The adsorption of Mn(II) by insolubilized humic acid

Industrial wastewater and mine drainage are an important source of pollution from potentially toxic elements (PTEs). These elements entering the environment are fixed or enriched by physical or chemical interactions with organic matter in water, which affects their morphology, migration, transformation and wider ecotoxicity (Li et al. 2013a; Lan et al. 2017). There are two types of bonding according to the properties of organic matter and metal ions (Schulten & Schnitzer 1993): one is the formation of an ionic bond to alkali metal ions such as Na(I), K(I), and another is the generation of complexes with heavy metal ions such as Cu(II), Pb(II) and Cd(II). There has been a lot of research on the reaction mechanism between organic matter and heavy metal ions such as Cu(II), Pb(II), Cd(II), Zn(II), etc. (El-Eswed & Khalili 2006; Oldham et al. 2017), but on the mechanism of interaction between organic matter and manganese is still scarce. The manganese ion is extremely unstable and harmful in water. If the human body absorbs too much manganese, it will cause poisoning and affect the central nervous system. In severe cases, it can cause irritability, hallucinations, and so on, resulting in manganese mania (He et al. 2005; Jiang et al. 2018). The World Health Organization (WHO) limits the maximum concentration of manganese in drinking water to 0.5 mg/L. At present, the treatment methods for manganese-containing wastewater mainly include chemical oxidation, bioremediation, electrolysis, adsorption and coagulation sedimentation (Patil et al. 2016; Yong et al. 2017; Hou et al. 2020). Among them, the adsorption method has the characteristics of easy operation, fast reaction speed, no introduction of other chemical substances, and avoiding secondary pollution, so it has a good application prospect (Huang et al. 2019).

Humic acid is widely distributed as a component of natural organic substances. As a potential material for water treatment, it has the advantage of being environmentally benign, of low toxicity and cost, and containing a variety of functional chemical groups, such as carboxyl, alcoholic hydroxyl, phenolic hydroxyl, aldehyde, ketone, and ethers (Manzak et al. 2017). The existence of these active groups determines a high reactivity with metal ions in aqueous solution through adsorption or complexation. When manganese ions in the water interact with the functional groups of humic acid, they will be adsorbed in the network structure of the humic acid macromolecule to form manganese-humic acid complexes (Oldham et al. 2017), strongly stabilized. Fawwaz Khalili et al. (Khalili & Al-Banna 2015) extracted insolubilized humic acid (NaIHA) from local soil to characterize it, and explored its adsorption properties for radioactive elements U(VI) and Th(IV). It is found that the adsorption characteristics of NaIHA for two metal ions conform to the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) adsorption isotherm models, and the adsorption mechanisms include electrostatic adsorption and coordination. Li Chen et al. (Chen et al. 2020) investigated the adsorption-desorption characteristics of humic acid colloid (HAC) for vanadium (V) and the interaction between them through a series of experiments. It was found that six groups on HAC participated in V(V) adsorption, the adsorption process follows the Langmuir isotherm adsorption model and first-order kinetic model. However, high solution pH (7–11) and temperature (20–40 °C) will greatly reduce HAC adsorption capacity.

In this study, eco-friendly and non-toxic insoluble humic acid (IHA) was used as adsorbent to remove Mn(II) from aqueous solution. While exploring the adsorption effect and mechanism, a low-cost and efficient method for the treatment of manganese-containing industrial wastewater was also proposed. The effects of factors such as initial Mn(II) concentration, pH and the dose of IHA on Mn(II) adsorption by IHA were studied. The adsorption kinetics and model for isothermal adsorption of manganese on IHA were investigated. Solid phase characterization methods such as FTIR, SEM, and EDS were used to study the interaction between humic acid and metal manganese ions.

All chemicals used in this study were of analytical grade and were not further purified before use. All solutions were made up with deionized water. Insolubilized humic acid and manganese(II) sulfate monohydrate (MnSO₄·H₂O) were purchased from Sinopharm Chemical Reagent Co., Ltd (China, Shanghai). Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were purchased from XiLong Science Co., Ltd (Shantou, China).

The IHA before and after adsorption of Mn(II) was dried and milled, and then KBr powder with a mass ratio of about 100:1 was added; after the mixture was fully milled again, it was pressed, and the FTIR spectra of the samples were recorded by Thermo Fisher Scientific Nicolet 6700 FTIR spectrometer instrument in the scanning range of 400–4,000 cm⁻¹. SEM-EDS was obtained from Hitachi S-4800 scanning electron microscopy, which is also equipped with an energy dispersive spectroscope.

The insolubilized humic acid, with a dry weight of 1.00 g, was added to a 200 ml simulated water sample with a Mn(II) mass concentration of 100 mg/L. At a temperature of 298.15 K, the effect of pH (2.5–7.5) on Mn(II) adsorption by IHA was investigated (Figure 1).

It can be seen from the figure that with the increase of pH, the removal efficiency and adsorption capacity of Mn(II) gradually increased, from 23.17% to 61.69%, and 6.67 mg/g to 17.77 mg/g, respectively. The process of heavy metal ion forming a complex with humic acid: is affected by the competition between H_mA dissociation and MA^n−m complex equilibrium (Li et al. 1997). At different pH, the degree of dissociation of carboxyl groups and phenolic hydroxyl groups contained in humic acid varies. Research has shown that pH greatly affects the complexation reaction between humic acid and metals, and its complexing ability and stability increase with increasing pH (Hu et al. 2017). When pH < 4.5, the removal of Mn(II) by IHA increased rapidly with the increase of pH, from 23.17% at pH = 2.5 to 54.87% at pH = 4.5, which may be because when the pH is at lower levels, the high concentration of H⁺ in the solution is more competitive with the adsorption sites than Mn(II), and it more easily reacts with the active groups on the surface of humic acid, so that the metal ions are easily dissociated from the complex. When the pH is 4.5–7.5, the removal efficiency increases slowly. On the one hand, the adsorption of H⁺ decreases and is released; at the same time, the H⁺ on the functional groups such as carboxyl group and hydroxyl group is also dissociated; therefore, the negative charge on the humic acid increases and the adsorption sites increase (Ding et al. 2019), so that the humic acid and Mn(II) can undergo a complex reaction to reduce the free Mn(II) concentration in the solution. Also, the active functional group of humic acid tends to complex with manganese ions at higher pH, which is more conducive to the formation of humic acid-manganese complexes, thereby reducing the concentration of Mn(II) in solution (Barnie et al. 2018).

Under the conditions of Mn(II) mass concentration of 100 mg/L, pH = 5.5, and temperature of 298.15 K, the effect of the dose of IHA (0.2–1.2 g) on Mn (II) removal was investigated (Figure 2).

It can be seen from the figure that with the increased dose of IHA, the removal efficiency of Mn(II) gradually increases, and the adsorption capacity gradually decreases. At low dose (0.20–0.80 g), the removal efficiency increased rapidly from 19.79% to 50.43%, with the adsorption capacity decreasing by 13.25 mg/g. This is because insolubilized humic acid contains a large number of active functional groups, in which the hydrogen in the functional groups such as -COOH and -OH is ionized to make it negatively charged (Basu et al. 2019). The negatively charged colloid has a strong binding capacity with the metal Mn(II) in the solution (Wei et al. 2019), so when the dose is gradually increased, the effective adsorption sites also relatively increase, so that the removal efficiency of manganese ions in the solution gradually increases. When the dose was 0.80–1.20 g, the removal efficiency increased slowly from 50.43% to 58.7%, and the adsorption capacity decreased by 4.4 mg/g. When the dose is high, the effective adsorption site is surrounded by a large number of insolubilized humic acid particles, and the electrostatic repulsion force between particles increases (Biswas et al. 2020), resulting in a decrease in the removal. However, due to the strong adsorption capacity of IHA, at high dose, a large number of adsorption sites can still effectively capture manganese ions (Cheng et al. 2015), thereby forming a complex of humic acid and manganese through coordination complexation, effectively reducing free manganese ions in solution.

The simulated surface water sample was adjusted to pH = 5.5, and insolubilized humic acid with a dry weight of 1.00 g was added into a series of simulated surface water samples containing Mn(II) of different concentrations (10–200 mg/L). The adsorption experiments were carried out at 298.15 K, 308.15 K and 318.15 K, and the effect of initial concentration of Mn(II) on the adsorption effect of IHA was studied (Figure 3).

As shown in Figure 3, both the temperature and initial concentration of the adsorption reaction will affect the adsorption performance of the adsorbent. At the same temperature, with the increase of the initial Mn(II) concentration in the solution, the removal efficiency of manganese by IHA decreased, while the adsorption capacity increased. In addition, at the same initial concentration of Mn(II), with the gradual increase of temperature, the removal efficiency of IHA on Mn(II) is also gradually improved. It can be seen that higher temperature is conducive to the removal of Mn(II) by IHA, which is assumed to be an endothermic reaction. When the concentration of Mn (II) is low, the humic acid with negative charge combines with the Mn(II) with positive charge, and the free Mn(II) in the solution is surrounded by a large number of adsorption points with opposite charge, so the removal is high. After the initial stage, which depends on the concentration of Mn(II), the system slowly forms a stable complex of humic acid-Mn(II). When the concentration of Mn(II) is high, the active sites on the surface of the humic acid are occupied by a large number of Mn(II), and gradually reach saturation. At the same time, the increase of the positive charge on the surface of humic acid molecules causes a repulsive force and reduces the complexing ability of humic acid with Mn(II) (Li et al. 2019). Studies have shown that when the concentration of metal ions is high, the precipitation of humic acid-Mn(II) complex will occur, which affects the molecular structure of humic acid and reduces its reaction with Mn(II) (Engebretson & von Wandruszka 1998).

Aliquots of insolubilized humic acid with dry weights of 0.6, 0.8, 1.0 and 1.2 g were added to a series of simulated surface water samples containing Mn(II) with an initial concentration of 100 mg/L. The pH of the solution was adjusted to 5.5, and the adsorption kinetics experiment was carried out.

Figure 4 shows the relationship between the removal of Mn(II) and the adsorption time. The efficiency of removal of Mn(II) by IHA with the increase of contact time, and a higher removal efficiency can be obtained in a shorter time. Then with the increase of reaction time, the removal efficiency of IHA to Mn(II) increased slowly, the effective sites were occupied by Mn(II) gradually, and finally reached the adsorption equilibrium (Yang et al. 2014). The extremely fast adsorption rate in the initial stage indicates the strong affinity of IHA for Mn(II), which may be due to the presence of a large number of functional groups (Guo et al. 2017). At the same time, the dose of IHA has a significant effect on the process of reaching equilibrium in the adsorption reaction. When the dose is 0.6, 0.8 g, the time required to reach equilibrium is 300, 420 min, respectively; while the dose of IHA was 1.0 and 1.2 g, the time to reach adsorption equilibrium was 240 min. It can be seen that with the increase of the dose, the time for the completion of the adsorption reaction is also reduced, leading to a more rapid equilibrium for the adsorption reaction.

The results of the fit to the adsorption kinetic model are shown in Figures 5 and 6 and Table 1. It can be seen from Figure 5 that the pseudo-first-order kinetic model fits the experimental data well in the initial rapid adsorption stage, but gradually deviates from the curve as the adsorption stage slows (Li et al. 2008). It can be seen from Figure 5 and the parameters in Table 1 that under the condition that the dose of IHA is 0.6, 0.8, 1.0 and 1.2 g, the simulation of the humic acid adsorption Mn(II) process by the pseudo-second-order adsorption kinetic model is better than that by the pseudo-first-order adsorption kinetic model, and the equilibrium adsorption capacities q_e obtained by the pseudo-second-order kinetic equation are 20.77, 19.04, 16.45 and 14.82 mg/g respectively, which are closer to the measured values. It shows that the pseudo-second-order kinetic model can better reflect the adsorption process of IHA to Mn(II), and it is speculated that the adsorption process is mainly chemical adsorption (Huang et al. 2009; Li et al. 2011).

It can be seen from the fitting curve of the internal diffusion model in Figure 6 that under different doses of IHA, the fitting of q_t to t^1/2 can be divided into two stages. In the initial stage, a large number of vacant active sites on IHA can be used to adsorb Mn(II); at this time, the adsorption reaction is mainly controlled by the ion exchange between Mn(II) and the functional groups on the surface of IHA and the diffusion of the external liquid membrane. The second stage is the slow adsorption; that is, as the binding sites are gradually occupied, a concentration gradient will form between the surface and the interior of the humic acid, which promotes the diffusion of Mn(II) into the interior of the humic acid molecule, and the adsorption rate is controlled by the intraparticle diffusion rate (Yang et al. 2018). From the different slopes of the first and second stages, it is known that there is a gradual stage in the adsorption process, the surface adsorption process is controlled by the thickness of the boundary layer. The internal diffusion rate constant Ki₂ is much smaller than Ki₁, which shows that the internal diffusion rate of particles is slow (Li et al. 2013b). Besides, the curve of q_t to t^1/2 is double-line in the whole time range, and the fitting curve does not pass through the origin, which shows that the adsorption process is not only affected by the internal diffusion of particles, but also under the common control of other adsorption stages (Li et al. 2013b). The adsorption rate of Mn(II) by IHA is determined by the boundary layer effect and the external mass transfer effect (Dong & Wang 2016).

From Figure 7(a) and 7(b), it can be seen that both Freundlich and Langmuir equations can fit the adsorption process well. The correlation coefficient R² in both models is greater than 0.99, which indicates that under this experimental condition, single-layer adsorption and heterogeneous surface adsorption coexist, and the adsorption process of insolubilized humic acid on Mn(II) is complicated, involving a variety of adsorption mechanisms (Zheng et al. 2018). Under the temperature conditions of 298.18 K, 308.18 K, and 318.18 K, the maximum adsorption capacity of insolubilized humic acid for Mn(II) fitted by the Langmuir model for manganese ions is 41.81 and 48.30 and 52.87 mg/g, respectively, which is close to the actual adsorption capacity. At the same time, it can be seen that the increase of temperature is conducive to the adsorption reaction, the main reason being that the temperature increase causes the adsorption equilibrium to move to the right. In addition, the values of R_L in the Langmuir model and 1/n in the Freundlich model are between 0 and 1, indicating that insolubilized humic acid has a higher affinity for Mn(II) (Acar & Malkoc 2004; Zhang et al. 2018).

The results of adsorption thermodynamic parameters are shown in Table 3. It can be seen from ΔG > 0 that the adsorption of Mn(II) by insolubilized humic acid is a non-spontaneous reaction process, ΔH > 0 indicates that the adsorption process is an endothermic reaction (Kimuro et al. 2019), and the increase of temperature is conducive to the adsorption process, which also confirms that the adsorption capacity of IHA increases gradually with the increase of temperature during the experiment. When the initial concentration of Mn(II) is 5 mg/L, ΔG < 0, which indicates that the degree of freedom of Mn(II) decreases on the solid-liquid interface, and Mn(II) initially occupies the effective adsorption site on HA at low concentration, but with the increase of concentration ΔG > 0, which indicates that the disorder of solid-liquid interface increases during the adsorption of Mn(II) by IHA. Since the entry of Mn(II) into the liquid phase is a process of increasing the degree of freedom, the rate of increase is greater than the rate of decrease in the degree of freedom caused by the adsorption of Mn(II) on IHA. The water molecules on the IHA surface dissociate due to adsorption, which leads to the increase of free molecules in the whole system (Yang et al. 2018).

The FTIR spectroscopy of the IHA before sorption and after sorption were obtained in the wavenumber range of 4,000–500 cm⁻¹. The broad and strong characteristic absorbance peaks of IHA at 3,371.54 cm⁻¹ are mainly caused by -OH stretching vibration on the carboxyl group, alcohol hydroxyl group and phenolic hydroxyl group (Chen et al. 2015). The characteristic peak of the functional groups changed after IHA adsorbs Mn(II). -OH located at the wavelength of 3,371.54 cm⁻¹ can play a hydrogen bonding role when IHA adsorbs Mn(II) (Boguta et al. 2019); after absorption, the intensity of the -OH stretching vibration peak in the complex of humic acid and manganese ions weakened, indicating that Mn(II) reacted with -OH on IHA (Erdogan et al. 2007). It is speculated that the mechanism of insolubilized humic acid adsorption on Mn(II) includes surface hydroxyl exchange. At 1,579.28 and 1,383.38 cm⁻¹, they are the antisymmetric stretching vibration peak and the symmetric stretching vibration peak of COO-, respectively. Compared with before and after adsorption, it is speculated that the carboxyl group participated in the complexation, and part of the COOH was transformed into the carboxylate COO- (Boguta & Sokołowska 2016). 1,033.56 cm⁻¹ is the characteristic peak of C-O low strength stretching vibration of ether (Zhang et al. 2015). The change in functional groups indicates that the carboxyl group and hydroxyl group participate in the adsorption of Mn(II) on IHA. The present study agrees with the findings of Md. Aminul Islam et al. (Islam et al. 2020). The adsorption of Mn(II) on IHA includes chemical and physical effects, and combined with adsorption kinetics studies, this process is mainly chemical adsorption.

Scanning electron microscope was used to observe the surface morphology of insolubilized humic acid before and after adsorption. Figure 9(a) is a sample obtained by adding IHA to a deionized aqueous solution containing no Mn(II), after the same treatment time as the Mn(II) adsorption experiment. It can be seen from Figure 9 that the appearance and morphology of IHA before and after adsorption of manganese ions are significantly changed. It can be seen from Figure 9(a) that the surface of the adsorbent insolubilized humic acid is uneven, rough, and clustered between the particles, which increases the specific surface area of the adsorbent and helps it adsorb metal manganese ions (Zhang et al. 2020). Comparing Figure 9(a) and 9(b), it can be seen that after adsorption, insolubilized humic acid loses the full cluster structure formed by large particles, thus presenting the form of small particles, and a large number of small particles and fragments are produced. This is due to the adsorption reaction between Mn(II) and insolubilized humic acid, and the functional groups on the surface of humic acid participate in the adsorption of Mn(II) (Jiang et al. 2015; Chen et al. 2020). EDS spectra of HA-Mn(II) complex surface is shown in Figure 10. It can be seen that the content of C element in the sample is extremely high, and the appearance of elements such as Al and Si may be caused by impurities in the commercially available humic acid (Wang et al. 2020). At the same time, the appearance of Mn element in the spectrum shows that IHA successfully adsorbed Mn(II) from aqueous solution.

In this study, insoluble humic acid (IHA), a natural organic substance with abundant reserves in nature, was used to remove Mn(II) from aqueous solutions. Batch adsorption experiments showed that IHA has an excellent adsorption effect on Mn(II), and pH had a significant impact on the adsorption performance of IHA. Under the condition of pH = 5.6 and temperature of 298.15 K, the maximum adsorption capacity of IHA was 52.87 mg/g. According to the results of adsorption kinetics and isotherm adsorption characteristics of IHA for Mn(II), the adsorption process conformed to the pseudo-second-order model and Langmuir model, and the adsorption of Mn(II) on IHA was mainly a chemical reaction. Thermodynamic analysis showed that the adsorption is a non-spontaneous endothermic reaction. FTIR analysis showed that the functional groups on IHA reacted with Mn(II). Due to the advantages of low-cost and eco-friendliness, IHA can be feasible for some certain practical wastewater treatment.

This work was supported by the National Natural Science Foundation of China (No. 41973078) and the Ministry of Education in China Project of Humanities and Social Science (2019JJ40081).

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