Characterization of Sorbents and Neutral Mine Drainage

We used ground fine bentonite obtained from the Kopernica site, zeolite obtained from Nižný Hrabovec, and stabilized dewatered digested sludge (SDDS) obtained from the wastewater treatment plant in Vlkanová. Basic criteria were chosen for the sample collection - low humidity and fineness of the samples. The samples had to be odorless - the smell could indicate the presence of another material (bentonite, zeolite) or insufficient stabilization (sludge).

Stabilized sludge was used specifically to remove pathogenic microorganisms that could cause hygienic complications when this sorbent is applied to the aquatic ecosystem. The sludge was dried and sieved to a fraction below 2 mm. After stabilization, the excess sludge is dewatered on sludge presses after the anaerobic stage. After this filtration, all its storage and disposal.

Neutral mine drainage was taken from the Voznická dedičná štôlňa adit in Štiavnica-Hodruša mine district (Central Slovakia) - 48º27'N and 18º42'E. Generated neutral mine drainage represents a relict of mining processes in the area. The estimated total historical production of the mines in the mining district is estimated at 4 000 t of Ag and 80 t of Au. Base metal mining, which was active from the 19^th century until 1992, yielded approximately 70 000 t of Zn, 55 000 t of Pb, and 8 000 t of Cu²⁺ [42]. The average pH over 2 years in neutral mine drainage was 7.24.

For sorption processes, we used solutions of mine water with modified concentrations of Cu²⁺ and Zn²⁺ for the needs of 5 input concentrations. Mine drainages are environmental pollutants that flow into the aquatic environment. Chemicals for adjusting the input concentration of the samples were obtained from the Technical University of Zvolen.

Characterizations of sorbents and neutral mine water are summarized in Table 1 and Table 2.

The crystal size microscopic images of the used sorbents 37 x 40 x 60 µm for zeolite, 500 x 200 x 80 µm for sludge, and 1100 x 1000 x 630 µm for bentonite, measured with a VHX-7000 digital microscope (Keyence).

Sorption of Heavy Metals

The adsorption was carried out at 27ºC and 10ºC in Erlenmeyer flasks. The sample volume was 100 ml in each flask. Before sorption, a 500 ml solution was made for each input concentration – a total of 5 input concentrations. We took 100 ml to determine the initial concentration. Then 100 ml of solution was added to each flask and mixed with 5 g.dm^-3 of sorbent. After 30, 60, 90, and 120 minutes, the sorption was stopped by filtering out the sorbent in individual bottles and then the concentration of metals – copper or zinc was measured. Temperature reduction to 10ºC was carried out in an incubator. The sorption of each input concentration was repeated 5 times for the relevance of the evaluation – a total of 6 measurements were made for each input concentration for each sorbent at each temperature. The addition of CuSO₄ was used to vary the input concentrations for Cu sorption. For changes in input concentrations for Zn sorption the addition of ZnCl₂ was used. Sorption was carried out by continuous stirring to ensure better contact with the aqueous phase with the sorbent.

Determination of Heavy Metal Concentrations During the Sorption Process

The concentrations of metals were determined by atomic absorption spectrometry (AAS). An AAS AVANTA Σ flame atomization spectrometer (GBC Scientific) was used for the determination of metals. A hollow cathode lamp with a supply current of 3.00 mA was used as the radiation source. Air/acetylene was used as the flame type at flow rates of 11.50 dm³. min^-1 for air and 1.10 dm³.min^-1 for acetylene. The relative errors of the AAS measurements were less than 5 %. The instrument operation as well as the evaluation of the results was carried out with the GBC Avanta software ver. 2.0.

Determination of Specific Surface

The determined each sorbent̕s specific surface according to Blaine – ČSN EN 196-6. The fineness of materials is expressed as the specific surface calculated from the time required for a given amount of air to flow through a compacted bed of a measured material of a given size and porosity. The method̕s principle is based on comparing the time taken for a defined amount of air to pass through a barrier so prepared. This time is then compared with the time for a reference material having a known specific surface area. The specific surface is proportional to the square root of the time required for air to flow through the compacted material. At the same time, the bulk density was determined using a pycnometer, which is necessary to determine the specific surface of fine materials.

Determination of Heavy Metal Concentrations of Sorbents and Neutral Mine Drainage

The AES-ICP atomic emission spectrometry with inductively coupled plasma method was used to determine copper, manganese, iron, lead, cadmium, and aluminum. The EA-TCD - -elemental analysis with thermal conductivity detection method was used to determine zinc.

Adsorption Capacity

From the measured concentrations, the adsorption capacity at equilibrium (qe) [45, 46], the amount of metal adsorbed per unit sorbent at time t (qt), and the percentage removal efficiency of Cu²⁺ and Zn²⁺ ions from the solution (Ads. %) were calculated.

The adsorption capacity at equilibrium and at time t, respectively, was calculated according to Equation (1):

Where: q_e is adsorption capacity at equilibrium and at time t (mg.g^-1), c₀ is the initial concentration of ions in solution (mg.dm^-3), c_e is the equilibrium concentration of ions in solution or the concentration of ions in solution at time t (mg.dm^-3), V is the volume of solution (dm^-3) and m is the mass of adsorbent added (g).

Fig. 1.

Microscopic image - bentonite.

Fig. 2.

Microscopic image - zeolite.

Fig. 3.

Microscopic image - sludge.

Percentage of Metal Ion Removal Efficiency

The percentage removal efficiency of metal ions from the solution was calculated according to Equation (2):

Experiments focused on the adsorption of copper and zinc were carried out with natural unmodified adsorbents in a closed system under constant stirring of the suspension at laboratory temperature. We monitored the progress of sorption depending on the sorbent used.

Freundlich and Langmuir Adsorption Isotherms

To express the dependence of the metal ion adsorbed amount on its equilibrium concentration in solution, Freundlich and Langmuir‘s isotherms were constructed for all adsorbents used. The isotherms were evaluated at 5 input concentrations.

Freundlich Adsorption Isotherm

The effect of initial metal concentration on adsorption is described by adsorption isotherms. Several empirical and semiempirical relationships have been proposed for the analytical expression of the isotherms, of which either the Freundlich or Langmuir isotherm is the most suitable for adsorption from solutions.

The Freundlich isotherm is usually valid for physical adsorption and for adsorption on heterogeneous surfaces with different active sites. It can be expressed by the relation:

To verify that the experimental data satisfies this isotherm, the relationship is linearized:

Where: K_f (mg.g^-1) is a constant related to the adsorption capacity and n is an empirical parameter expressing the adsorption intensity, which varies with adsorbent heterogeneity.

Langmuir Adsorption Isotherm

The Langmuir isotherm is usually valid for chemisorption or electrostatic adsorption, where only a monomolecular layer is formed on the adsorbent surface and all active centers are equivalent. The Langmuir isotherm is expressed by the relation:

respectively in linearized form:

Where: q_m (mg.g^-1) gives the maximum monolayer adsorption capacity and b is the equilibrium constant dependent on the sorption energy.

Determination of the Specific Surface of Fine Materials

The specific surface is calculated from the following equation at a given porosity of the material and a temperature of 20 ± 2ºC:

Where: – K is the instrument constant found from the

measurement of the reference material, – e porosity = 0,5, – t is the time interval for the leveling of the gauge

level of the two determined points, – η is the dynamic viscosity of air at a given

temperature, – ρ is the bulk density (g . cm^-3) of the test material.

The result is in cm²/g when the values are entered correctly.

Parameters of Adsorption Isotherms and Specific Surface Area

The parameters of adsorption isotherms present the ability and conditions for the sorption of individual sorbents. The overall picture of sorption is complemented by the specific surface area mechanism. Therefore, all sorbents at both temperatures with both Freundlich and Langmuir adsorption isotherm parameters were presented with the specific surface area mechanism in a summarized table. These findings can then be confronted with the adsorption capacity of the monolayer qm as seen in Table 3.

Metal Removal Depending on the Input Concentration

The temperature has a significant effect on the sorption process, there we decided to lead the experiment at two temperatures and compare the results Figs 4-9.

Metal Removal Depending on Time

Monitoring the kinetics of the sorption process is necessary from the point of view of the overall picture of the sorption progress. It is important to observe when sorption starts or at what time the process equilibrium of sorption occurs

Sorption of Copper and Zinc on Bentonite

When evaluating the linearity, we have to conclude that copper sorption on bentonite or zinc was not linear. Reduced temperature improved the sorption for both metals by more than 10 %. The decrease in temperature also led to an increase in the adsorption capacity of the monolayer qm. The progression of sorption was gradual for both zinc and copper at both laboratory and reduced temperature as a function of time to steady state. Liu and Zhou [47] sorbed copper and nickel using bentonite. At pH 7, copper sorption of 70 % was achieved. Our results approximately correspond with this finding.

Melichová and Hromada [48] achieved Cu²⁺ sorption of approximately 90% using natural bentonite. Atkovska et al. [49] achieved the removal efficiency in respect of zinc cations above 98 %. On the other hand, Sen and Gomez [50] achieved a zinc removal rate of approximately 40 % on bentonite. Chang et al. sorbed Cu²⁺ and Ni²⁺ using bentonite and bentonite/GO composite [51]. In the study, approximately 70 % of Cu²⁺ was sorbed after 180 min of sorption at 25±1ºC. The results show that the steady-state sorption occurred after 120 min. Comparable sorption was achieved in our study only at reduced temperature. Otherwise, the efficiency of copper sorption on bentonite was mostly at half level. Even Cheng et al. did not always observe a linear progression for copper sorption on bentonite (bentonite/GO). Cao et al. compared [52] the use of bentonite, bento-zeolite, and artificial zeolite for the sorption of Cu²⁺ and Zn²⁺ from wastewater. They achieved Cu²⁺ and Zn²⁺ removal efficiencies of 73.63%±2.58 and 54.46%±1.34, respectively. The better sorption of Cu²⁺ over Zn²⁺ was also confirmed by our research. The sorption of Cu²⁺ was also in agreement with our results, while the sorption of Zn²⁺ was higher compared to our results. Certain variations in the efficiency may, of course, be due to different sorbent compositions, and different conditions - temperature, and pH. Kakaei et al. discussed the sorption of heavy metals using bentonite and modified bentonites [53]. Their adsorption capacity of monolayer q_m was more than one order of magnitude higher compared to our values. This may be due to the different process conditions compared to our experiment.

Fig. 4.

Cu sorption on bentonite.

Fig. 5.

Zn sorption on bentonite.

Fig. 6.

Cu sorption on zeolite.

Fig. 8.

Cu sorption on SDDS.

Fig. 7.

Zn sorption on zeolite.

Fig. 9.

Zn sorption on SDDS.

Fig. 10.

Cu sorption on bentonite at laboratory temperature.

Fig. 12.

Zn sorption on bentonite at laboratory temperature..

Fig. 14.

Cu sorption on zeolite at laboratory temperature.

Fig. 11.

Cu sorption on bentonite at 10ºC.

Fig. 13.

Zn sorption on bentonite at 10ºC..

Fig. 15.

Cu sorption on zeolite at 10ºC.

Nowadays, some studies consider the modification of bentonite in combination with different materials. For example Rashid et al. [54] used a composite of bentonite and dead fungal biomass. The adsorption of Zn(II) was recorded to be 56, 48.23, 32.27, 24.64, 19.77, and 16.45 using 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3 (g) adsorbent dose. Tahervand and Jalali [55] tested metal sorption on soil amended with bentonite, zeolite, and calcite. Zinc sorption on bentonite-amended soil, zeolite-amended soil, and calcite-amended soil at pH 2 were 3.5, 3.7, and 5.0 times that sorbed on control soil, respectively. Zinc sorption on calcite-amended soil ranged from 38.5 to 99.6% at pH 2 and 9, respectively with a mean of 64.7%. Modification of natural bentonite is required to improve its adsorption capacity and surfactants are widely used for such modification. Surface modification by the cationic surfactant (Bencylhexadecyldimethyl ammonium chloride, BCDMACl) improved adsorption capacities (q_max 50.76 and 35.21 mg.g^-1 for Cu²⁺ and Zn²⁺, respectively); about 2.5 and 2.0 fold improvements over natural bentonite. Kumararaja et al. [57] used chitosan-g-poly(acrylic acid)-bentonite composite for immobilization of heavy metals from contaminated soils. The adsorption pattern followed the Langmuir isotherm model, and the maximum monolayer capacity was 88.5, 72.9, 51.5, and 48.5 mg.g^-1 for Cu²⁺, Zn²⁺, Cd^2,+, and Ni²⁺, respectively.

Fig. 16.

Zn sorption on zeolite at laboratory temperature.

Fig. 18.

Cu sorption on SDDS at laboratory temperature.

Fig. 20.

Zn sorption on SDDS at laboratory temperature.

Fig. 17.

Zn sorption on zeolite at 10ºC.

Fig. 19.

Cu sorption on SDDS at 10ºC .

Fig. 21.

Zn sorption on SDDS at 10ºC.

Sorption of Copper and Zinc on Zeolite

In the case of copper sorption on zeolite, the reduced temperature did not contribute to better sorption. For neither metal was the sorption pattern at both temperatures linear, depending on the input metal concentration? For zinc sorption on zeolite, the reduced temperature promoted sorption. The improvement in zinc sorption was confirmed by an increase in the adsorption capacity of the monolayer q_m. For copper sorption, the most efficient sorption occurred during the first 30 min of sorption. Zn²⁺ sorption on the zeolite gradually increased throughout the sorption. Svilovic et al. [58] carried out copper sorption at different temperatures using zeolite. An increase in Cu sorption was found as a function of increasing temperature. Our experiment proved the opposite effect of temperature on the progress of sorption. Shaheen et al. achieved in their work [59] using zeolite, the sorption of Cu²⁺ and Zn²⁺ at rates of 80% and 60 %, respectively. The lower sorption of Zn²⁺ compared to Cu²⁺ was confirmed by our research. Elboughdiri and Garcia in their study [60] used natural zeolite to remove various heavy metals from industrial wastewater. At a sorbent concentration of 50 g.l^-1, they achieved Cu²⁺ removal rates of approximately 80%. Interestingly, the same Cu²⁺ removal was achieved in our study with ten times less amount of sorbent for the same volume of wastewater. The Cu²⁺ removal efficiency might have fluctuated due to the zeolite composition or other properties of the zeolite. Wang et al compared in their work [61] metal sorption on natural zeolite and SiO₂ encapsulated natural zeolite (SiEZ). With the addition of 3 g.l zeolite^-1 (close to the addition of sorbent in our study), Cu²⁺ sorption and Zn sorption were obtained at 81% and 86%, respectively. Better sorption of Zn over Cu was demonstrated, which is the portion of our finding. The level of Cu²⁺ removal was very close to our value. Rocha et al. demonstrated in their work a progression of sorption with increasing temperature, which contradicts our findings [62]. Sorption according to our hypothesis can be improved by decreasing the temperature, since in that case the desorption effect is reduced. Moreover, Rocha et al. showed an increase in q_m with increasing temperature (their q_m values were several times higher compared to our study), which also contradicts our study. Also, other studies reported natural zeolite as a sorbent for the removal of heavy metals and other compounds [63-66].

Sorption of Copper and Zinc on SDDS

For both Cu²⁺ and Zn²⁺ sorption on SDDS, the sorption efficiency increased with decreased temperature. The reduced temperature also promoted an increase in the adsorption capacity of the monolayer. The only linear progression was observed for zinc sorption on SDDS at 10ºC. Both Cu²⁺ and Zn²⁺ sorption proceeded gradually throughout the experiment. In our study, better sorption of Cu²⁺ compared to Zn²⁺ was also found, but the sorption levels were lower at about 60% and 40%, respectively. The drying of the sludge reduced the sorption of both metals in the study by Ong et al. This sorption level of Cu²⁺ (45%) and Zn²⁺ (30%) is more consistent with our results. Dewatered sludge was used in our study. The sorption of Zn and Cu²⁺ by the sludge was approximately 30% lower compared to the work of Kurniawan et al. [67]. According to other studies, activated sludge can also be used for the removal also elements like U⁶⁺ [68] or other chemical compounds like tetracycline [69], silver nanoparticles [70], or dyes (aniline blue, Nile blue, Bismarck brown Y and safranin O) [71]. Xu et al. [72] investigated sludge-based adsorbents for heavy metal removal, which can be prepared by carbonization, physical activation, and chemical activation.

Some studies investigated the adsorption of heavy metals on biochar derived from anaerobically digested sludge mainly for Pd removal [73-75]. However, this could be the aim of the next research.

Comparison of Metal Sorption on Different Sorbents

In the past, positive sorption on sludge has been demonstrated [76], however, the highest sorption of copper was observed when bentonite and zeolite were used. The highest sorption of zinc was obtained by sorbing on zeolite. The highest adsorption capacity of the monolayer was found with zeolite both at the laboratory and at reduced temperature. In general, copper was sorbed better than zinc. The hydration radius, among other factors, may influence this phenomenon. When we compare the hydration radius of Cu and Zn according to Calvo [77] the hydration radius of Cu²⁺ and Zn²⁺ reaches 4.19 and 4.30, respectively.

Different sorbents have quite variable removal efficiency. Ahmed et al. [78] reported the removal efficiency of activated dolomite at 80 % and bone char at 76%. Aden et al. [79] stated removal efficiency for bark and fly ash at 54%. Other studies declared the removal efficiency of sludges at level 46% [80], nano bentonite 74% [81], hydroxyapatite 62% [82], minerals such as goethite 46% [83], and modified asphaltite ashes 48% [84].

Sorption in most cases, in contrast to other studies [51-53, 58, 85] was not linear. This phenomenon may have been due to the continuous mixing of the sorbent with the neutral effluent solution. The intense contact of the phases may have promoted metal desorption, thus decreasing the removal intensity of both metals from the solution.