Manganese Ions Removal from Industrial Wastewater

In this study the removal of Mn (II) ions from wastewater using magnetite nanomaterial was investigated. Some factors influencing the wastewater treatment process were studied such as: treatment time, pH and the concentration of Mn (II) ions from wastewater. The results showed that using magnetite nanomaterial adsorbent lead to a wastewater treatment efficiency higher than 97%. Langmuir and Freundlich models were applied to describe the adsorption process. The correlation coefficients (R) showed that both models are applicable to the experimental data obtained.


Introduction
The industrial wastewaters polluted with heavy metal ions, which are toxic, is a huge environmental issue [1]. Heavy metals ions affect seriously human health if they are entering in the human body [2][3][4][5]. The sources of heavy metal ions pollution are industrial effluents from metal plating and finishing, rubber processing, mining, agriculture and many others [6]. This category of pollutants has toxic actions for our bodies because of their bioaccumulation in the tissues [7][8][9][10]. According to the NTPA 001/2002 standard which contains the limits of loading with pollutants of the industrial and domestic wastewater discharged in the natural environment, the concentration of manganese ions should not exceed 1.00 mg/L. Moreover, the limit of Mn 2+ accepted in drinking water is 0.05 mg/L according to the Water Law 458/2002. Nowadays, many methods are used for depollution of industrial wastewater, but some of them are not effective for removing manganese ions or they may be too expensive.
The purpose of this study was to remove low concentrations of manganese ions from wastewaters using the magnetite nanomaterial (Fe3O4). Also, was studied the influence of contact time, pH and Mn (II) ions concentration of the wastewater upon the efficiency of the depollution process.
The utilisation of nanosized magnetic oxide particles have many advantages such as: high surface area which gives high capacity of pollutants retaining from wastewater compared with other conventional adsorbents, reduced cost and easily separation from wastewater using magnetic field at the end of the process [11]. For this reason, new methods of depollution implying the use of relatively inexpensive adsorbents are researched (

Materials
For this study, wastewater solutions having different concentrations (0.70, 1.00, 1.20, 1.40, 1.60, 1.80 and 2.00 mg/L) of Mn(II) were used. The wastewaters were studied at two different pH values. The pH was established by adding sodium hydroxide or hydrogen chloride. The nanomaterial used for this study was magnetite (Fe3O4) and its obtaining method is presented elsewhere [17].

Methods
To remove manganese ions from 100 mL wastewaters, an amount of 0.2 g of magnetite was added. These wastewaters were homogenized at room temperature. During the experiments was measured the concentration of Mn(II) ions using a photometer PhotoLab S12.
Adsorption isotherms were used for experimental data modeling. The removal efficiency (ƞ, %) and equilibrium adsorption amount (qe, mg/g) of Mn (II) ions were calculated using the following formulas: where: Co-represents the initial concentration of Mn(II) ions, mg/L; Cf-represents the final concentration of Mn(II) ions, mg/L.
where: Co-represents the initial concentration of Mn(II) ions, mg/L; Ce-represents the equilibrium concentration of Mn(II) ions, mg/L; V -represents the volume of the wastewater, L; W -represents the amount of the magnetite nanomaterial, g.

3.1.Effect of pH
The removal of Mn(II) ions was studied at two pH values (8 and 11.5), and the results are shown in Figure 1. It can be observed that in the case of the concentration of 0.70 mg/L the removal efficiency increases with the increase of the pH from 91.4% to 92.8% (Figure 1a). In the case of the https://doi.org /10.37358/Rev. Chim.1949 Rev. Chim., 71 (7) (Figure 1b). For the wastewater having 1.20 mg/L concentration Mn (II) the removal efficiency increases with increasing pH from 90.0% to 97.5% (Figure 1c). These observations lead to the conclusion that a basic wastewater is a more favorable medium for the removal of manganese ions from industrial wastewater. a b c

Effect of contact time
The effect of contact time was studied for the removal of Mn (II) ions from wastewater and the results are shown in Figure 2. It can be seen that the removal efficiency of pollutant removal from wastewater increases with increasing time up to 500 min after which it remains constant, resulting in a removal efficiency of 90.0% for pH 8 and 97.5% for pH 11.5.

3.3.Adsorption isotherms
Two models were applied in the study of removal of Mn(II) ions from wastewater, Langmuir and Freundlich.

Langmuir model
The Langmuir model is presented in the form of the following equation: where: KL -is the Langmuir isotherm constant, L/mg; Qmax -is the maximum quantity, mg/g as the amount corresponding to complete monolayer coverage; Ce -is the concentration at equilibrium, mg/L; qe -is the amount of metal adsorbed per gram of the adsorbent at equilibrium, mg/g. Using the following equation was calculated the equilibrium parameter RL: where: RL -value indicates that the process is unfavourable if RL>1, linear if RL=1, favourable if 0< RL<1 and irreversible if RL=0; KL is the Langmuir constant; C0 -is the initial concentration, measured in mg/L.

Freundlich model
Freundlich model is presented as the equation: = * 1/ (5) where: qe -is the quantity of metal adsorbed by adsorbent at equilibrium, mg/g; KF -is the Freundlich isotherm constant, mg/g; Ce -is the equilibrium concentration of adsorbate, mg/L; n -is the adsorption intensity; The Langmuir and Freundlich models based on experimental data were graphically represented (Figure 3,4). The constants of both models are presented in Table 2. It is observed that the experimental values fit the isotherms adequately. In the case of the Langmuir model, its applicability indicates the monolayer coating of the magnetite surface by the manganese ions. The Langmuir constant RL is in the range 0-1, indicating that the retention process of Mn (II) ions is favorable. The linear graph between log (Ce) and log (Qe) confirm the applicability of the Freundlich model.  The results suggest that this type of magnetic nanomaterial could be successfully used for manganese ions removal from industrial wastewaters. The possibility of fast separation by magnetic field of magnetite from the wastewater at the end of the treatment and the high removal efficiency recommend it as a useful adsorbent. Also, future regeneration studies will be undertaken in order to establish the life duration use of the magnetite adsorbent. It is a very high probability that the magnetite nanomaterial can be adapted to the removal of other types of heavy metals from wastewater.

Conclusions
The study focused on the removal of manganese ions from wastewater. The removal of Mn(II) ions from wastewater using Fe3O4 nanomaterial reveal that the process was pH dependent and the maximum efficiency was observed at pH 11.5 resulting in a yield of 97.50%. At a lower pH (8) the maximum depollution efficiency was 91.00%. The treatment time required to reach the maximum capacity of the magnetite nanomaterial in order to remove the Mn(II) ions was 500 min, then the Mn(II) ions concentrations remained constant in the wastewaters. The R 2 values indicate that adsorption process take place onto heterogeneous material, according to Freundlich model. Values of n lower than 1 indicates possible binding between magnetite and manganese ions.
Aknowledgement: This work was supported by a grant of the Romanian Ministry of Research and Innovation, CCCDI-UEFISCDI, project number 26PCCDI/01.03.2018, "Integrated and sustainable processes for environmental clean-up, wastewater reuse and waste valorization" (SUSTENVPRO), within PNCDI III.