Comparative Adsorption of Cu(II), Zn(II), Cd(II), and Mn(II) from Aquatic Solution and Neutral Mine Drainage Using Paper Sludge

Abstract

The use of paper sludge as a waste stream from industrial facilities represents a significant environmental challenge due to its quantity and heterogeneous composition. The aim of the study was to evaluate the adsorption characteristics of paper sludge in neutral mine effluents and aquatic solutions of metal ions: Cu(II), Zn(II), Cd(II), and Mn(II). The main novelty of the research is a comparison of the adsorption process in synthetically prepared aquatic solutions and neutral mine drainage from field sampling. The adsorption process of the monitored metals was evaluated in terms of adsorption capacity, parameters of the Freundlich and Langmuir adsorption isotherm, and the separation factor. The adsorption capacity of paper sludge of all metals is significantly lower in neutral mine drainage (NMD) compared to adsorption in aquatic solution. The adsorption capacity of Zn(II) in aqueous solution reaches equilibrium over time, similarly to Cu(II), with values ranging from 0.2 to 1.6 mg/g. For Cd(II), a slight increasing trend in the adsorption capacity of paper sludge is observed at higher initial concentrations (3–5 mg/L) over a contact time of 90–120 min. In general, aqueous solutions of metal ions exhibited higher adsorption capacities compared to NMD, with the highest value recorded for Cu(II) at 4.742 mg/g. As the concentration values in the original solution increased, a decline in K_R (from 268% to 137% at a C0 range of 4–20 mg/L) was observed. In the mine drainage with the addition of Zn(II), K_R values were also lower compared to those in aquatic solutions. The reduction in K_R became more pronounced with increasing initial concentration, showing a decrease of 29.9% to 38.9% at C0 levels ranging from 2 to 10 mg/L. The separation factors for Cu(II), Zn(II), and Cd(II) were lower in NMD, indicating better metal separation from real mine waters. The results confirm the potential of paper sludge as a low-cost adsorbent for the treatment of heavy metal contaminated waters.

1. Introduction

Pulp and paper sludge is the main organic residue generated in the process of wastewater treatment in the pulp and paper industry. The annual world production of paper and cardboard is estimated to be 400 million tons. Paper and cardboard production is projected to reach up to 550 million tons by 2050, which could increase the production of this type of sludge by 48–86% in comparison to current levels [1]. The total volume of paper sludge produced in relation to paper production is currently around 40% [2].

In the past, the field of research on reducing the amount of produced paper sludge focused on the possibilities of its recycling. The suitability of using paper sludge as an additive to mortar was investigated and confirmed [2]. However, the study identified some limits to this method of recycling—adding paper sludge waste above 15% is not recommended for application in construction, and the parameters of air and water retention were not within the limits when applying mortars. Cusidó et al. studied the suitability of adding paper sludge to clay bricks, confirming the suitability of recycling paper sludge in this form [3]. Increasing the content of paper sludge in the clay mixture provides the material with improved thermal and acoustic insulation properties, but, on the other hand, it reduces its mechanical strength. In their research, Kizinievič et al. [4] evaluated the recycling of paper sludge in the form of clay bricks. Based on that study, the maximum appropriate addition of paper sludge to clay bricks was determined at the level of 15%. A higher content of the paper sludge additive has a negative effect on the mechanical properties of the clay body and frost resistance. Based on the above-mentioned findings, it is necessary to focus research on other options for recycling paper sludge than in the field of building materials.

This study aims to evaluate the adsorption characteristics of paper sludge for selected heavy metals, also referred to as potentially toxic elements (PTEs), in neutral mine effluents and synthetically prepared aquatic solutions of metal ions. The use of this type of waste as an adsorbent for water remediation has already been investigated in the past [5]). After pyrolysis, the paper sludge biochar was able to adsorb risky elements, such as Cu, Zn, and As [6]. However, in this study, the paper sludge was pretreated pyrolytically. Gorzin and Abadi’s research focused on creating low-cost activated carbon from paper mill sludge to eliminate Cr(VI) ions in aqueous environments. The results suggested that this carbon material, made from paper mill waste, holds potential as an effective adsorbent for removing Cr(VI) ions from water [7]. In the studies by Jaria et al. [5] and Xu et al. [6], the adsorption process using paper sludge after pyrolytic conversion was investigated. According to Xu et al., PMSB (biochar from paper mill sludge) demonstrated significant adsorption ability for Cu, Zn, and As, attributed to its large pore structure, high carbonate content, and OH groups with straightforward bonding [6]. In their study, Jaria et al. [5] emphasized that the operation mode of factories can influence the characteristics of the final materials derived from precursors, highlighting the importance of accounting for this variability when considering paper mill sludge as a raw material. A composite adsorbent made from chitosan and paper sludge has been developed for the removal of heavy metals, such as Cu(II) and Cr(III). This material exhibits high adsorption capacities and is capable of regeneration for repeated use, making it a cost-effective option for wastewater treatment [8]. Additionally, paper mill sludge can be thermally treated to produce calcium-rich biochar, which effectively removes phosphorus from wastewater through chemisorption-dominated mechanisms [9]. According to a recent study [10], both pristine and iron-modified hydrochar (PSH and PSH-Fe15) derived from paper waste sludge demonstrated promising potential as low-cost adsorbents for Cr(VI) removal from aqueous solutions. The use of paper mill sludge as a low-cost adsorbent under various conditions has already been confirmed [8]. In our study, the adsorption of PTEs using sludge without pretreatment was monitored, which is one of the novelties of the research. The reason for this difference is the effort to reduce input costs (financial and material) in the pretreatment of paper sludge. The obtained results of metal adsorption using untreated paper sludge were compared with already conducted studies using modified and untreated paper sludge. Another novelty of our research is the comparison of the used adsorbates. In previous studies, the research was focused on monitoring adsorption in solutions of metal ions [11,12,13], acidic mine effluents [14,15,16], or synthetically prepared multi-metal solutions [17,18]. We decided to compare neutral mine effluents with solutions of metal ions in order to monitor the possible competition of other metals already present in NMD (neutral mine drainage) during the adsorption process. In our research, we compared the adsorption of PTEs in two types of adsorbates—synthetically prepared aquatic solution of distilled water with specific concentrations of PTEs and neutral mine drainage. PTEs were added to NMD to ensure the same initial concentration. In addition to this, we consider another novelty of the study the investigation of the adsorption process in neutral mine drainage compared to acid mine drainage, which is much better investigated in the scientific field [19,20,21,22,23,24]. NMD is typical not only for areas of Slovakia [25,26] but also for other areas around the world [27,28,29], which makes research on the purification of this type of water from heavy metals of global importance. The practical significance of the research lies in supporting the recycling of paper sludge and, at the same time, protecting the environment by reducing the environmental burden of mine effluents. Only monitoring the adsorption process directly from field samples can contribute to the overall view of the suitability of the materials used for metal adsorption in real conditions.

2. Materials and Methods

2.1. Atomic Absorption Spectrometry (AAS)

The flame atomic absorption spectrometer AVANTA Σ (GBC Scientific, Perai, Malaysia) was used to determine the metal ions of Cu(II), Zn(II), Cd(II), Cd(II), and Mn(II) during the adsorption process. The source of radiation was a hollow cathode ray tube with a current of 3.00 mA. For the analysis, a Flame Atomic Absorption Spectrometer GBC 933 AA (AAS-GBC Scientific Equipment Ltd., Dandenong, Australia) was used. Air/acetylene with a flow rate of 11.50 L/min for air and 1.10 L/min for acetylene was used as a flame type. The relative errors of AAS measurements were less than 5%. The operation of the device and the evaluation of the results were carried out using the software GBC Avanta ver. 2.0.

2.2. Determination of pH

The pH parameter was determined according to STN ISO No. 10390 [30]. For the analysis, a lab pH meter inoLab^® pH 7110 (Xylem Analytics Germany Sales GmbH & Co. KG, Ingolstadt, Germany) was used.

2.3. Adsorption of Metal Ions

The aqueous solutions were prepared as single-element solutions in the following concentrations. For Cu: 4, 8, 12, 16, and 20 mg/L; for Zn: 2, 4, 6, 8, and 10 mg/L; for Cd: 1, 2, 3, 4, and 5 mg/L; for Mn: 2, 4, 6, 8, and 10 mg/L. The solutions were prepared using CuSO4·5H₂O, ZnSO4·7H₂O, Cd(NO₃)2·4H₂O, and MnCl₂·4H₂O salts. The NMD solutions were prepared in the same way, but, instead of distilled water, real mining water was used. The chemicals were obtained from CENTRALCHEM, Bratislava, Slovakia.

The adsorption process was conducted at a constant temperature of 20 °C with a solution volume of 100 mL and an adsorbent concentration of 5 g/L under continuous stirring. Samples for analysis were taken at time intervals of 30, 60, 90, and 120 min after filtering the adsorbent. The experiments were conducted in 5 repetitions, and the pH was not buffered.

2.4. Microscopic Images of Crystal Size

A VHX-7000 digital microscope (Keyence, Osaka, Japan) was used for the measurement. The structure of the paper sludge was monitored with magnification of 30×, 100×, and 300×.

2.5. Sampling Sites of Neutral Mine Drainage

Neutral mine drainage was taken from the Voznická hereditary adit in the mining district of Štiavnica-Hodruša (central Slovakia—the village of Voznica) at 48°27′ N and 18°42′ E. The Voznica hereditary adit was focused on the extraction of non-ferrous metals—copper, zinc, and lead. This water is a remnant of mining processes in the area. The location of the sampling site within the Slovak Republic is shown in Figure 1.

2.6. Characteristics of Neutral Mine Drainage

In neutral mine effluents from the monitored area, an increased concentration of Cu(II) of 32.8 μg/L was determined (limit 1.1–17.2); Zn(II): 5356.7 μg·L (limit 7.8–52 μg/L); Cd(II): 20.55 μg/L (limit 0.08–0.25 μg/L); and Mn(II): 3520.7 (limit 300 μg/L). For this reason, these PTEs were selected for adsorption with the use of paper sludge. Mine effluents in the monitored locality have a neutral pH character. Metal contents, including limit values according to legislation, are listed in

Table 1. Metal elements’ concentrations and pH in the NMD sample.

PTEs [μg/L]	Cu * [μg/L]	Zn * [μg/L]	Cd * [μg/L]	Mn * [μg/L]	Ni [μg/L]	Pb [μg/L]	Al [μg/L]	Fe [μg/L]	pH [–]
LV	Cu 1.1 μg/L for 1st and 2nd hardness classes and 8.8 μg/L for 4th and 5th hardness classes	7.8 μg/L for 1st and 2nd hardness classes and up to 52 μg/L for 4th and 5th hardness classes	0.08 μg/L for 1st hardness class and up to 0.25 μg/L for 5th hardness class	Total Mn 300 μg/L	20 μg/L	7.2 μg/L	200 μg/L	Total Fe 2 mg/ L	–
MV	18.2	5356.7	20.55	3520.7	8.3	1.8	152	12.3	7.6

Notes: LV—limit value according to the Regulation of the Government of the Slovak Republic No. 269/2010 Coll. MV—measured value. * Elements exceeding the concentration limit.

.

2.7. Characteristics of Paper Sludge

The paper mill from which the sludge originates specializes in the production of hygienic paper, primarily using recycled paper. The ratio of recycled paper to new (virgin) cellulose, which is supplied by an external company, is adjusted based on the quality and composition of the recycled material. The recycled paper is disintegrated and deinked and then mixed with virgin cellulose, with the recycled paper making up more than 70% of the raw material. The paper sludge is generated during the water treatment process at the plant, with the water primarily coming from the disintegration and deinking processes. This production process does not use sulfate technology. Currently, the paper sludge (

Table 2. Physical and chemical characteristics of paper sludge obtained from the paper mill.

Property	Sludge	Unit
Dry matter	95.30	%
pH	6.83	-
Cadmium	0.48	mg/kg
Lead	7.93	mg/kg
Nickel	3.7	mg/kg
Magnesium	432	mg/kg
Calcium	29,231	mg/kg
Potassium	142	mg/kg
Phosphorus	33.7	mg/kg
Manganese	11.5	mg/kg
Copper	1.02	mg/kg
Zinc	25.8	mg/kg
Iron	111	mg/kg
Sodium	45.6	mg/kg
CaO	26.84	%
Al2O3	4789	%
Carbonates	53.2	%
Corg	19.7	%
Nitrogen	4380	g/kg

) is used as an additive in the brick industry, but this method of processing will be discontinued in 2025, creating the need to seek alternative solutions for its further use.

Microscopic analysis was used to improve knowledge about the structural properties of the adsorbent (Figure 2, Figure 3 and Figure 4).

2.8. Pretreatment of Paper Mill Sludge (Physicochemical Steps)

The paper mill sludge used as an adsorbent was pretreated as follows.

Drying of the Sludge

Fresh paper mill sludge, obtained directly from the wastewater stream of the paper mill before the biological treatment stage, was first air-dried at laboratory temperature (approximately 22 ± 2 °C). The sludge was spread in a thin layer on clean plastic trays and regularly mixed to ensure uniform drying. Drying continued until a constant weight was achieved. The dry matter content after drying was approximately 95.3%.

Crushing of the Sludge

The dried sludge was then mechanically crushed using a porcelain laboratory mortar and pestle. Crushing removed large pieces and produced a finer, more homogeneous material suitable for further adsorption tests. The aim of this mechanical pretreatment was to achieve uniform particle distribution, allowing for better contact between the adsorbent and the adsorbed metal ions.

Sieving of the Sludge

The crushed sludge was sieved using laboratory sieves by selecting a particle size fraction of less than 0.5 mm. This fraction size was chosen to ensure a sufficiently large active surface area, thereby maximizing adsorption efficiency and improving the homogeneity of the sample.

Homogenization

The fraction of sludge obtained from sieving (<0.5 mm) was gently mixed to ensure homogeneity before use in the experiments.

The prepared sludge was then stored in sealed plastic containers until its use in adsorption experiments.

2.9. Calculations

2.9.1. Adsorption Capacity

From the measured concentrations, the adsorption capacity at equilibrium (q_e) and the amount of metal adsorbed per unit of sorbent at time t (qt) from the solution q_e were calculated.

The adsorption capacity at equilibrium and at time t, respectively, was calculated according to Equation [31]:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{\left({\mathrm{c}}_{\mathrm{o}}-{\mathrm{c}}_{\mathrm{e}}\right)·\mathrm{V}}{\mathrm{m}}}$$

(1)

where c_o is the initial concentration of ions in the solution (mg/L), c_e is the equilibrium concentration of ions in the solution or the concentration of ions in the solution at time t (mg/L), V is the volume of the solution (L), m is the mass of adsorbent added (g), and q_e is the amount of adsorbed heavy metal per unit of sorbent mass (mg/g).

2.9.2. Freundlich and Langmuir Adsorption Isotherms

To express the dependence of the adsorbed amount of a metal ion on its equilibrium concentration in solution, Freundlich and Langmuir isotherms were constructed for the paper sludge adsorbent. The isotherms were evaluated at 5 initial concentrations.

The adsorption isotherms for PTEs were obtained from a dependency where q_e (mg of soluted substance adsorbed per gram of adsorbent) is a function of c_e (mg of soluted substance in 1 L of solution after adsorption) under equilibrium conditions at a constant temperature of 298 K. Time intervals for evaluating the adsorption capacity were 30, 60, 90, and 120 min. Adsorption took place at pH∼7.

2.9.3. Freundlich Adsorption Isotherm

The effect of the initial metal concentration on adsorption is described by adsorption isotherms. Several empirical and semiempirical relationships have been proposed for the analytical expression of 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 adsorption on heterogeneous surfaces with different active sites. It can be expressed by the relationship [32]

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{f}}·{\mathrm{c}}_{\mathrm{e}}^{{\displaystyle \frac{1}{\mathrm{n}}}}$$

(2)

To verify that the experimental data fit this isotherm, the relationship is linearized [32]:

$${\mathrm{l}\mathrm{o}\mathrm{g}\mathrm{q}}_{\mathrm{e}}={\mathrm{l}\mathrm{o}\mathrm{g}\mathrm{K}}_{\mathrm{f}}+{\displaystyle \frac{1}{\mathrm{n}}}{\mathrm{l}\mathrm{o}\mathrm{g}\mathrm{c}}_{\mathrm{e}}$$

(3)

where K_f (mg/g) is a constant related to the adsorption capacity and n is an empirical parameter expressing the adsorption intensity, which varies with the heterogeneity of the adsorbent.

2.9.4. 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 [32]. The Langmuir isotherm is expressed by the relationship [32]

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{m}}·\mathrm{b}·{\mathrm{c}}_{\mathrm{e}}}{1+\mathrm{b}·{\mathrm{c}}_{\mathrm{e}}}}$$

(4)

or in the linearized form [32]

$${\displaystyle \frac{{\mathrm{c}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}}={\displaystyle \frac{1}{\mathrm{b}·{\mathrm{q}}_{\mathrm{m}}}}+{\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{m}}}}·{\mathrm{c}}_{\mathrm{e}}$$

(5)

where q_m (mg/g) gives the maximum monolayer adsorption capacity and b is the equilibrium constant dependent on the sorption energy.

2.9.5. Separation Factor

The Langmuir constant b (L/mg) is used to calculate K_R, a dimensionless separation factor given by the equation [33]

$${\mathrm{K}}_{\mathrm{R}}={\displaystyle \frac{1}{1+\mathrm{b}{\mathrm{C}}_0}}$$

(6)

where C₀ is the initial concentration (mg/L).

K_R (also referred to as K_L) is a constant related to the energy of adsorption (Langmuir constant) [34]. The K_R value for a favorable course of adsorption is within the interval of 0–1. Values close to 0 indicate the irreversibility of the process, and values close to 1 indicate the deterioration of separation by adsorption [33]. The K_R value indicates that the adsorption character is either unfavorable if K_R > 1, linear if K_R = 1, favorable if 0 < K_R < 1, or irreversible if K_R = 0 [34].

3. Results and Discussion

3.1. Adsorption Capacity

Adsorption capacity was evaluated for metal ions of Cu(II), Zn(II), Cd(II), and Mn(II) in aquatic solutions of metal ions and neutral mine drainage (Figure 3, Figure 4, Figure 5 and Figure 6).

The average values of adsorption capacities with standard deviations are presented in

Table 3. Average and standard deviation of adsorption capacity according to time and initial concentration for Cu(II) adsorption.

Time	Initial Concentration [mg/L]	Mine Drainage		Water Solution
		Mean [mg/g]	Std.Dev. [mg/g]	Mean [mg/g]	Std.Dev. [mg/g]
30	4	0.240	0.002	0.422	0.001
30	8	0.103	0.011	0.823	0.005
30	12	0.402	0.006	1.271	0.004
30	16	0.284	0.005	1.261	0.052
30	20	0.627	0.001	1.430	0.002
60	4	0.265	0.005	0.514	0.004
60	8	0.194	0.009	0.832	0.005
60	12	0.493	0.005	1.271	0.004
60	16	0.488	0.001	1.250	0.011
60	20	0.609	0.023	1.441	0.011
90	4	0.255	0.001	0.516	0.002
90	8	0.133	0.004	0.885	0.008
90	12	0.296	0.013	1.343	0.014
90	16	0.465	0.004	1.305	0.012
90	20	0.367	0.011	1.442	0.011
120	4	0.235	0.002	0.518	0.001
120	8	0.135	0.002	0.892	0.000
120	12	0.138	0.006	1.416	0.004
120	16	0.108	0.013	1.390	0.000
120	20	0.000	0.000	1.589	0.002

,

Table 4. Average and standard deviation of adsorption capacity according to time and initial concentration for Zn(II) adsorption.

Time	Initial Concentration [mg/L]	Mine Drainage		Water Solution
		Mean [mg/g]	Std.Dev. [mg/g]	Mean [mg/g]	Std.Dev. [mg/g]
30	2	0.116	0.008	0.198	0.026
30	4	0.399	0.008	0.170	0.001
30	6	0.382	0.002	0.422	0.027
30	8	0.376	0.005	0.914	0.001
30	10	0.291	0.006	1.512	0.001
60	2	0.113	0.001	0.209	0.004
60	4	0.443	0.017	0.323	0.001
60	6	0.895	0.007	0.602	0.012
60	8	0.503	0.010	1.002	0.002
60	10	0.458	0.009	1.533	0.001
90	2	0.151	0.001	0.239	0.001
90	4	0.460	0.010	0.372	0.001
90	6	1.002	0.001	0.618	0.010
90	8	0.603	0.005	1.008	0.008
90	10	0.470	0.014	1.534	0.002
120	2	0.274	0.002	0.258	0.004
120	4	0.636	0.007	0.377	0.001
120	6	1.018	0.004	0.743	0.010
120	8	0.862	0.002	1.089	0.021
120	10	0.694	0.002	1.614	0.002

,

Table 5. Average and standard deviation of adsorption capacity according to time and initial concentration for Cd(II) adsorption.

Time	Initial Concentration [mg/L]	Mine Drainage		Water Solution
		Mean [mg/g]	Std.Dev. [mg/g]	Mean [mg/g]	Std.Dev. [mg/g]
30	1	0.038	0.004	0.188	0.001
30	2	0.125	0.002	0.239	0.001
30	3	0.141	0.007	0.455	0.005
30	4	0.206	0.014	0.337	0.004
30	5	0.219	0.004	0.562	0.004
60	1	0.071	0.001	0.208	0.002
60	2	0.165	0.002	0.280	0.002
60	3	0.293	0.002	0.518	0.001
60	4	0.260	0.006	0.381	0.010
60	5	0.242	0.002	0.597	0.002
90	1	0.082	0.001	0.209	0.005
90	2	0.163	0.001	0.331	0.004
90	3	0.370	0.001	0.536	0.008
90	4	0.321	0.009	0.586	0.004
90	5	0.294	0.008	0.749	0.005
120	1	0.090	0.001	0.211	0.006
120	2	0.186	0.001	0.347	0.003
120	3	0.399	0.003	0.592	0.008
120	4	0.356	0.003	0.625	0.002
120	5	0.246	0.000	0.808	0.002

and

Table 6. Average and standard deviation of adsorption capacity according to time and initial concentration for Mn(II) adsorption.

Time	Initial Concentration [mg/L]	Mine Drainage		Water Solution
		Mean [mg/g]	Std.Dev. [mg/g]	Mean [mg/g]	Std.Dev. [mg/g]
30	2	0.001	0.001	0.226	0.001
30	4	0.055	0.004	0.539	0.014
30	6	0.076	0.004	0.758	0.006
30	8	0.002	0.001	0.938	0.003
30	10	0.172	0.005	1.730	0.001
60	2	0.005	0.003	0.288	0.008
60	4	0.032	0.004	0.569	0.001
60	6	0.130	0.007	1.003	0.002
60	8	0.001	0.001	1.124	0.010
60	10	0.277	0.017	1.740	0.014
90	2	0.017	0.004	0.291	0.004
90	4	0.066	0.001	0.626	0.004
90	6	0.194	0.003	1.005	0.001
90	8	0.026	0.009	1.134	0.001
90	10	0.314	0.007	1.905	0.001
120	2	0.063	0.010	0.314	0.002
120	4	0.087	0.005	0.656	0.017
120	6	0.238	0.001	1.017	0.002
120	8	0.091	0.005	1.212	0.007
120	10	0.354	0.002	1.724	0.012

.

3.2. Adsorption Isotherms

Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13 and Figure 14 present the Freundlich and Langmuir adsorption isotherms for the adsorption of Cu(II), Zn(II), Cd(II), and Mn(II) from aquatic solution and mine drainage.

The parameters of the graphs of Freundlich and Langmuir adsorption isotherms for aquatic solution of metal ions and NMD are shown in

Table 7. Parameters of Freundlich and Langmuir adsorption isotherms in the adsorption of Cu(II), Zn(II), Cd(II), and Mn(II) from aquatic solution and mine drainage.

Adsorbate Ion		Isotherm Constants
	Aquatic Solution				Mine Drainage
	Langmuir Constants
	qm [mg/g]	b [L/mg]	Equation	R2	qm [mg/g]	b [L/mg]	Equation	R2
Cu(II)	4.742	0.102	y = 2.0586 + 0.211 × x	0.789	0.0002	−0.4593	y = −11,733.6892 + 5390.6591 × x	0.554
Zn(II)	2.472	0.694	y = 0.579 + 0.4089 × x	0.767	0.880	1.207	y = 0.9412 + 1.1361 × x	0.754
Cd(II)	2.685	0.351	y = 1.0613 + 0.3727 × x	0.744	0.307	5.537	y = 0.5897 + 3.2613 × x	0.863
Mn(II)	3.413	0.1244	y = 2.354 + 0.2934 × x	0.856	0.455	0.072	y = 31.183 + 2.1601 × x	0.077
	Freundlich Constants
	Kf [mg/g]	n	Equation	R2	Kf [mg/g]	n [-]	Equation	R2
Cu(II)	0.450	1.238	y = −0.3467 + 0.8074 × x	0.973	1.1429	−0.3039	y = 0.0583−3.291 × x	0.453
Zn(II)	0.932	1.564	y = −0.0314 + 0.6351 × x	0.984	0.505	3.072	y = −0.2967 + 0.3252 × x	0.188
Cd(II)	1.435	1.212	y = −0.1569 + 0.8251 × x	0.989	0.266	2.567	y = −0.575 + 0.3891 × x	0.439
Mn(II)	2.539	1.364	y = −0.4044 + 0.7324 × x	0.993	0.036	1.267	y = −1.4398 + 0.7875 × x	0.526

.

The separation factor was compared at five initial concentrations for selected PTEs for aquatic solutions of metal ions, as well as NMD (

Table 8. Separation factor in the adsorption of Cu(II), Zn(II), Cd(II), and Mn(II) from aquatic solution and mine drainage.

Adsorbate Ion	Adsorbate	Initial Concentration [mg/L]
		KR
		4	8	12	16	20
Cu(II)	aquatic solution	0.709	0.550	0.449	0.379	0.328
	neutral mine drainage	−1.194	−0.374	−0.222	−0.157	−0.122
		2	4	6	8	10
Zn(II)	aquatic solution	0.418	0.265	0.193	0.152	0.126
	neutral mine drainage	0.293	0.172	0.121	0.094	0.077
		1	2	3	4	5
Cd(II)	aquatic solution	0.740	0.588	0.487	0.416	0.363
	neutral mine drainage	0.153	0.083	0.057	0.043	0.035
		2	4	6	8	10
Mn(II)	aquatic solution	0.801	0.668	0.573	0.501	0.446
	neutral mine drainage	0.875	0.778	0.700	0.636	0.583

).

3.3. Evaluation of Adsorption Capacity

The adsorption capacity of paper sludge of all metals is significantly lower in NMD in comparison to the adsorption in aquatic solution. This means that the influence of other metals in NMD is obvious, and a competitive character is observed. It was proven already in the past that paper sludge is a suitable alternative for the removal of metals, such as Pb(II), As(III), As(V), Cr(III) and Cr(VI) [35], Zn(II) [36], Cu(II), and Ag(I) [17], from wastewater.

By evaluating the adsorption of Cu(II) in an aquatic solution, a balanced capacity over time was determined. As the initial concentration of the adsorbed medium increases, the adsorption capacity rises slightly.

In case of Cu(II) adsorption, lower values of adsorption capacity of approximately 1.6 mg·g⁻¹ were recorded (c₀ 20 mg/L) in comparison to the mentioned study where the values were 30 mg·g⁻¹ (c₀ 30 mg/L) for PMSB and approximately 12.5 mg/g (c₀ 30 mg/L) for DPMSB. The reason for this difference may be the modification of adsorbents, the heterogeneity of adsorbents, or other conditions of the adsorption process (initial concentration, ratio of adsorbent–adsorbate, pH).

Compared to aquatic solutions, this unambiguity of adsorption capacity is lost in NMD. Lower initial concentrations (4–8 mg/L) are able to keep the equilibrium capacity (0.1–0.25 mg/g), although it is several times lower. At higher initial concentrations (12–20 mg/L), a sharp decrease in adsorption capacity was observed after 60 min of adsorption. This is explained by the cessation of adsorption and the subsequent leaching of the adsorbed Cu(II) ions. The likely cause is the competition from the ions present in the NMD. Based on Calace et al.’s study [17], a decrease in adsorption capacity in the adsorption of Cu(II) in a mixed solution of metals was also observed in comparison to an aquatic solution containing only one ion of a metal—Cu(II) (decrease from 9.53 to 2.54 mg·g⁻¹). Xu et al. observed the saturated adsorption capacity of sludge–chitosan material for Cu(II) at 114.6 mg/g [8].

The adsorption capacity of Zn(II) in an aquatic solution is balanced over time, similarly to the case of Cu(II), as they acquire values of 0.2–1.6 mg/g. With increasing levels of c₀, an increase in adsorption capacity was observed during the adsorption process. This phenomenon was also confirmed by the research of Chaukura et al. [36]. In our study, lower values of Zn(II) adsorption capacity were recorded compared to the research of Xu et al. [6], which used modified adsorbents of paper sludge. The modification of sludge-based adsorbents is the reason for the higher adsorption capacity values.

In NMD, ions of Zn(II) are adsorbed on average twice less compared to adsorption in an aquatic solution. Adsorption capacities have an increasing trend depending on time. After 120 min of the adsorption process, they acquire values of 0.25–0.9 mg/g.

The most significant decrease in adsorption capacity shows the highest c₀ of 10 mg/L. This phenomenon was also observed in the case of Cu(II); the highest concentrations of adsorbed metals probably have spatial barriers in the adsorption.

In aquatic solution, Cd(II) ions have an increasing trend of the adsorption capacity of paper sludge at higher initial concentrations of Cd(II) 3–5 mg/L. The trend is clearly higher. Yoon et al. investigated the adsorption of the binary system of the solution of As(V) and Cd(II) [18] using pyrolytically treated paper sludge. The research confirmed higher levels of adsorption capacity, which could be due to the modification of the adsorbent, or the different nature of the adsorbate. Modification of paper sludge does not ensure an improvement of adsorption capacity in all circumstances. In one study [37], it was confirmed that the chemical treatment process of the adsorbent with NaOH resulted in an improvement of the adsorption capacity. On the other hand, when modifying paper sludge using NaOH and C₄H₆O₆ (tartaric acid), a reduction in adsorption capacity for heavy metals was observed compared to untreated paper sludge. When using NaOH and C₄H₆O₆, there was a decrease of the final pH. This variation caused the competitive effect of the cation [37]. Based on the research focused on the adsorption of metals from aquatic solutions, it was confirmed that as the pH increases, the sorption capacity increases, too [17]. It follows from this that the acidic chemical modification of paper sludge as an adsorbent can cause a decrease in adsorption capacity in comparison to an untreated adsorbent.

In NMD, the adsorption capacity of Cd(II) decreased approximately twice in comparison to aquatic solution at all initial concentrations of c₀. The same phenomenon was confirmed by Calace et al. [17]. In their study, a decrease in adsorption capacity in a mixed solution compared to an aquatic solution from 10.23 to 2.36/1.68 mg/g was observed. This represents approximately a fivefold decrease in concentration value.

When observing the trend of the adsorption capacity of Mn(II) in an aquatic solution, a balanced trend depending on time was observed. With increasing levels of c₀, the values of adsorption capacities increased slightly. In NMD (compared to the aquatic solution), the most significant decrease of the adsorption capacity of Mn(II) compared to the previous metals was observed. With adsorption of Cu(II), Zn(II), Cd(II), the decrease of adsorption capacity was 2- to 3-fold, while in the adsorption of Mn(II), the 5–10-fold decrease was observed. The lowest adsorption capacity values were observed at low initial concentrations (2–6 mg/L). Higher initial concentrations of c₀ in Mn(II) adsorption were several times lower, but they increased slightly over time. The maximum adsorption capacity of Mn(II) from NMD was measured at 0.35 mg/g and from the aqueous solution at 1.9 mg/g.

Table 9. Results of adsorption capacities from comparative studies according to the Langmuir isotherm.

References	Adsorbent	qmax Cu (mg/g)	qmax Zn (mg/g)	qmax Cd (mg/g)	qmax Mn (mg/g)	Temperature (°C)	pH/Key Conditions
Our results	Untreated paper sludge	4.742	2.472	2.685	3.413	25	pH ≈ 7; 5 g/L adsorbent
[38]	Sludge–clay granules (GSC)	2.76	1.23	1.53		25	pH 5.0
[39]	Water–hyacinth biochar + nano-MnO2	103.9	68.4	151.4		25	pH > 6 (not specified)
[40]	KMnO4-modified walnut shell biochar (MWSC)	30.18	58.96	44.94		25	pH 5.0
[41]	Guanidyl-modified cellulose (Gu-MC)	82	77	69		25	pH 6.0
[42]	Porous Jute/PAA hydrogel			401.7		20	pH 6.0 (Cd only)

presents the results of adsorption capacities from comparative studies.

The study focused on a one-time evaluation of the adsorption capacity of untreated paper sludge; however, for practical applications of such a material, its long-term behavior needs to be taken into account. Recent research shows that paper sludge can effectively capture heavy metals, such as Cu, Zn, Ni, and Pb, and even without chemical modification, it demonstrates significant sorption capacity. For instance, Merah et al. (2023) demonstrated high adsorption of methylene blue using untreated paper industry sludge [43]. Although untreated biosorbents, such as agricultural waste or fruit peels, retain high adsorption capacity without chemical modification, their reusability and mechanical stability tend to be limited.

Although the regeneration of the adsorbent was not examined in this study, the literature suggests that simple elution techniques (e.g., HCl, NaOH) can enable biosorbents to be reused over several cycles without significant loss of efficiency. Staszak and Regel-Rosocka (2024) report that even untreated biosorbents can retain their sorption capacity across multiple cycles if appropriate regeneration methods are used [44]. Another study found that regeneration using diluted acids is effective in maintaining the adsorption properties of sludge-based biosorbents [45].

An important environmental consideration is the potential leaching of adsorbed metals, particularly under changing pH or redox conditions. Rosazlin et al. (2010) showed that when recycled paper mill sludge is applied to soil, trace amounts of heavy metals, such as Cu, Zn, Ni, Pb, and As, can leach into lower soil layers, especially if the sludge is not fully mineralized [46]. This finding highlights the need for careful monitoring of possible secondary environmental impacts when using untreated biosorbents.

3.4. Evaluation of Adsorption Isotherms

The parameters for the Langmuir and Freundlich models (Table 7) of the observed metals were obtained through linearization of equilibrium isotherms. The coefficient of determination (R²) points to a more appropriate description of the adsorption of metal cations from an aquatic solution by the Freundlich isotherm (0.97–0.99), which means multilayer adsorption. This fact was also confirmed in a study by Xu et al. [6] when using biochar from paper sludge (PMSB). This fact indicates that the adsorption of Cu, Zn, and As using a modified adsorbent [6], as well as our adsorbent, was not only present on one molecular layer but also on multimolecular layers [47]. Likewise, the suitability of the description of adsorption by the Freundlich isotherm was confirmed in the research of adsorption from aquatic solution using two types of biochar synthesized from paper sludge during the removal of Zn(II) [36], chemically processed paper pulp [48] when removing Zn(II), or pyrolytically modified paper sludge [18] when removing Cd(II).

For adsorption from a mine effluent, a more suitable description is the Langmuir equation of the isotherm (0.55–0.86), i.e., it is a surface adsorption with a higher heterogeneity of the adsorbent in a single layer. The conformity with the results of our research was confirmed in study Calace et al., where at the sorption of a solution containing Ag(I), Cd(II), Cu(II), Pb(II) and Cr(VI), the thermodynamic curves best correspond to the Langmuir model, while the experimental data for the individual heavy metal Cu(II) best correspond to the Freundlich model [49].

The size of the monitored PTEs according to their ionic radii is from the smallest: Cu (II)—72 pm, Zn (II)—74 pm, Mn (II)—80 pm, and Cd (II)—97 pm. The smallest cations should be adsorbed faster and to the largest possible amount compared to larger cations because they can pass through the micropores of the adsorbent. Among the monitored metals, Cu(II) as the smallest cation with the highest electronegativity (according to Pauling: Cu—1.9; Cd—1.69; Zn—1.65 and Mn—1.55) has more favorable adsorption than other monitored PTEs, based on smaller sorption spherical interferences [50]. Our results confirm that Cu(II) was adsorbed the fastest in both the aqueous solution and NMD, already after 60 min, compared to Zn(II), Cd(II), and Mn(II), where equilibrium was reached after 90 min (Cd(II)) or as late as 120 min (Zn(II) and Mn(II)). From the point of view of the adsorption rate, the adsorbed metals can be arranged in the following order: Cu(II) > Cd(II) > Zn(II) > Mn(II).

Parameter q_m of the Langmuir isotherm is related to the adsorption capacity of the adsorbent, concerning the specific dissolved substance. In comparison of this parameter of the monitored metals in aquatic solution and in NMD, it was found out that the values are several times higher in adsorptions from aquatic solution. The adsorption capacity values (mg·g⁻¹) of paper mill sludge for metals from aqueous solutions were measured in the following order: Cu(II)—4.742, Mn(II)—3.413, Cd(II)—2.685, Zn(II)—2.472. Based on a study [51], the value of the maximum adsorption capacity q_m at the Mn(II) adsorption level of 12.7 mg/g was recorded using sewage sludge. The higher q_m values compared to our results could have been due to the difference of adsorbents.

Due to the increased number of competing cations, the metals in NMD as a multi-component mixture were more difficult to adsorb. The adsorption capacities q_m of monitored metals were reduced several times compared to single-component aquatic solutions of ions at two to eight times. The biggest difference was found out in the adsorption of Cu(II).

Alatalo et al. [35] observed higher values of q_m in comparison to our values in the adsorption of Pb(II) from an aquatic solution using industrial paper sludge (q_m = 11.78 mg/g). The reason for this difference is probably due to the better absorbability of Pb(II) ions compared to the metal ions Cu(II), Zn(II), Cd(II), and Mn(II). Another reason for this difference may be the ratio of liquid–solid substances, as well as the difference in the pH values of metal solutions, as both of these factors significantly affect the q_m parameter [49]. In one study [37], lower levels of the maximum adsorption capacity of untreated paper sludge of 0.113 mg/g were monitored (parameter q_m) compared to our results for adsorption of Cd(II) from an aquatic solution of 2.685 mg/g. This is confirmed by the fact that the modification of paper sludge does not automatically lead to the improvement of heavy metal adsorption under specific conditions.

In the case of the second smallest observed cation Zn (II), an increased intensity of adsorption from NMD was detected. The n parameter of the Freundlich isotherm rose from 1.564 in the adsorption from the aquatic solution to 3.072 from NMD. Mn(II), on the other hand, reduces the intensity of adsorption from the mine drainage and has the highest decrease in adsorption capacity from the aquatic solution compared to NMD. Cd(II) has the largest ionic radius and the most difficult probability of adsorption in competition with the present cations. However, the highest value of the parameter b = 5.537 L/mg of the Langmuir isotherm indicates the presence of its stable complex on the adsorbent. This fact was also confirmed in the previous section on the adsorption rate of metal ions, where the slowest adsorption for Cd(II) was expected due to the size of the ionic radii, but this metal had better results in terms of adsorption rate than Zn(II) and Mn(II).

Parameter n of the Freundlich isotherm indicates the favorability of adsorption. All obtained n values are in the interval of 1–10, which is positive for the given adsorption process [52]. In the aquatic solution, the n values were approximately the same for all metals, which implies that these conditions are favorable and do not disturb the heterogeneity of the adsorbent surface. Similar values of parameter n were also recorded for Cd(II) adsorption using biochar from paper sludge 3.55 [18] compared to our results of 1.212.

The effect of the mine drainage on the adsorption of the monitored metals is significant compared to the aquatic solution. The main reason is the multi-component solution and the influence of competing ions in adsorption.

The negative value of the Langmuir constant b for Cu(II) was caused by a desorption process after 60 min. The adsorption of Cu(II) from mine water occurred within the first hour and was subsequently, most likely due to the influence of competing ions, displaced back into the solution. Copper does not form stable complexes with the adsorbent in this environment.

3.5. Evaluation of the Separation Factor

The obtained K_R values are shown in Table 8. The separation factors decrease with increasing initial concentration, which means that the lowest amounts of metal ions are the easiest to separate. The separability from NMD is worse than from the aqueous solution for Zn(II) and Cd(II). Negative K_R values for Cu(II) from NMD indicate the irreversibility of the process (a chemical reaction between Cu(II) and the adsorbent), as was also noted in the previous evaluation. Mn(II) behaves oppositely—it is slightly more separable from NMD than from the aqueous solution.

3.6. Recyclability and Stability of the Adsorbent

In this study, the recyclability and mechanical stability of the untreated paper sludge adsorbent were not experimentally evaluated. However, based on the recent literature, similar untreated biosorbents have shown the potential for regeneration through mild acid or base elution without significant loss of adsorption capacity [44].

Research on untreated paper mill sludge and nanocellulose-based adsorbents indicates that mechanical degradation and partial loss of active binding sites may occur after repeated use cycles [43]. Despite this, regeneration protocols using diluted HCl or NaOH have proven effective for maintaining the performance of similar biosorbents over multiple adsorption–desorption cycles [45].

The recyclability and long-term stability of untreated paper sludge as an adsorbent remain a critical factor for real-world applications. Although the present study did not include regeneration tests, existing studies suggest that similar biosorbents can be effectively regenerated up to five times with minimal performance loss. However, structural weakening and decreased binding site availability have been reported after several cycles, particularly in non-modified materials [44].

Moreover, attention should be paid to potential leaching of previously adsorbed metals during recycling, especially under variable pH conditions [53]. Therefore, future studies should include cyclic adsorption–desorption experiments and standardized leaching tests (e.g., TCLP) to fully assess the environmental safety and durability of the material.

4. Conclusions

This study demonstrates the effective utilization of untreated paper sludge, an industrial waste, as a low-cost adsorbent for removing Cu(II), Zn(II), Cd(II), and Mn(II) from contaminated water. The significance of this work lies in its real-world application. Unlike many studies limited to synthetic solutions or requiring chemically modified sorbents, here, the raw paper sludge was applied directly to actual neutral mine drainage (NMD) with no pretreatment. This novel approach highlights a sustainable waste-to-resource strategy for water remediation, showing that a readily available waste material can successfully mitigate heavy metal pollution under field-relevant conditions.

Our results confirm that Cu(II) was adsorbed the fastest in both the aqueous solution and NMD already after 60 min compared to Zn(II), Cd(II), and Mn(II), where equilibrium was reached after 90 min (Cd(II)) or as late as 120 min (Zn(II) and Mn(II)). The adsorption capacity values (mg·g⁻¹) of paper mill sludge for metals from aqueous solutions were measured in the following order: Cu(II)—4.742, Mn(II)—3.413, Cd(II)—2.685, Zn(II)—2.472. The adsorption capacity of Zn(II) in an aquatic solution is balanced over time; similarly to the case of Cu (II), they acquire values of 0.2–1.6 mg/g. In the aquatic solution, Cd(II) ions have an increasing trend of the adsorption capacity of paper sludge at higher initial concentrations of Cd(II) 3–5 mg/L.

Key findings indicate notable differences in adsorption performance between idealized laboratory prepared solutions and the complex NMD matrix. In single-metal aqueous solutions, the paper sludge achieved relatively high adsorption capacities for the target metals, benefiting from the absence of competing ions. In contrast, in the multi-component NMD, the effective uptake for each metal was reduced, reflecting competition and interference from the real water matrix. Isotherm modeling further underscored these differences, as the Langmuir isotherm provided the best fit for the adsorption of individual metals in simple synthetic solutions, suggesting monolayer coverage on a homogeneous set of sites. However, for the neutral mine drainage, adsorption data were better described by the Freundlich model, which captures the heterogeneous surface and multi-layer adsorption phenomena. This shift in isotherm behavior implies that the presence of multiple metals and other constituents in NMD leads to a more complex adsorption process, deviating from ideal monolayer adsorption observed in the controlled single-solute systems. The description of the equilibrium for NMD is described by the Langmuir isotherm, and Cd(II) reaches the highest parameter at b = 5.5357 L/mg, which is a sign of the most favorable stable adsorbate complex on paper sludge.

The competitive adsorption experiments revealed that certain metal ions were preferentially taken up by the paper sludge in the presence of others. In particular, Cu(II) exhibited the highest affinity among the four metals under multi-metal conditions, often sequestering to the largest extent and thereby limiting the removal of the other metal ions to some degree. Conversely, Mn(II) showed the lowest uptake when in competition, consistent with its more challenging removal at neutral pH. Overall, the observed trend in a multi-metal system (with Cu(II) generally outperforming Zn(II), Cd(II), and especially Mn(II) in uptake) highlights the selectivity of the sludge’s adsorption sites. These findings provide insight into how co-existing contaminants can influence treatment efficiency, an important consideration for practical remediation of mixed-metal wastewaters like mine drainage.

In summary, this work establishes that untreated paper sludge can serve as an effective and sustainable adsorbent for heavy metal removal in both controlled aqueous solutions and real neutral mine drainage. The use of an unmodified waste material directly in a complex environmental water matrix is a novel and practical contribution to water treatment research. The distinct adsorption behaviors observed between single-metal and multi-metal systems underscore the importance of testing candidate materials under realistic conditions. Taken together, the results highlight the potential of converting paper industry sludge into a value-added material for environmental cleanup, thereby advancing circular economy principles. Future studies building on these findings should explore long-term field applications and scaling up of this approach, but the present results clearly demonstrate the feasibility and promise of using waste paper sludge for real-world heavy metal remediation.