Efficiency of a flocculent consisting of bentonite clay and fly ash for the removal of pollutants in AMD

Acid mine drainage (AMD) has globally been identified as a serious human and environmental hazard, particularly in areas in the neighbourhood of operating and ceased mining operations. Its detrimental effect is attributed to high sulphide content resulting from minerals when rocks are exposed to oxidizing conditions mostly in coal and gold (Finkelman 2007), including copper mining and the chemical industry. A large quantity of AMD is generated during mining and floor cleaning, thus posing a danger to the ecosystem; that is, discharging hazardous inorganic trace elements emerging from various mining operations and combustion (Cravotta 2008; Silva et al. 2010). Sulphide minerals present in the AMD also play an enormous role in environmental degradation through the sulphide oxidation process (Giere et al. 2006). AMD is characterized by a low pH, high salinity levels, broad range of toxic metal ions and high sulphate, iron, aluminium and manganese content. Pyrite (FeS₂) and pyrrhotite (FeS) are sulphide species commonly found in coal (Kuyucak et al. 2006). Their existence in AMD is attributed to oxidization during exposure to oxygen in an aqueous medium to form ferrous ions and sulphuric acid, a process that occurs underground where mining takes place and overflows to the surface after heavy rainfall in the form of acid water, hence termed AMD.

As a consequence, its quality does not qualify to be discharged to any water body without pre-treatment, an exercise that has been demonstrated to incur extra costs to existing operations. There are a number of research projects that have been exploited employing various technologies such as precipitation, distillation, reverse osmosis, adsorption, bioremediation, wet oxidation, advanced oxidation, carbon nanotubes, coagulation-electro oxidation, neutralization and Fenton's reagent, and so on (Ntwampe 2013; Skousen et al. 2016; Ntwampe et al. 2015a, 2015b; Ntwampe et al. 2016a; Ntwampe et al. 2017a). Various challenges were identified from some effective technologies whereas others exhibited high operating costs or required costly, sensitive and sophisticated equipment that needs highly technical and skilled manpower (Ntwampe 2021a). Based on the afore-mentioned challenges, it is imperative to continue exploiting alternative reagents which are cost-effective in order to promote AMD treatment to the mining and chemical industries. Such outcomes may play a very pivotal role particularly to rural communities where access to water and chemicals is a challenge; as that will be a relief as the primary source of potable water is unanalysed groundwater from boreholes (deep wells), shallow wells and springs (Dzwairo et al. 2006) which may contain water-borne hazardous substances.

In view of the fact that most emerging technologies focus more on commercial reagents, other types of reagents such as natural and waste materials have also exhibited positive prospects. Clay, dust and coal ash have been identified as some of cost-effective materials; provided they are applied in a form of mixed reagent (Ntwampe 2016a). However, a combination of bentonite clay and fly ash has not been conducted. Similarly to AMD, fly ash poses health hazard to humans and environment as it leaches from the stockpiles containing high concentration of silica and calcium; which can elevate the pH to 12. It is considered as ferro-alumino silicate consisting of glass spheres with tiny particles size in a range of 20–80 μm hosting various elements such as Ca, K, Na, Fe, Al and Si within a matrix (Mattigod et al. 1990). Chemical composition of fly ash is actually dependent upon the mineralogy of the parent coal, whereby the most predominant mineralogy of the coal used in this experiment includes iron oxidees, alumina, calcium oxide silica with a varying quantity of unburned carbon; including high amount of alkalinity emanating from a fraction of lime. The danger emanating from fly ash is leachate from disposed slurry after wastewater treatment and heavy rainfall as it is piled up to form a heap of harden rock after carbon extraction during combustion. Most of the leachate includes toxic metals, cations and anions from ash heaps which is detrimental to the ecosystem (Gitari et al. 2006).

On the other hand, bentonite clay which is mined in South Africa generally contains both calcium and magnesium montmorillonite. Clay has previously been evaluated as a reagent for turbidity adsorption (Ntwampe et al. 2016a). Montmorillonite has a large specific surface area with a net specific charge and is responsible for the large exchange capacity of the clay (Syafalni et al. 2013). It is also characterized by its bonding and swelling properties including plasticity which enable it to react with other substances easily (Luckham & Rossi 1999). Montmorillonite is composed of a central sheet of octahedral aluminium ions between two tetrahedral sheets (T-O-T) of silicon ions; these sheets are linked by oxygen atoms. The configuration of the bentonite clay is responsible for its efficiency during adsorption/absorption (Syafalni et al. 2013). The application of flocculent-B and C in this study is based on the removal of toxic metals which include As, Co, Zn and Pb; the rationale being based on their detrimental effects to humans and the ecosystem (Benzaoui et al. 2017).

The objective of this study is to investigate the cost-effectiveness of simplified synthetic flocculent prepared by a combination of bentonite clay and fly ash in AMD treatment without pH adjustment. The rationale behind the study is to determine sorption capacity and physico-chemical effects of the mineralogical composition of fly ash for the removal of turbid materials, organic compounds and toxic metals in acidic wastewater (AMD).

The sample used for the experiment was collected from Krugersdorp (South Africa) in a 25 litres plastic drum. The sample was air-tied and stored at room temperature. The quality of the sample was modified by adding 3.23, 2.5, 2.8 and 8.9 g/moL of As, Co, Zn and Pb respectively. The pH, conductivity and turbidity of raw AMD sample were 2.25, 6.27 mS/cm and 276 NTU respectively. The sample contained the following major elements: Cu, Zn, Co, Ni, Mn, Ti, Pb, Al, Fe, Se, Na, Mg, Ca and K as shown in Table 1.

The physical and chemical properties of the AMD, namely the pH, EC and turbidity are illustrated in Table 2.

Bentonite clay was obtained from the Yellowstar Bentonite mine, a bentonite mining and supplying company situated in Parys in the Free State (SA). The bentonite was mined from a quarry located in Koppies (SA). The pH, conductivity and turbidity of bentonite clay solution before processing were 2.79, 2.71 mS/cm and 43.9 NTU respectively. Chemical composition is shown in Table 3.

Coal fly ash (CFA) samples were collected from power generation utility (Lethabo Power Station) situated in the Northern Free State Province in South Africa and stored in tightly in lockable plastic containers.

In the first set of experiment, bentonite clay (BC) and fly ash (FA) were pulverised to 160 μm particle size respectively and rinsed with distilled water to remove impurities using 0.45 μm pore membrane (flocculent-A). Two other flocculants (flocculent-B and C) were prepared by pulverising the same reagents to the same size and mixed. The sample (flocculent-B) was rinsed using distilled water whereas flocculent-C was rinsed using tap water; both with FA:BC ratio of 3:1 as obtained from the trail test (flocculent-A) (Table A1). The solid mixtures were then inserted in an oven separately to dehydrate it at 60 °C for 24 hrs, after which the mixtures were cooled and pulverised to the same particle size.as being ready for dosage. The rationale for rinsing the AMD sample using demineralised water was to remove unwanted salts in the sample, whereas the use of potable water was to remove salts using a lenient method. The pH, EC and turbidity of the flocculent were 5.2, 6.4 mS/cm and 109 NTU respectively.

In this study, coagulation-flocculation treatment was conducted by pouring 200 mL of AMD sample into five 500 mL glass beakers and dosed with a varying amounts of reagents of flocculent-A as mentioned above. Two similar sets of experiments were conducted by dosing AMD samples with 1.0–3.0 g of a flocculent-B (FA:BC of 3:1) and similar dosage to AMD samples (flocculent-C). The samples were treated in a jar test at 250 rpm for 2 minutes and reduced to 100 rpm for 10 min, allowed to settle for 1 hour after which the pH, conductivity, turbidity and toxic metal content were measured, see below. The choice of aforementioned rpm were based on the standard procedure for a jar test (Satterfield 2005).

Flocculent-A – this experiment served as a trial to determine optimal dosage to consider when conducting actual research experiments stated below. In this experiment, AMD samples were dosed with increasing (1.0–3.0 g bentonite clay) and decreasing (3.0–1.0 g) fly ash to determine the best FA:BC ratio for the removal of pollutants in the AMD sample.

Flocculent-B – this experiment involved treatment of AMD using 20–60 mL dosage of a flocculent consisting of a combination of bentonite clay and fly ash rinsed with distilled water, as determined in a trial test afore-mentioned.

Flocculent-C – this experiment involved treatment of AMD using 20–60 mL dosage of a flocculent consisting of a combination of bentonite clay and fly ash rinsed with tap water as determined in a trial test aforementioned.

A MetterToledo Seven Multimeter (made in Germany) pH meter with an electrode filled with 09u; the tip was used. The equipment was calibrated with standard solutions with the pH of 4.0 and 7.0 before use.

An EDT instrument FE 280 conductivity-meter (made in Japan) was used and calibrated with a 0.1 KCl standard solution.

A Merck Turbiquant 3000T Turbidimeter (made in Japan) was used to determine turbidity or the suspended particles in the supernatant using NTU as a unit of measure. It was calibrated with 0.10, 10, 100, 1,000 and 10,000 NTU standard solutions.

Inductively Coupled Plasma (ICP), a Perkin Elmer Optima DV 7000 ICP-OES Optical Emission Spectrometer (made in USA) was used to measure the concentration of the toxic metals in the supernatant. Upon anticipation of a high concentration of the toxic metals, the ICP equipment was calibrated with a standard solution between 100 and 1,000 ppm of the salts.

Ion chromatography Dionex ICS 5000 (Sunnyvale, made in US) equipped with IonPac AS12-AS anion column and suppressed conductivity detector to measure the chlorides and sulphates.

A KYKY-EM3200 Digital Scanning Electron Microscope (model EM3200) (made in China) was used.

The x-ray diffraction (XRD) patterns of the samples were analysed using a Rigaku Miniflex II Desktop x-ray diffractometer (made in Japan) with Cu Kα radiation. A step size of 0.02° at a speed of 4° (2θ)/min over 10–80° was applied. The minerals in VM treated between 950 and 1300 °C were quantified by Siroquant software.

Equilibrium adsorption is defined by an isotherm equation whereby the surface properties and attraction of the adsorbent are expressed by its parameters. The Freundlich and Langmuir isotherms are applied to determine the relation between the amount of the adsorbate sorbed by the adsorbent and their equilibrium concentrations in aqueous solution. The Langmuir isotherm is applied to determine whether the one monolayer only shaped through reaction, existence of corresponding positions, fixity of adsorbate and absence of an interaction between adsorbate-adsorbent. The Langmuir isotherm reflects high sorption while the Freundlich isotherm reflects the infinity position (Ugwu & Igbokwe 2019) when there is variance between two models.

Figure 1 represents the pH, EC and turbidity of AMD treatment dosed with flocculent-A, flocculent-B and flocculent-C that is, unprocessed increasing bentonite clay and decreasing fly ash; a combination of processed bentonite clay and fly ash rinsed with distilled water and tap water respectively.

The pH of the AMD samples (flocculent-A) with increasing trend of bentonite clay and decreasing fly ash (i.e. 1.0–3.0 and 3.0–1.0 respectively) showed an increasing trend from 5.2 (raw AMD) to a range of 2.8–4.2, which is indicative of the potential capacity of the fly ash in the neutralization of the AMD. Such a decreasing trend of the pH with increasing dosage of basic fly ash is attributed to the presence of metal oxide such as CaO and MgO. Precipitation of some of the metals in the AMD such as iron and aluminium contributes extensively to the removal of pollutants. Relating removal efficiencies of the pollutants to destabilization-hydrolysis, a precursor of sorption mechanism, Fe present in AMD undergoes hydrolysis to form four hydrolysis species. These species range from cationic to anionic state, providing iron with a reaction efficiency covering both cations and anions. The pH range for flocculent-C (4.0–4.9) falls within a range suitable for co-existence of all four species, namely Fe³⁺, Fe(OH)²⁺, Fe(OH)₂⁺ and Fe(OH)₃ in aqueous solution (Flynn 1984; Ntwampe et al. 2017b). The presence of these species guarantees optimal removal of the pollutants (Ntwampe 2019; Ntwampe & Moothi 2019) as reflected in Figure 1 and Table 4. The observation citing adsorption as one of the mechanisms confirms that removal of pollutants in this study is a physico-chemical phenomenon. Nonetheless, the distribution of the species is controlled by equilibrium equations of the low molecular weight hydrolysis species; whereby those formed during hydrolysis have an area of dominance with some overlap (Kamaly et al. 2016; Ntwampe et al. 2016b). On the other hand, aluminium hydrolysis also contributed to the decreasing trend of the pH with increasing dosage of fly ash. Although aluminium chemistry is not as explicit as iron, the first species, a small fraction of Al(OH)²⁺ is formed at pH range 3–6; then another species (Al(OH)₂⁺) emerges at pH within 4 and 7 followed by the most dominant insoluble species, Al(OH)₃ in a pH range 4–9; normally acting as a buffer in the region of 3.5–4.0 (Uhlmann et al. 2004). Ubiquity of pyrite as an iron mineral in the metal ores is a main source of acid generation in the mines together with Ca/Mg carbonate minerals, which are contributory sources to pH regulation due to their high dissolution rate, which is faster than sulphide oxidation (Huang et al. 2011). Following the pH changing trend (Figure 1), it is evident that the buffering effect is attributed to the basic condition by the presence of carbonates when reacting with sulphides in the AMD.

On the other hand, EC, which is a measurement of the total dissolved solids in the solution, is also associated with the ionic strength of the colloidal suspension and the solubility of the solute(s); this is indicative of the rate of reaction. According to Figure 1, EC showed an indirect proportional changing trend with fly ash; that is, it increases with decreasing fly ash from 5.47 mS/m (raw AMD) to a range of 3.1–4.4 mS/cm. This also shows that the flocs formed during hydrolysis as reflected by the decreasing trend of the pH reduced ionic metals, inorganic matter and other pollutants present in the AMD (Table 1). On the contrary, the pH of the samples with flocculants-B and C showed a direct proportional changing trend (3.9–2.7 and 4.1–2.6 respectively) with the fly ash; this is attributed to increasing amount of fly ash the dosage was maintain in an increasing trend. The results showed the EC of the samples with flocculent-B was slightly lower compared to those of samples with flocculent-C; an observation suggesting that rinsing with distilled water demineralized some of the pollutants. This trend was also observed in the EC of the samples with flocculant-B and C, where similar physico-chemical reactions occurred.

Residual turbidity of the samples with flocculent-A showed an increasing trend with increasing dosage of flocculent-A, from 232 NTU (raw AMD) to a range of 12.1–14.8 NTU. On the other hand, residual turbidity of the samples with flocculants-B and C is slightly lower compared to the corresponding samples with flocculent-A. However, residual turbidity of the samples with flocculent-C is lower compared to that of the samples with flocculent-B. It is suggested the observations obtained from the samples with flocculent-A and B were attributed to limited rate of reactions between the flocculants and the AMD samples. In the case of flocculent-A, which was unprocessed, the mixture between bentonite clay and fly ash lacked sorption bondage, which is a combination of the physico-chemical properties of both reagents. On the other hand, the efficiency of flocculent-B was negatively affected by the reduced reactivity of a reagent as some of the constituents were leached in distilled water during rinsing. Flocculent-C, which was rinsed using tap water, released impurities and absorbed some salts present in tap water. According to residual turbidity (Figure 1), the samples with FA:BC 3:1 and 2.5:1.5 (flocculent-C) corresponding to pH 4.9 and 4.0 yielded the lowest residual turbidity of 6.2 and 7.8 NTU, respectively; however, corresponding samples with flocculent-B yielded a higher residual turbidity of 10.4 and 11.6 NTU, respectively. The results shown by the combination of bentonite clay and fly ash (flocculent-A, B and C) clearly indicate that they couldn't yield high removal for turbid materials (Fig. A1). Bentonite clay and fly ash dosed separately yielded poor residual turbidity in the ranges 143–184 and 150–192 NTU, respectively. Based on insignificant changing trends of the pH and conductivity in treated AMD using bentonite clay and fly ash, poor removal for turbidity and toxic metal is probable (Fig. A1). Pulverization to optimal particle size plays a pivotal role as it liberates minerals bound together in complex inactive compounds to simple reactive minerals that stimulate physico-chemical reactions from the primary stage of destabilization-hydrolysis, adsorption, co-precipitation, inter-particle bridging and sedimentation (Ntwampe 2013; Ntwampe 2021a). Optimal turbidity reduction (Figure 1) suggests that plasticity and swelling are the main properties of bentonite clay, including basicity and oxidation potential, which characterise fly ash. Apart from adsorption capacity of bentonite clay, in the presence of SiO₂, Al₂O₃, Fe₂O₃, CaO₂ are prone to form a complex reactive complex, particularly in the presence of bentonite clay characterized by a porous and reactive surface. These chemicals reacted during drying in an oven and behave as physico-chemical catalysts due to their specific surface area including that for unburned carbon (0.1–0.5 m³/g and 10²–4 × 10⁴ m²/g respectively) (Wang et al. 2018) and the oxide component (Figure 1). The performance of the fly ash is ameliorated by aforementioned factors, which include optimal sorption of the pollutants due to high specific areas (Ram & Masto 2014) and high density of the fly ash (1.0–1.78 g/cm³), higher than the liquid medium, as they contributed towards optimal sorption and embedment of the pollutants during settling due to high settling velocity. Unburned carbon also plays a role during physico-chemical reactions in the system as it has a high electronegativity of 2.5, which is indicative of high reactivity. It is also suggested that its mineral content, such as SiO₂ (existing as both amorphous and crystalline forms), Al₂O₃ and CaO (Vassilev & Vassileva 2007; Ahmaruzzaman 2010) are also attributed to optimal removal of pollutants. Based on the collaboration between the pH and residual turbidity (Figure 1) it is evident that demineralization of a reagent reduces the efficiency of the removal of pollutants in the wastewater. The observations also reveal that the pH values of the FA:BC ratio 3:1 and 2.5:1.5 are typically ideal for the removal of pollutants in the AMD; this is also indicative of lower solubility of toxic metals employing both FA:BC ratios and pH range 4.0–4.9. It also implies that solubility product of the metal hydroxides was constant at these pH ranges, resulting in decreasing solubility of the soluble pollutants (metals and toxic metals), Table 4.

The removal efficiencies of metals associated with temporary hardness; that is, Ca²⁺ and Mg²⁺ removal treated with 20 mL of FA:BC ratio 3:1, flocculent-B showed a reduction from 197 to 112 and 82 to 45 mg/L, relatively moderate removal efficiencies (55.8 and .59.6% respectively). It has to be noted that in cases where some of the metals do not show significant removal, that may be attributed to leaching of those metals. On the other hand, the removal efficiencies of metals associated with permanent hardness (Cl⁻ and SO₄²⁻) treated under similar conditions was 68.9 and 89.1% respectively. Such a high removal of SO₄²⁻ can be associated with dissolution of CaO present in fly ash and subsequent formation of gypsum; higher pH of the same FA:BC ratio is indicative of dissolution of CaO in fly ash. In the case of Cl⁻ removal using flocculent-C, where Cl⁻ ions migration is attributed to capillary suction and diffusion effect; a similar conditions occurring on an alkaline concrete structure (Zeng et al. 2011). Three main characteristics associated with fly ash, namely pozzolanic, morphological, and microaggregate filling effects, whereby the former is triggered by Ca(OH)₂ formed during fly ash hydration. Secondary hydration reaction between the activated ingredients, such as SiO₂ and Al₂O₃, in fly ash, and Ca(OH)₂ produces C–S–H gel, calcium aluminate hydrate products, and so on. These products may absorb more free chloride ions, reducing the deposition of chloride ions in sludge (Huang et al. 2017). This also suggests that chloride binding capacity is improved with the addition of fly ash; otherwise the hydrates can fill large size pores in the flocculent matrixes and thus reduce porosity, narrow pore diameter, and block the connectivity of the pores, all of which subsequently slow the diffusion and migration of chloride ions (Huang et al. 2020). In the case of bentonite clay, pollutants removal occurs by ion exchange, porous surface, intercalation, cations exchange capacity, adsorption processes including natural organic compounds. The efficiency of bentonite for the removal of pollutants is stimulated by the presence of mineral salts (Sato et al. 2016) and fly ash (Figure 1; flocculent-C); and that inference is invoked by the observation obtained in the study by Huang et al. (2020) where the bonding effect of limestone calcinated with clay was investigated. A fraction of metals particularly divalent and trivalent such as calcium, magnesium, iron, aluminium, etc., are subject to those physico-chemical reactions. The removal of sulphate was coupled with the formation of gypsum incorporated into Al, Fe-oxyhydroxysulfates. Such a formation enabled easy sorption by the flocculent-C, particularly by intercalation and adsorption attributed to bentonite clay (Ntwampe 2020).

Table 3 illustrates the concentrations of the metals in The AMD after treatment using flocculent-C. The selection of these samples over those from the other two experiment was based on focusing on the objective of the study and limit number of tables.

The results (Table 3) showed an optimal removal of mineral salts in the AMD using flocculent-C, also related to pH suppression by fly ash constituent aforementioned. It is clear that efficient removal of pollutants is not only attributed to the sorption capacity of the flocculants, but also metal precipitation as reflected by the increasing rate of removal with increasing dosage ratio of FA:BC, residual turbidity ranging from 12.8 to 6.2 (Figure 1). The removal efficiencies of non-toxic salts such as Ca, Mg, Cl⁻ and SO₄²⁻ (Table 3) confirm that their compounds contributed to turbidity showing to be 84.2; 79.2; 87.9 and 89.2% respectively. Toxic metals also showed high removal efficiencies, suggesting that their removal is attributed to several mechanisms, namely mineral phase precipitation and sorption to high surface area Fe and Al oxyhydroxy sulphates more likely to be associated with attenuation of toxic metals such as Cu²⁺ and Zn²⁺ species, an observation invoked by Gitari (2014). Contrary to the available findings (Britton 1956) the results showed that the pH of minimum solubility of the hydroxides/oxyhydroxides of Fe³⁺, Fe²⁺, Al³⁺, Zn²⁺, Cu²⁺ and Ni²⁺ is in a range 2.5–4.6 (Figure 1).

Figures 2 and 3 represent an adsorption isotherm applying Langmuir and Freundlich models for Cu, Zn, Co and Pb respectively. Based on the limited graphs required, it was decided to represent those four toxic metals as they are classified as some of the metals that are toxic to aquatic life (Ntwampe 2021b).

Adsorption behaviour of flocculent-C using the Langmuir model could also not be predicted due to the lack of fitness of the experimental data, and that was substantiated by low R² for Cu, Zn, Co and Pb (i.e. 0.893, 0.816, 0.729 and 0.681 respectively).

The adsorption behaviour of flocculent-C using Freundlich model predicted because of the fitness of experimental data. The R² for Cu, Zn, Co and Pb (i.e. 0,996, 0,994, 0,99 and 0,989) are close to unity which also indicates that the adsorption process was favourable under prevailing conditions, and the thermodynamic process revealed that the process by flocculent-C was spontaneous and the adsorption was endothermic and physical in nature.

Figure 4 is a graph representing adsorption capacity of flocculent-C applying pseudo second-order plotting the ratio of time and adsorption vs. contact time whereas the correlation regression (R²) was determined using both x and y values. Pseudo-first order could not be presented due to limitation of the graphs; however, it is not an ideal fit for experimental data. The pseudo-second order parameters q_e and k₂ were calculated from the intercepts and the slope of t/q_t and the time.

The pseudo-second order model showed rate equation R² values for Cu, Zn, Co and Pb adsorption onto flocculent-C were close to unity, i.e. 0.995, 0.992, 0.996 and 0.984 respectively. On the other hand, the pseudo-first order model showed unfavourable adsorption capacity under prevailing conditions.

Table 5 shows the parameters of AMD with flocculent-C applying Langmuir and Freundlich models.

Table 6 shows the parameters of AMD with flocculent-C applying pseudo-second order.

Figure 5 shows the removal efficiencies of toxic metals (Cu, Zn, Co and Pb) present in the AMD sample using flocculent-C.

Figure 7 illustrates the adsorption of toxic metals onto flocculent-C (adsorbent) with different contact times (0–100 min) using 120 mg sample. The retention for Cu ions onto the adsorbent increases with increasing contact time and reaches a saturation phase at around 60 minutes, from which time the retention becomes almost constant. This was typically a two-phase process, the first phase, contact time less than 60 minutes whereby adsorption is attributed to the availability of a large number of active sites as well as rapid diffusion of metal ions, from the solution to the surface of the solid. In the second phase, the adsorption reached saturation equilibrium of the adsorbent sites on the adsorbent. Saturation phase for Zn ions was around 80 min, Co and Pb around 80 and 70 min respectively. A similar explanation regarding adsorption mechanisms is similar to that stated about Zn ions afore-mentioned.

Figure 6 shows the XRD graph of the sludge of the AMD treated with flocculent-C using FA;BC ratios 3:1 and 2.5:1.5 respectively, as stated that the dosage ratios are effective for the removal of turbid materials.

Both XRD graphs showed two characteristic peaks at 2θ positions of 21° and 38° Figure 6(a) and 6(b) respectively. Figure 6(a) is within intensity of 1,000–2,000 counts whereas Figure 6(b) is at the same characteristic peak position within intensity of 1,000–1,700 counts.

Figure 7 shows the SEM images of the AMD solution treated with flocculent-B and C.

The SEM microgram (Figure 7(a), flocculent-B) showed large ‘sponge-like’ flocs brought together to a dense hard structure. There are few little voids around the structure which is indicative of rigid cohesion between bentonite clay and fly ash. On the other hand, the SEM microgram (Figure 7(b), flocculent-C) also showed a cake-like structure with small and large cake-like porous and non-porous sites spread throughout the slide. The micrograph shows porosity and crystalline view throughout which suggests that there are still some pores ready for sorption. SEM micrograph for flocculent-A could not be included in this paper due to limited graphs required, which also did not exhibit any significant purpose as it showed poor sorption capacity.

The objective of this study was to determine the effect of fly ash when mixed with bentonite clay for the removal of pollutants present in AMD. The study involved two main sets of experiments involving processing of the mixture using distilled water and tap water respectively; and compares their efficiencies. The preparation of the flocculent was based on FA/BC mass ratio; whereby each reagent exhibited its efficiency based on characteristics.

The observations showed that increasing FA/FC mass ratio of fly ash in a flocculent is directly proportional to removal efficiency of the pollutants. The basic property is one of the factors that ameliorated metal solubility suppression as shown by removal of toxic metals, including the removal of permanent hardness (chloride and sulphate). Basic component of fly ash contributed to hydrolysis of iron and aluminium salts; it also has high turbid materials removal efficiency (increasing mass ratio yielded increasing rate of removal). On the other hand, bentonite contributed to the removal of non-toxic metals by ionic exchange and CEC. Demineralized water used for rinsing of the reagents during processing posed a negative impact in sorption capacity as it removes reactive minerals in the reagents (bentonite clay and fly ash). Tap water added to sorption capacity of the flocculent when using it for rinsing during processing, particularly when the reagents contain mineral salts as they contribute during physico-chemical reactions. Toxic metals present in the AMD sample reached saturation point after 60 min depending on the adsorption capacity of the adsorbent (flocculent-C). That can also be explained that adsorption capacity of the adsorbent exhibited its optimal efficiency after 60 min. Based on the effects of porosity, plasticity and swelling, which characterise bentonite clay, they contributed to sorption efficiency (SEM), which also showed that adsorption is a physico-chemical phenomenon.

All relevant data are included in the paper or its Supplementary Information.