG installation operated by GIG-PIB (Poland).

2. Materials and Methods

UCG Wastewater Characteristic

Wastewater from oxygen-blown UCG of bituminous coal from the “Wesoła” Coal Mine was used in this study.

Table 1. Average physical and chemical characteristics of the UCG wastewater.

Parameter	Value
pH [-]	8.193
Conductivity [mS/cm]	1600.5
Redox, [mV]	−113.3
COD [mg dm3]	189.03
CN− [mg/dm3]	0.971
S2− [mg/dm3]	0.215
Trace elements
Al [mg/kg]	2.520
As [mg/kg]	<0.02
Cd [mg/kg]	<0.02
Co [mg/kg]	<0.05
Cr [mg/kg]	<0.025
Cu [mg/kg]	<0.025
Fe [mg/kg]	0.179
Mn [mg/kg]	0.333
Mo [mg/kg]	<0.05
Ni [mg/kg]	0.492
Pb [mg/kg]	<0.05
Sb [mg/kg]	0.080
Ti [mg/kg]	<0.02
Zn [mg/kg]	0.213
Sum [mg/kg]	3.817
BTX
Benzene [mg/dm3]	0.210
Toluene [mg/dm3]	0.080
Ethylbenzene [mg/dm3]	0.004
m-xylene [mg/dm3]	0.004
p-xylene [mg/dm3]	0.010
Isopropylbenzene [mg/dm3]	0.002
o-xylene [mg/dm3]	0.010
Sum [mg/dm3]	0.320

Note: mean values of 3 measurements.

collates basic physicochemical properties of the wastewater. Further details regarding the process and results of UCG experiments can be found in [36,37,38].

The investigated coagulation process is based on the use of iron ions, so the concentration of iron in UCG wastewater equal to 0.179 mg Fe/kg must be treated as a reference. Among trace elements, only the concentrations of Al, Ni, Mn, Fe, Zn, and Sb (listed in decreasing order of detected concentration) were determined above the lower limit of detection (LDL). Similarly, the concentrations of BTX compounds were close to the LDL, whereas the PAH were below this threshold. Therefore, there is a high degree of uncertainty in the analysis of the applicability of the two coagulation variants as methods for the removal of organic pollutants.

3. Analytical Methods

3.1. Determination of pH, Electrochemical Conductivity, and Redox Potential

Determination of the pH of the wastewater, its electrochemical conductivity, and redox potential was carried out using a laboratory multimeter CX505 produced by Elmetron (Zabrze, Poland). Dedicated electrodes were used to measure each of the parameters.

3.2. Determination of COD

The determination of COD was carried out spectrophotometrically using dedicated HACH-LCK 014 cuvette tests. In this method, oxidisable substances react with a solution of potassium dichromate in sulphuric acid in the presence of silver sulphate as a catalyst. The presence of chlorides is masked by mercury sulphate. The determination is based on the measurement of the intensity of the green colour caused in the solution by Cr³⁺ ions. Before analysis, the sample was diluted 10- and 25-fold. Then, 0.5 cm³ of the diluted sample was introduced into the test cuvette, and the samples were mineralised (digested) for 15 min at 170 °C. After the mineralisation was completed and the sample was cooled down, the colour intensity was measured at the wavelength of λ = 605 nm using the DR6000 HACH Lange spectrophotometer (Loveland, CO, USA).

3.3. Determination of the Content of Phenols

The content of phenols was determined as the so-called phenol index by the photometric method using dedicated HACH-LCK 354 cuvette tests. The method uses the reaction of phenols contained in the sample with 4-nitroaniline, which forms yellow complexes with phenols. First, 2 cm³ of the diluted sample and 0.2 cm³ of sodium nitrite solution were introduced into a test cuvette containing 4% hydrochloric acid. Then, after 2 min, the sodium carbonate solution was added to the cuvette. The colour intensity was measured after another 2 min at a wavelength of λ = 476 m using a DR6000 HACH Lange spectrophotometer.

3.4. Determination of the Content of Free Cyanides

The content of free cyanides was determined by the photometric method using dedicated HACH-LCK 315 cuvette tests. In this technique, cyanide ions contained in the sample react with chlorine, and the resulting chlorinated cyanides, in the presence of barbituric acid, form violet complexes with pyridine. 1 cm³ of the sample was introduced into a test cuvette containing potassium dihydrogen phosphate, chloramine B, and sodium hydrogen phosphate, and 1,3-dimethylbarbituric acid was added. Then, after mixing, 1 cm³ of pyridine solution was introduced into the cuvette, and after 3 min, the colour intensity was measured at the wavelength of λ = 588 m using the DR6000 HACH Lange spectrophotometer.

In this text, the discussion on cyanides refers always to their free form.

3.5. Determination of the Content of Ammonium Nitrogen

The content of ammonium nitrogen was determined by the photometric method using dedicated HACH-LCK 302–305 cuvette tests. In an alkali environment (pH 12.6), ammonia reacts with hypochlorite and salicylic ions in the presence of sodium nitroprusside (a catalyst) to yield indophenol blue.

0.5 cm³ of sample was introduced to the analytical cuvette containing a mixture of hipochlorites and salicylic ions. Next, nitroprusside was added, and the sample was left for 15 min, after which the intensity of solution colour was measured using a DR6000 HACH Lange Spectrophotometer at wavelength λ = 550 nm.

3.6. Determination of the Content of Sulphides

The determination of sulphides was carried out spectrophotometrically using dedicated HACH-LCK 653 cuvette tests.

In this method, sulphide ions contained in the sample react with p-aminodimethylaniline to form leucomethylene blue, which, in the presence of iron (III) ions, turns into methylene blue.

4 cm³ of the sample was introduced into a test cuvette containing a 30% solution of sulphuric acid with the addition of p-aminodimethylaniline. Then 0.5 cm³ of a solution of 3% sulphuric acid containing iron (III) ions was added. After 10 min, the colour intensity was measured at the wavelength of λ = 665 nm using the DR6000 HACH Lange spectrophotometer.

3.7. Determination of the Content of Cations and Anions

The content of cations and anions was determined using a two-channel capillary ion chromatograph, Dionex ICS-5000 (Thermo Scientific, Waltham, MA, USA). The method is based on the separation of negatively or positively charged ions using ion exchange chromatography. Analytical column IonPack AS-25 (250 mm × 2 mm) together with protective column IonPack AG-25 were used for the separation of anions, while analytical column CS-16 (250 mm × 3 mm) and a protective column CG-16 were used for the analysis of cations. The eluent flow was maintained at 0.250 cm³/min for anions and 0.500 cm³/min for cations. The column and detector compartments were maintained at 30 °C and 20 °C, respectively. The eluate leaving the column is continuously analysed by a conductivity detector. Chromatograms were recorded isocratically—25 mM sodium hydroxide (for analysis of anions) and 25 mM methanesulfonic acid (for analysis of cations) were generated electrolytically in situ during the analysis. The measurement data is processed by the Chromeleon^® 6.7 (Dionex) data management system.

3.8. Qualitative and Quantitative Analysis of Trace Elements

Samples were analysed according to ISO 11885:2007 Water quality—Determination of selected elements by inductively coupled plasma optical emission spectrometry (ICP-OES) [39]. Samples were digested in an Ethos One (Milestone, Milan, Italy) microwave digestion system using 2 cm³ of sample and 6 cm³ of nitric acid (67% from Sigma Aldrich, St. Louis, MO, USA) was added to each Teflon vessel. The vessels were heated at 200 °C for 25 min in a microwave oven. The concentrations of Al, As, Cd, Co, Cu, Cr, Fe, Mn, Mo, Ni, Pb, Sb, Ti, and Zn were determined using iCAP 6500 Duo (Thermo Scientific, Waltham, MA, USA). To calibrate the spectrometer, standard solutions were prepared from single-element solutions (SCP Science Company, Baie D’Urfé, QC, Canada) with a standard concentration of 1000–10,000 mg/dm³.

3.9. Qualitative and Quantitative Analysis of Volatile Organic Compounds (VOCs)

100 cm³ of the sample was mixed with 25 cm³ of hexane and mixed using a magnetic stirrer for 1 h. Afterwards, the separated organic phase was evaporated under reduced pressure and analysed by GC/FID.

The chromatographic analysis was performed using ThermoScientific Trace GC Ultra equipped with a capillary column ZB-WAXplus L = 60 m, ID = 0.32 mm, FT = 0.25 µm, and a flame-ionisation detector (FID).

3.10. Qualitative and Quantitative Analysis of the Polyaromatic Hydrocarbons (PAHs)

270 cm³ of the sample was treated using the solid phase extraction technique (SPE) and Supelclean LC-18 SPE 500 mg 6 cm³ cartridges. The following steps of the SPE were performed. The cartridge was activated using methanol (2 steps × 2 cm³ each), followed by rinsing with deionised water (3 × 1 cm³). Subsequently, the sample was fed through the cartridge. Elution of the analytes was performed in two steps: (1) using acetone (2 × 2 cm³) and (2) using benzene (2 × 2 cm³). The thus obtained organic phase was analysed by GC/FICD.

The chromatographic analysis was performed using Trace GC ThermoFinnigan equipped with a capillary column ZB-6 L = 60 m, ID = 0.32 mm, FT = 0.25 µm, and a flame-ionisation detector (FID).

4. Introduction of Cyanides and Sulphides to the UCG Wastewater

In solutions not specifically prepared or treated to stabilise their cyanide or sulphide content, these contaminants tend to exhibit unstable behaviour, resulting in changes in their speciation and concentrations over time. Based on experience, wastewaters generated in coal gasification systems can contain more than 15 mg/dm³ of cyanides and 40 mg/dm³ of sulphides. However, in the case of the tested stream of UCG wastewater, due to the time gap between the gasification experiments and coagulation studies, the samples contained only traces of these contaminants. Specifically, the sulphide concentration did not exceed LDL (<0.1 mg/dm³), while the cyanide concentration was measured to be 1 mg/dm³. Therefore, it was decided to artificially introduce the aforementioned concentrations of cyanides and sulphides into the UCG wastewater before the coagulation experiments.

At the pH of the treated UCG wastewater, i.e., 8–9, sulphides mainly exist in the solution in the form of undissociated, soluble H₂S (gas) and HS⁻. During EC, the pH increases, leading to an increase in the amount of available free sulphide ions. On the other hand, chemical coagulants containing iron ions have an acidic pH, which may lead to the emission of gaseous hydrogen sulphide. To avoid the need to artificially increase the pH of the UCG wastewater, sulphides were added to the solution only in the case of EC experiments. Cyanides are known to form complexes over time and react with heavy metals, but they can also undergo oxidation with hydrocarbons available in the solution. Due to the much less volatile nature of cyanides, they were introduced for both chemical and electrochemical coagulation tests. They were added just before each experimental series, and before division to individual objects.

Table 2. Summary of measured concentrations of CN⁻ ions in UCG wastewater—chemical coagulation.

	Mean	Min	Max	St. dev.
CN−, [mg/Nm3]	14.65	13.70	15.80	0.79

presents the variability of cyanide concentrations in the objects subjected to chemical coagulation.

During EC trials, both cyanides and sulphides were introduced into the wastewater. The concentration of cyanides was maintained at a level similar to chemical coagulation tests, while the amount of sulphides introduced was 40 mg/dm³.

Table 3. Summary of measured concentrations of CN⁻ and S²⁻ ions in UCG wastewater—electrocoagulation.

	Mean	Min	Max	St. dev.
CN−, [mg/Nm3]	14.19	13.40	15.30	0.71
S2−, [mg/Nm3]	37.92	29.85	45.15	5.39

presents the variability of cyanide and sulphide concentrations in the objects subjected to EC.

During the batch EC tests, the procedure for measuring sulphides in wastewater was refined. Despite introducing an identical mass of Na₂S, there was a substantial spread in the data, which was attributed to the continuous removal of sulphide from the solution and the emission of small amounts of gaseous H₂S. Additional sources of variability were linked to the heterogeneous hydration of the salt and the very small quantities of the added reagent. For this reason, the average value shown in Table 3 was used as the baseline in the subsequent analysis of sulphide removal effectiveness.

5. Calculation of Removal Efficiency/Reduction of the Value of Component i

$${\mathsf{\eta }}_{\mathbf{i}}=\mathbf{1}-{\displaystyle \frac{{\mathbf{c}}_{\mathbf{i}\_\mathbf{o}\mathbf{u}\mathbf{t}\mathbf{l}\mathbf{e}\mathbf{t}}}{{\mathbf{c}}_{\mathbf{i}\_\mathbf{i}\mathbf{n}\mathbf{l}\mathbf{e}\mathbf{t}}}}\left[\mathbf{\%}\right]$$

(1)

where ${\mathrm{c}}_{\mathrm{i}\_\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}$ is the concentration of component i at the inlet to the reactor [mg/dm³] and ${\mathrm{c}}_{\mathrm{i}\_\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}$ is the concentration of component i at the outlet from the reactor [mg/dm³].

The effectiveness of treatments was determined based on the removal efficiency of cyanides, heavy metals, and organic pollutants (BTX). Additionally, changes of such parameters as redox or COD were also analysed as secondary measures.

6. Feedstocks

6.1. Coagulants

Four main types of iron-based coagulants were used in this study, namely acidic solutions of iron (II) and iron (III) sulphates and chlorides.

Table 4. Table summarising the basic properties of the coagulants used.

Trade Name	Chemical Specie	Density, kg/dm3	Total Fe Content, % w/w
PIX-100	FeCl2	1.2659	9.76
PIX-100 COP	FeSO4	1.0552	1.75
PIX-113	Fe2(SO4)3	1.5262	11.39
PIX-116	FeCl3	1.3143	10.26

presents a comparison of the basic characteristics of herein utilised coagulants. The last column represents the total Fe concentration in the coagulants (sum of all valences).

6.2. Electrodes

Anode was manufactured from flat plate of Armco pure iron steel (<99.5% Fe grade 04 J according to PN 89/H-84023/02, C_max = 0.035%, Mn_max = 0.25%, Si_max = 0.02%, P_max = 0.025%, S_max = 0.030%, Cr_max = 0.10%, Ni_max = 0.10%, Cu_max = 0.10%, Al_max = 0.02–0.07%), while cathode was made of stainless steel (AISI 304).

7. Chemical Reagents

For pH adjustments, a commercially available 50% sodium hydroxide solution with analytical grade purity was used. Cyanide and sulphide ions were introduced into the solution using potassium cyanide (KCN) and hydrated sodium sulphide (Na₂S), supplied by Thermo Scientific and Warchem Sp. z o.o., respectively. Due to the hygroscopic nature of Na₂S, its actual state of hydration was assessed during the tests, as described below.

8. Chemical Coagulation Setup

8.1. Sample Notation

Samples taken prior or after treatments, as well as characteristic parameters collected during the study, are described using a coding method presented in

Table 5. Coding of samples—chemical coagulation tests.

Notation of the Tests and Samples	Coagulant_Dose [mg Fe/dm3]_pH before Coagulation
example	PIX100_10.0_6.3

.

8.2. Test Procedure

Jar tests were conducted during experiments on the chemical coagulation process of UCG wastewater. The main goal of coagulation was to maximise the removal of organic compounds, metalloids, and cyanides. The setup consisted of four identical sets of beakers, each having a capacity of 2 litres. The beakers were placed on magnetic stirrers, and the type and length of the stir bars used were selected to enable both high turbulence and slow, gentle mixing required for proper coagulation during the different stages of the procedure. As a result, 40 mm rods with a diameter of 5 mm were found to perform best. No additional flocculant (e.g., polymeric) was added to enhance clarification.

8.2.1. Coagulation Procedure

• pH correction;
• Measurement of basic parameters of raw wastewater: pH, conductivity, redox;
• Introducing 1000 cm³ of wastewater into each of the beakers;
• Measurement of preset doses of coagulant and their introduction into syringes;
• Start of the mixing and coagulation program;
• Measurement of basic parameters in the effluent after sedimentation is finished;
• Sampling and lab analyses of the supernatant of the treated wastewater.

8.2.2. Mixing Program

• Initiation of mixing; 1200 rpm for 1 min; very intensive mixing;
• Coagulant injection;
• 700 rpm for 5 min; medium intensity mixing;
• 400 rpm for 5 min;
• 200 rpm for 10 min; minimal intensity mixing, minimal agitation of the liquid;
• Cessation of mixing;
• Sedimentation—120 min.

8.3. Preliminary Tests on the Effect of Coagulant Dosage on the Change in Solution pH

pH adjustments necessary to maintain the optimal working range for each specific coagulant were determined experimentally for each case (type of coagulant and dosage). Generally, it was observed that the optimal pH range for maximum effectiveness of iron sulphates and chlorides fell between 4 and 9. pH corrections were made only when the resulting pH of the solution fell outside this optimal range, or when no signs of coagulation-flocculation were observed. No external source of alkalinity was added to any of the samples.

9. Electrocoagulation Setup

In this study, the batch EC tests were performed using an approach similar in nature to the above-described Jar Tests. The experiments were carried out in a system consisting of two beakers of 2 dm³ each, an electrode support, and a Twintex TP-2305 dual-channel power supply (30 V DC, 2 × 5 A). Both electrodes had an area of 20 cm² each and were spaced 2 cm apart during all experiments. Since the currents used in this study were very small (<0.5 A), to increase the resolution of the measurement, an additional measurement system consisting of a set of multimeters was used in addition to the measurement indicated by the power source. The measurements thus had a resolution of 1 mA/mV and a precision of 0.03%. The beakers were thermostated in a water bath at 19.7 ± 0.3 °C.

Sample Notation

Samples and characteristic parameters collected during the study were described using a coding method presented in

Table 6. Coding of samples—electrocoagulation tests.

Test and Sample Notation	el_Dose [mg Fe/dm3]_Current [mA]_Time [min]_pH before the Process
example	el_d60_i58_t60_pH 8.5

.

10. Results and Discussion

10.1. Chemical Coagulation of UCG Wastewater

10.1.1. FeCl₂ as a Source of Fe Ions—PIX-100

Table 7. Chemical coagulation of UCG wastewater using FeCl₂.

	PIX100_1.0_7.9	PIX100_1.5_7.9	PIX100_2.0_7.9	PIX100_2.5_7.9
Dose, [mg Fe/dm3]	123.55	185.33	247.10	308.88
pH after coag., [-]	5.88	5.73	5.41	3.70
Conductivity, [mS/cm]	2.003 (−25.15%)	2.241 (−40.02%)	2.573 (−60.76%)	2.894 (−80.82%)
Redox, [mV]	24 (−121.18%)	36 (−131.76%)	52 (−145.88%)	152 (−234.12%)
CN−, [mg/dm3]	10.8 (21.17%)	10.4 (24.09%)	11.1 (18.98%)	10.7 (21.9%)
COD, [mg/dm3]	186.2 (2.62%)	194.2 (−1.57%)	204.2 (−6.8%)	216.2 (−13.08%)

Notes: Average of measured value (n = 3) (removal/reduction eff.)—negative value indicates an increase in the specific property. The concentration of cyanide measured in the effluent prior to treatment was equal to 13.7 mg/dm³.

collates the results of using FeCl₂ as a source of Fe ions. None of the tested coagulant doses (123–309 mg Fe/dm³) resulted in a reduction in the concentration of cyanide ions greater than 24%. With increasing doses of the coagulant, an increase in the values of redox potential, COD, and conductivity of the wastewater was observed. The highest tested coagulant doses resulted in the greatest clarity of the solution and the fastest sedimentation; however, an increase in dose led to an increase in COD—for the dose of 307 mg Fe/dm³, COD increased by 13%. In chemical coagulation, this observation is attributed to the introduction of an excess of iron, chloride, and sulphate ions, as well as secondary contaminants brought into the wastewater with the coagulant itself. This notion will be further explored in the following paragraphs.

10.1.2. FeSO₄ as a Source of Fe Ions—PIX-100 COP

For the PIX-100 COP coagulant, much lower doses were introduced into the treated UCG wastewater stream (37–92 mg Fe/dm³) to assess the effectiveness of using very low doses of divalent iron. As shown in

Table 8. Chemical coagulation of UCG wastewater using FeSO₄.

	PIX100 COP_2.0_8.2	PIX100 COP_3.0_8.2	PIX100 COP_4.0_8.2	PIX100 COP_5.0_8.2
Dose, [mg Fe/dm3]	36.93	55.40	73.86	92.33
pH after coag., [-]	7.10	6.74	6.44	6.43
Conductivity, [mS/cm]	1.6931 (−5.79%)	1.7402 (−8.73%)	1.7625 (−10.12%)	1.7563 (−9.73%)
Redox, [mV]	−46 (−59.41%)	−25 (−77.94%)	−7 (−93.82%)	−7 (−93.82%)
CN−, [mg/dm3]	12.5 (15.54%)	12.3 (16.89%)	11.9 (19.59%)	11.9 (19.59%)
COD, [mg/dm3]	181.2 (5.72%)	181.2 (5.72%)	180.2 (6.24%)	180.2 (6.24%)

Note: The concentration of cyanide measured in the effluent prior to treatment was equal to 14.8 mg/dm³.

, even a dose of approximately 74 mg Fe/dm³ resulted in a 19.6% reduction in cyanide. Similar to PIX-100, increasing the dosage led to a rise in the redox potential and conductivity of the wastewater. However, unlike PIX-100, lower doses of PIX-100 COP caused a slight decrease (5–6%) in COD. Table 8 collates the results of using FeSO₄ as a source of Fe ions.

10.1.3. Fe₂(SO₄)₃ as a Source of Fe Ions—PIX-113

PIX-113 was the first of the two coagulants tested that were based on trivalent iron ions. Theoretically, the increased ion charge should result in significantly higher effectiveness in neutralising negatively charged colloids. However, an undesirable consequence is the need to adjust pH to maintain the process within the optimal range when using equivalent doses of Fe. pH corrections were applied to all samples where the dose exceeded 190 mg Fe/dm³.

Similar to the Fe²⁺-based coagulants, no significant improvement in cyanide removal efficiency was observed, with the maximum reduction of 21% recorded at a dose of 260 mg Fe/dm³. Increasing the dose of PIX-113 led to a rise in the conductivity of the wastewater, but the impact on the redox potential was ambiguous. As with PIX-100 COP, a slight decrease (5–6%) in the COD value of the wastewater was also observed.

Table 9. Chemical coagulation of UCG wastewater using Fe₂(SO₄)₃.

	PIX113_0.7_8.3	PIX113_1.1_10.0	PIX113_1.5_10.0	PIX113_1.9_10.0
Dose, [mg Fe/dm3]	121.68	191.22	260.75	330.28
pH after coag., [-]	5.46	8.88	7.71	6.28
Conductivity, [mS/cm]	1.7646 (−10.25%)	2.383 (−48.89%)	2.696 (−68.45%)	2.847 (−77.88%)
Redox, [mV]	50 (−144.12%)	−149 (−31.47%)	−76 (−32.94%)	2 (−101.76%)
CN−, [mg/dm3]	12.1 (16.55%)	12.9 (18.35%)	12.5 (20.89%)	12.6 (20.25%)
COD, [mg/dm3]	177.2 (7.32%)	175.2 (3.31%)	169.2 (6.62%)	169.2 (6.62%)

Note: The concentration of cyanide measured in the effluent prior to treatment was equal to 15.8 mg/dm³.

collates the results of using Fe₂(SO₄)₃ as a source of Fe ions.

10.1.4. FeCl₃ as a Source of Fe Ions—PIX-116

The last coagulant tested was ferric chloride (PIX-116), which was expected to exhibit the highest coagulation activity. pH adjustments were necessary for all tested samples.

Interestingly, the impact of PIX-116 across all analysed categories did not differ significantly from the other coagulants. At the lowest dose, a 5–7% lower efficiency in cyanide removal was observed, while only a marginally higher efficiency in COD removal was noted (1–2%).

Table 10. Chemical coagulation of UCG wastewater using FeCl₃.

	PIX116_0.9_10	PIX116_1.3_10	PIX116_1.7_10	PIX116_2.1_10
Dose, [mg Fe/dm3]	121.36	175.30	229.24	283.18
pH after coag., [-]	8.97	8.59	7.30	5.84
Conductivity, [mS/cm]	2.381 (−48.77%)	2.623 (−63.89%)	2.93 (−83.07%)	3.068 (−91.69%)
Redox, [mV]	−154 (−35.88%)	−132 (−16.47%)	−57 (−49.71%)	27 (−123.82%)
CN−, [mg/dm3]	13.3 (12.5%)	12.6 (17.11%)	12.2 (19.74%)	12.2 (19.74%)
COD, [mg/dm3]	177.2 (6.83%)	173.2 (8.94%)	175.2 (7.89%)	174.2 (8.41%)

Note: During this measurement series, the cyanide concentration in the effluent before coagulation was 15.2 mg/dm³.

collates the results of using FeCl₃ as a source of Fe ions.

10.1.5. Effect of Coagulant on the Removal of Cyanide Ions

For all tested doses and forms of Fe used in the chemical coagulation experiments, only a slight decrease in cyanide concentration was measured. The maximum reduction of 24% was achieved using the coagulant PIX-100 (FeCl₂) at a dose of 185 mg Fe/dm³.

The collected experimental data indicate that using herein tested chemical coagulants, the upper limit of cyanide removal efficiency from UCG wastewater is confined to the range of 22–24%. Importantly, none of the tested configurations of process parameters succeeded in reducing the cyanide concentration in the solution below the limit of 10 mg/dm³. Figure 1 presents the effect of dose for all four tested here chemical coagulants on removal of cyanides from UCG wastewater.

10.1.6. Effect of Coagulant on the Removal of Metals and BTX

Analyses of heavy metal and BTX content in the chemically coagulated wastewater were conducted using the supernatant obtained after two hours of sedimentation. The samples were not filtered before analysis. Overall, the coagulation and flocculation processes were rapid and efficient, as indicated by iron levels below the detection limit in 3 out of 4 analysed samples. The results are presented in

Table 11. Summary of the removal efficiency for trace elements and BTX—most favourable configurations of the tested chemical coagulants.

	PIX100_1.5_7.9	PIX100 CPO_4.0_8.2	PIX113_1.5_10.0	PIX116_2.1_10.0
Dose, [mg Fe/dm3]	185.33	73.86	260.75	283.18
CN−, [mg/dm3]	10.4 (24.09%)	11.9 (19.59%)	12.5 (20.89%)	12.2 (19.74%)
COD, [mg/dm3]	194.2 (−1.57%)	180.2 (6.24%)	169.2 (6.62%)	174.2 (8.41%)
Trace elements
Al, [mg/kg]	<0.125 (>95.04%)	<0.125 (>95.04%)	<0.125 (>95.04%)	<0.125 (>95.04%)
Fe, [mg/kg]	75.9 (−42,302.23%)	<0.125 (>30.17%)	<0.125 (>30.17%)	<0.125 (>30.17%)
Mn, [mg/kg]	1.15 (−245.35%)	0.452 (−35.74%)	0.103 (69.07%)	1.63 (−389.49%)
Ni, [mg/kg]	0.545 (−10.77%)	0.526 (−6.91%)	0.546 (−10.98%)	0.566 (−15.04%)
Sb, [mg/kg]	>0.02 (>75%)	>0.02 (>75%)	>0.02 (>75%)	>0.02 (>75%)
Zn, [mg/kg]	0.871 (−308.92%)	0.079 (62.91%)	0.014 (93.43%)	3.7 (−1637.09%)
Sum of metals, [mg/kg]	78.677 (−1961.23%)	1.327 (65.23%)	0.933 (75.56%)	6.257 (−63.92%)
BTX
Benzene, [mg/dm3]	0.16 (23.81%)	0.75 (−257.14%)	0.06 (71.43%)	0.66 (−214.29%)
Toluene, [mg/dm3]	0.13 (−62.5%)	0.15 (−87.5%)	-	-
Ethylbenzene, [mg/dm3]	0.41 (−10,150%)	0.18 (−4400%)	0.01 (−150%)	0.02 (−400%)
m-xylene, [mg/dm3]	0.05 (−1150%)	0.02 (−400%)	-	-
p-xylene, [mg/dm3]	0.05 (−400%)	0.38 (−3700%)	0.01 (0%)	-
Isopropylbenzene, [mg/dm3]	0.04 (−1900%)	0.06 (−2900%)	-	-
o-xylene, [mg/dm3]	0.04 (−300%)	0.51 (−5000%)	0.01 (0%)	0.22 (−2100%)
Sum of BTX, [mg/dm3]	0.88 (−700%)	2.05 (−1763.64%)	0.09 (18.18%)	1.11 (−909.09%)

, which compiles data from all four tested coagulants at doses that achieved the highest removal efficiency.

When analysing metals and metalloids, all tested coagulants reduced Al and Sb concentrations to below the detection limit (<LDL). Coagulants with sulphate as the counter ion demonstrated higher efficiency in removing trace elements. However, these coagulants also led to cross-contamination of the treated wastewater with Ni and Mn. Specifically, PIX-100 introduced Cu into the solution, while PIX-116 introduced Zn. These findings suggest that while pure reagents can achieve high efficiency in heavy metal removal, industrial or technical grade coagulants may unintentionally introduce trace elements into the wastewater.

The impact of the coagulants on BTX and PAH content in UCG wastewater was also analysed. Table 11 presents only the BTX results, as PAH levels were measured to be <LDL in all samples. For BTX, the use of chemical coagulants led to inconclusive results. The slight increases in the concentrations of individual BTX compounds can likely be attributed to both the very low initial levels and the heterogeneity of the effluent. Overall, the coagulants did not exhibit a positive effect on BTX removal.

10.2. Electrocoagulation of UCG Wastewater

As previously introduced, general considerations lead to the conclusion that the processes of chemical and electrocoagulation, when performed using the same cation, differ mainly in their effect on pH (during EC pH increases). Consequently, EC results in a significantly lower amount of cross-contaminants introduced. A drawback, particularly important from a technological perspective, is that compared to chemical methods, EC leads to the formation of finer flocs and thus, even with longer sedimentation times, results in less effective spontaneous sedimentation. Similarly to chemical coagulation, the efficiency of batch EC experiments was also assessed based on the removal efficiency of cyanides, sulphides, heavy metals, and organic compounds (BTX, PAH).

10.2.1. Effect of the Dose of Fe

The first set of EC experiments was designed to determine the effect of Fe dose. During these tests, the electrode dissolution time was kept constant at 60 min, and the pH of the effluent was not adjusted.

The first two tests in this series were conducted using constant-voltage mode (the current changed as a result of changing conductivity of the solution), while the subsequent two tests were carried out in constant-current mode (voltage of the source controlled to maintain a constant current flow). As no significant differences were observed between the modes, and the constant-current mode was easier to control, the remaining EC experiments were performed keeping the former method of control.

Table 12. Effects of varying Fe dose at a constant reaction time.

	el_d60_i58_t60 _pH 8.5	el_d120_i115_t60_pH 8.5	el_d180_i172_t60_pH 8.5	el_d240_i230_t60_pH 8.5
Current, [mA]	58	115	172	230
Voltage min–max, [V]	2.79	4.51	6.46–7.25	7.82–8.97
Time, [s]	3600	3600	3600	3600
Measured Fe dose/difference from Faraday’s law, [mg Fe/dm3/%]	56.8/6.25	110.6/7.93	165.2/8.05	226.2/5.85
Parameters of the effluent after electrocoagulation
pH, [-]	8.74	8.91	9.06	9.18
Conductivity, [mS/cm]	1.8339 (−14.58%)	1.7657 (−10.32%)	1.6886 (−5.5%)	1.5938 (0.42%)
Redox, [mV]	−142 (−25.29%)	−151 (−33.24%)	−160 (−41.18%)	−166 (−46.47%)
CN−, [mg/dm3]	3.74 (60.59%)	2.51 (73.55%)	1.48 (84.4%)	1.03 (89.15%)
S2−, [mg/dm3]	0.395 (98.96%)	0.405 (98.93%)	0.306 (99.19%)	0.217 (99.43%)
COD, [mg/dm3]	247.2 (13.02%)	238.2 (16.19%)	227.2 (20.06%)	222.2 (21.82%)

Note: During this series of experiments, the cyanide in the effluent before coagulation was 9.49 mg/dm³ while as the baseline for the calculation of sulphide removal the value of 37.9 mg/dm³ was used (Table 3).

collates all data characterising the conditions of the tests as well as the measured parameters.

Working in a closed system, a steady increase in the pH of the wastewater was observed as the dose of Fe introduced increased. For cyanides, a clear effect of the coagulant dose on the reduction efficiency was observed. The highest cyanide removal efficiency achieved during this series of tests reached 89%, indicating that a dose of 225 mg Fe/dm³ reduced the concentration of cyanides below 1 mg/dm³. Even more substantial results were obtained for sulphide ions, where more than 98% reduction was observed for all tested process variables. The only slight difference occurred when EC was performed at higher pH levels. Further details regarding these experiments are discussed in subsequent sections of this work.

Figure 2 clearly shows that EC with a dose of approximately 55–60 mg Fe/dm³ can reduce cyanide concentration by over 60%. This outcome is nearly three times more effective than the best result achieved through chemical coagulation. Further increasing the dose to 230 mg Fe/dm³ enhances cyanide reduction to 90%.

An additional parameter demonstrating the beneficial effect of EC was the decrease in COD. However, in none of the tested process configurations did the COD reduction exceed 28%. Similar to chemical coagulation, the redox potential increased with lower Fe doses, while the conductivity of the solution increased across the full range of Fe doses tested. It is important to note that, due to the mechanistic effect of forming very fine flocs, the change in conductivity is not straightforward to analyse.

10.2.2. Effect of the Electrode Dissolution Time at a Constant Dose of Fe

Subsequently, a series of experiments was conducted to determine the effect of EC time. This series of tests indicated that as the time of electrode dissolution increased, the resulting concentration of cyanides in the treated effluent also increased, leading to a decrease in removal efficiency. However, for cyanides, sulphides, and COD, clear and overlapping minima were observed, suggesting that for a dose of 104–115 mg Fe/dm³, a process time of 60 min is optimal.

Table 13. Effects of varying reaction time at a constant dose of Fe.

	el_d120_i230_ t30_pH 8.7	el_d120_i153_ t45_pH 8.5	el_d120_i115_ t60_pH 8.5	el_d120_i77_ t90_pH 8.6	el_d120_i57_ t120_pH 8.7
Current, [mA]	230	153	115	77	57
Voltage min–max, [V]	8.50–9.86	5.50–5.93	4.51	3.48–3.69	2.63–2.81
Time, [s]	1800	2700	3600	5400	7200
Measured Fe dose/difference from Faraday’s law, mg Fe/dm3	104.2/13.26%	109/9.06%	110.6/7.93%	113.7/5.76%	114.9/3.51%
Parameters of the effluent after electrocoagulation
pH, [-]	8.99	9.04	8.91	9.02	9.03
Conductivity, [mS/cm]	1.7945 (−12.12%)	1.7643 (−10.23%)	1.7657 (−10.32%)	1.7537 (−9.57%)	1.7891 (−11.78%)
Redox, [mV]	−156 (−37.65%)	−157 (−38.53%)	−151 (−33.24%)	−157 (−38.53%)	−158 (−39.41%)
CN−, [mg/dm3]	2.93 (69.13%)	3.38 (64.38%)	2.51 (73.55%)	3.62 (61.85%)	4.14 (56.38%)
S2−, [mg/dm3]	0.771 (97.97%)	0.563 (98.52%)	0.405 (98.93%)	0.595 (98.43%)	0.996 (97.37%)
COD, [mg/dm3]	256.2 (26%)	254.2 (16.44%)	238.2 (16.19%)	261.2 (19.43%)	266.2 (23.11%)

collates the results of varying reaction time at a constant dose of Fe.

10.2.3. Determination of Main Effects and Interactions between Time and Dose

The EC studies performed were further analysed to determine the existence of main effects as well as interactions between the controlled variables. For the case of time and dose, a positive interaction effect was found for cyanide removal from UCG wastewater. As expected, the dose had the dominant effect in this process. For COD, a negative effect of time and a positive effect of dose were observed. Notably, in this system, the interaction effect of dose and time had the most significant impact on COD reduction.

The following Figure 3 and Figure 4 and

Table 14. Compilation of normalised values of main effects and interactions between time and dose of Fe on the efficiency of cyanide and COD reduction in UCG wastewater—electrocoagulation.

Effect	Removal of CN	Removal of COD
MEtime	4.42	−9.81
MEdose	7.16	1.73
Interaction.Eff.	3.69	2.14

graphically and numerically illustrate the results of these analyses.

10.2.4. Effect of Effluent pH

Furthermore, a series of studies was conducted to determine the effect of changing the pH of the treated wastewater. At higher pH levels, an increase in the amount of iron hydroxide flocs produced is expected; however, at the same time, the amount of free Fe ions available in solution decreases.

The data presented in

Table 15. Influence of varying the pH of the electrocoagulated wastewater—at constant dose and time of Fe dissolution.

	el_d240_i230_t60_pH 8.5	el_d240_i230_t60_pH 9.5	el_d240_i230_t60_pH 10.5	-
Current, [mA]	230	230	230	-
Voltage min–max, [V]	7.82–8.97	8.78–10.61	7.50–10.25	-
Time, [s]	3600	3600	3600	-
Measured Fe dose/difference from Faraday’s law, [mg Fe/dm3/%]	226.2/5.85	213.7/11.05	215.5/10.3	-
Parameters of the effluent after electrocoagulation
pH, [-]	9.18	10.07	11.01	-
Conductivity, [mS/cm]	1.5938 (0.42%)	1.7268 (−7.89%)	2.384 (−48.95%)	-
Redox, [mV]	−166 (−46.47%)	−219 (−93.24%)	−272 (−140%)	-
CN−, [mg/dm3]	1.03 (89.15%)	4.56 (51.95%)	4.89 (48.47%)	-
S2−, [mg/dm3]	0.217 (99.43%)	2.578 (93.2%)	4.038 (89.35%)	-
COD, [mg/dm3]	222.2 (21.82%)	239.2 (26.22%)	244.2 (24.68%)	-

show that at a final process pH of 11, the removal efficiency of sulphides and cyanides decreased from previously observed levels of approximately 89% and 99% to 48.5% and 89%, respectively. Therefore, while producing more flocs can be crucial for separating suspended solids or grease in other effluents, this approach proves to be less effective for UCG wastewater.

10.2.5. Efficiency of Electrocoagulation on Removal of Metals and BTX from Wastewater

During the batch EC tests, the removal of elements such as aluminium, manganese, and zinc was primarily observed (reduction to below the limit of detection). However, electrode dissolution, along with the excess Fe introduced, was also associated with a slight increase in Ni and Sn content in the treated effluent. The maximum recorded concentrations of these elements were 0.77 mg/kg and 0.096 mg/kg, respectively.

Using the data as an example, it can be determined that doses of 225–325 mg Fe/dm³ introduced into UCG wastewater resulted in a residual concentration of Fe in the treated effluent in the range of 2–3 mg Fe/kg. This is expected, as under basic conditions Fe primarily forms very poorly soluble hydroxides.

In the case of BTX, a 2–50% decrease in concentration was observed for benzene, toluene, and ethylbenzene. Compared to chemical coagulation, these results are more consistent and conclusive.

Table 16. Results on the efficiency of removal of trace elements and organic compounds—electrocoagulation.

	el_d240_i230_t60_pH8.5	el_d240_i306_t45_pH 8.5	el_d180_i345_t30_pH 8.7	-
Current, [mA]	230	306	345	-
Voltage min–max, [V]	7.82–8.97	10.12–11.97	11.64–12.65	-
Time, [s]	3600	2700	1800	-
Measured Fe dose/difference from Faraday’s law, [mg Fe/dm3/%]	226.2/5.85	227/5.31	162.2/9.98	-
Parameters of the effluent after electrocoagulation
pH, [-]	9.18	9.22	9.16	-
Conductivity, [mS/cm]	1.5938 (0.42%)	1.6005 (0%)	1.7334 (−8.3%)	-
Redox, [mV]	−166 (−46.47%)	−168 (−48.24%)	−166 (−46.47%)	-
CN−, [mg/dm3]	1.03 (89.15%)	2.43 (74.39%)	2.25 (76.29%)	-
S2−, [mg/dm3]	0.217 (99.43%)	0.103 (99.73%)	0.647 (98.29%)	-
COD, [mg/dm3]	222.2 (21.82%)	229.2 (24.65%)	250.2 (27.73%)	-
Trace elements
Al, [mg/kg]	<0.125 (>95.04%)	<0.125 (>95.04%)	<0.125 (>95.04%)	-
Fe, [mg/kg]	1.94 (−983.8%)	2.55 (−1324.58%)	2.99 (−1570.39%)	-
Mn, [mg/kg]	<0.2 (>39.94%)	<0.2 (>39.94%)	<0.2 (>39.94%)	-
Ni, [mg/kg]	0.748 (−52.03%)	0.77 (−56.5%)	0.686 (−39.43%)	-
Sb, [mg/kg]	0.085 (−6.25%)	0.084 (−5%)	0.096 (−20%)	-
Zn, [mg/kg]	<0.02 (>90.61%)	<0.02 (>90.61%)	<0.02 (>90.61%)	-
Sum of metals, [mg/kg]	3.118 (18.31%)	3.749 (1.78%)	4.117 (−7.86%)	-
BTX
Benzene, [mg/dm3]	0.204 (2.86%)	0.1 (52.38%)	0.2 (4.76%)	-
Toluene, [mg/dm3]	0.269 (−236.25%)	0.07 (12.5%)	0.04 (50%)	-
Ethylbenzene, [mg/dm3]	0.01 (−150%)	0.002 (50%)	-	-
m-xylene, [mg/dm3]	0.02 (−400%)	0.03 (−650%)	-	-
p-xylene, [mg/dm3]	0.02 (−100%)	0.01 (0%)	0.02 (−100%)	-
Isopropylbenzene, [mg/dm3]	0.01 (−400%)	-	0.009 (−350%)	-
o-xylene, [mg/dm3]	0.03 (−200%)	0.09 (−800%)	0.04 (−300%)	-
Sum of BTX, [mg/dm3]	0.563 (−411.82%)	0.302 (−174.55%)	0.309 (−180.91%)	-

collates the efficiency of removal of trace elements and organic compounds—electrocoagulation.

11. Conclusions

The experimental results indicate the potential of both chemical coagulation and electrocoagulation for treating wastewater from the UCG process. While the removal of sulphides, cyanides, trace elements, and BTX was demonstrated, the subsequent paragraphs summarise the findings and discuss their implications.

11.1. Chemical Coagulation

Four commercially available coagulants, differing in the valency of the iron ion (II or III) and the counter ion (chlorides or sulphates), were characterised for their efficiency in the chemical coagulation of UCG wastewater. The following conclusions can be drawn from these experiments:

• For all tested doses and forms of Fe used in the coagulation process, only a slight decrease in cyanide concentration was observed. The maximum reduction of 24% was achieved with PIX100 (FeCl₂) at a dose of 185 mg Fe/dm³.
• The collected experimental data indicate that the upper limit of cyanide removal efficiency from UCG wastewater for all tested coagulants lies in the range of 20–24%.
• None of the tested process configurations were able to reduce the concentration of cyanides in the solution to below 10 mg/dm³.
• The optimum pH range for the use of all tested coagulants was determined to be 4–9.
• For the maximum tested doses of coagulants, effective coagulation, flocculation, and sedimentation were observed, resulting in residual Fe concentrations in the solution below the limit of detection (LDL).
• All tested coagulants led to a decrease in Al and Sb concentrations to below LDL; however, they also introduced secondary contamination with Ni and Mn. Additionally, PIX100 introduced Cu into the solution, while PIX116 introduced Zn.
• Higher trace element removal efficiency was observed with the sulphate coagulants.
• None of the tested coagulants demonstrated a clear effect on BTX removal.
• One of the risks associated with chemical coagulation for the removal of cyanide or sulphide is the need to apply very high doses of the coagulant. This not only introduces secondary pollutants but also counter ions into the wastewater. It is important to note that the sulphates and chlorides introduced along with the iron can lead to concentrations that exceed permissible limits, necessitating further treatment before the effluent can be safely released.

11.2. Electrocoagulation

Compared to the chemical coagulation, EC demonstrated the following advantageous characteristics:

• A dose as low as 60 mg Fe/dm³ led to over 60% cyanide reduction and more than 98% sulphide removal efficiency.
• The highest sulphide removal efficiency achieved was over 99.7%, with a residual S²⁻ concentration of 0.103 mg/dm³, at a dose of 240 mg Fe/dm³.
• Performing EC at a starting pH higher than 8.5 resulted in reduced removal efficiency for S²⁻ and CN⁻, likely due to decreased availability of free Fe ions in solution.
• Increasing the dose of Fe from 60 mg/dm³ to 240 mg/dm³ improved cyanide removal efficiency from 60% to 90%, resulting in a residual concentration of 1.03 mg/dm³.
• For COD, the highest removal efficiency achieved was only 26%.
• During batch EC experiments, the treated wastewater became enriched with Ni and Sn, which may be related to the components of the electrodes used. Other trace elements remained at levels below LDL.
• EC was effective in reducing Zn, Al, and Mn present in UCG wastewater. Notably, Mn was one of the contaminants introduced by chemical coagulants. Doses higher than 220 mg Fe/dm³ led to the complete removal of Zn.
• EC resulted in an increase in Fe content in the solution, which was dose dependent. The maximum concentrations of Fe in the treated effluent were measured at 2–3 mg/dm³. Given the production of insoluble iron hydroxides, effective flocculation and filtration should help manage this contamination effectively.
• For BTX, reductions of up to 50% were observed in benzene, toluene, and ethylbenzene. These results were more consistent than those achieved with chemical coagulants, although the residual concentrations of organics in UCG wastewater remained very low.

Since the two techniques were compared primarily in terms of their efficiency in removing cyanides and sulphides, we further elucidated these results by considering the mechanistic aspects of their removal.

For both chemical and electrochemical coagulation, the removal of sulphide ions generally involves the precipitation of metal sulphides, which are then coagulated by metal ions. In contrast, the mechanism of cyanide removal during chemical coagulation is more complex and can involve multiple intermediate products and pathways. One commonly discussed route for chemical coagulation is the adsorption of cyanides onto coagulant flocs; however, this mechanism typically achieves an efficiency of no more than 25%. Although similar discrepancies between the efficiency of cyanide removal through chemical and electrocoagulation techniques have been observed for other effluents [22], in UCG wastewater the well-established efficiency of ferrous sulphates could not be confirmed [40].

In summary, for the pretreatment of UCG wastewater prior to phytoremediation, electrocoagulation has demonstrated distinctive advantages. This technique generally provides better removal efficiency for various components while using significantly lower doses of coagulant. Moreover, due to the lack of possibility for cross-contamination of the effluent (e.g., introducing heavy metals and increasing its salinity at high doses of the coagulant), it is also intrinsically safer from the environmental perspective.

In the UCGWATERplus project, the research on electrocoagulation was part of a broader study aimed at evaluating the potential of a hybrid wastewater treatment system that integrates electrocoagulation, constructed wetlands, and solid-phase adsorption. Further results from this research are documented in the literature [36,37,41,42].