*Article* **Recovery of Cr(III) from Tannery Effluents by Diafiltration Using Chitosan Modified Membranes**

**Asmaa Zakmout 1,2 , Fatma Sadi <sup>2</sup> , Svetlozar Velizarov 1,\* , João G. Crespo <sup>1</sup> and Carla A. M. Portugal 1,\***


**Abstract:** The selective recovery of chromium remaining in tannery effluents after the leather tanning process is highly desirable to potentiate its reuse, simultaneously minimizing the ecotoxicity of these effluents. To the best of our knowledge, this work evaluates for the first time the ability of a chitosan-based membrane for selective recovery of chromium from a tannery wastewater by subsequent diafiltration and selective chromium desorption, envisaging their integration after tannery wastewater treatment by reverse osmosis (RO). A polyethersulfone (PES) microfiltration membrane top-coated with a chitosan layer (cs-PES MF022) was used for selective recovery of Cr(III), from concentrate streams obtained by treatment of synthetic and real tannery effluents through reverse osmosis (RO), through a diafiltration process. The diafiltration of the RO concentrates was conducted by an intermittent addition of water acidified to pH 3.6. The prepared cs-PES MF022 membranes were able to retain 97% of the total mass of Cr(III) present in the RO concentrates, from a real tannery effluent, with a selectivity of 4.2 and 5 in reference to NH<sup>4</sup> <sup>+</sup> and Cl−, respectively, 12.9 and 14.6 in reference to K and Na, and >45 in reference to Mg, Ca, and S. Such a high selectivity is explained by the preferential adsorption of Cr(III) onto chitosan, and by the relatively high permeability of cs-PES MF022 membranes to the other ionic species. Proof of concept studies were performed to investigate the desorption of Cr(III) at pH 2 and 5.8. A higher Cr(III) desorption degree was obtained at pH 2, leading to a final solution enriched in Cr(III), which may be re-used in tannery operations, thus improving the process economy and reducing the hazardous impact of the effluents discharged by this industry.

**Keywords:** tannery effluent; chromium recovery; chitosan membrane; diafiltration; selective adsorption; selective desorption

#### **1. Introduction**

Leather tanning requires the use of a large variety of chemicals, such as tannins, sulphates, phenolics, surfactants, and ion salts. In particular, Cr(III) is used in the form of chromium sulphate Cr2(OH)2(SO4)<sup>2</sup> for the conversion of collagen from skin into commercial leather. This process results in the production of high wastewater volumes with appreciable chromium content [1] that increases the ecotoxicological impact of the tannery effluents [2]. Wastewater treatment is thus mandatory before discharge this water stream, which must possess physicochemical characteristics that conform with local regulations for a safe discharge into the ecosystems. In addition, the recovery of chromium allowing for its subsequent reuse can reduce the costs associated with the leather tanning process.

Several physicochemical and biological approaches have been used to reduce the organic and inorganic content of tannery wastewaters, including coagulation/flocculation induced by specific chemical agents [3], electrocoagulation [4], solvent extraction [5],

**Citation:** Zakmout, A.; Sadi, F.; Velizarov, S.; Crespo, J.G.; Portugal, C.A.M. Recovery of Cr(III) from Tannery Effluents by Diafiltration Using Chitosan Modified Membranes. *Water* **2021**, *13*, 2598. https:// doi.org/10.3390/w13182598

Academic Editor: Margaritis Kostoglou

Received: 11 July 2021 Accepted: 15 September 2021 Published: 21 September 2021

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adsorption [6], aerobic or anaerobic biological treatment [7], incineration [8], chemical oxidation [9], membrane filtration [10–12], and via process integration, such as by combining coagulation with microfiltration [13] or electrodialysis [14,15]. Among the adsorptionbased methods, different adsorbents, such as clays [16,17], activated carbon derived from different biological [18] and non-biological materials [19] with distinct morphologies [20], and chitosan-based compounds [6,21–26] have proved to allow for an efficient capture of metals from target effluents.

Chitosan is an eco-friendly and inexpensive biomaterial obtained from the partial deacetylation of the acetoamine groups of chitin. Chitosan-based flakes/beads [6], nanofibres [22], membranes [12,23], or films [24] produced by chitosan crosslinking have been prepared and tested in terms of their capacity to adsorb metal ionic species. The outstanding capacity of chitosan to adsorb heavy metal ions [22], including copper [25,26] and chromium [23], such as Cr(III), has been mainly ascribed to the ability of amine to act as chelators of these ionic species. However, a potential interaction with hydroxyl groups should also be considered. The chelating efficiency was found to be dependent on the electric charge of the species involved and, thus, dependent on pH [6]. In strongly acidic solutions, Cr(III) is mainly present in the form of cationic species Cr3+, Cr(OH)<sup>2</sup> + , and Cr(OH)+2, whereas the Cr precipitation observed at more alkaline conditions (pH > 6) is explained by a decrease of the solubility of chromium hydroxides at increasing pH values. Thus, due to the prevalence of positively charged amine (R-NH<sup>3</sup> + ) and hydroxyl (R-OH<sup>2</sup> + ) groups of chitosan, the adsorption of the positively charged Cr(III) species is unfavoured at lower pH values, but increases with the deprotonation of these chemical groups as the pH increases. Nevertheless, Cr(III) removal by adsorption is still more efficient at a pH lower than 6, due to the absence of intense Cr precipitation [23]. Mirabedini et al. [24] produced magnetic chitosan hydrogels crosslinked with glyoxal, which showed pH dependent Cr(IV) removal efficiency from water, with optimal values of 80–90% reached at pH 4.

The high affinity of chitosan to heavy metals brings additional interest in the development of chitosan-based membranes for the removal of these metals from wastewaters, taking advantage of both, their adsorptive and water filtration capacity. In a work from Juang et al. [27], chitosan flakes were added to a filtration system to increase the efficiency of regenerated cellulose membranes to remove divalent ions, such as Cu(II), Co(II), Ni(II), and Zn(II). Chitosan flakes were shown to increase the removal degree of these metal ions six to tenfold under acidic conditions. In another work from Li et al. [28], electrospun chitosan membranes were prepared and used as membrane adsorbers for the removal of Cr(IV) from model aqueous solutions. Regarding the Cr(IV) loading capacity of these membranes, their bed saturation and efficiency showed a higher dependence on pH, solution flow rate, fluid flow distribution, and membrane packed patterns, as compared to the Cr(IV) concentration in the feed solution and bed length. The maximum bed loading capacity was found to be 16.5 mg Cr(IV)/g of chitosan under dynamic fluid conditions. Chitosan/polyethylene oxide (PEO) nanofibres (90:10) assembled to spunbonded polypropylene (PP) substrate containing 90% chitosan developed by Desai et al. [29] showed a Cr binding capacity of 35 mg Cr(VI)/g chitosan. The same authors have also shown that the binding capacity of chitosan/PEO nanofibres is dependent on the chitosan content in the chitosan/PEO blend and decreases with the increase of the fibre diameter [29]. In a previous work by Ghaee et al. [30], chitosan/cellulose acetate composite nanofiltration membranes with various amounts of cellulose acetate were prepared and evaluated in terms of their rejection capacity to copper. The membranes obtained exhibited a molecular weight cut-off of 830.74 Da, and a Cu rejection of 81.03%. Cellulose membranes, enriched with chitosan-silver ions, were designed by Căprărescu et al. [31] and used for the removal of iron ions from synthetic wastewaters with an electrodialysis system. The presence of silver ions increased the electric conductivity of the membranes, resulting in iron removal rates > 60% at an applied voltage of 15 V.

In our previous study [12], a polyethersulfone (PES) membrane coated with a thin chitosan layer—a chitosan modified membrane designated for simplicity as cs-PES MF022was prepared, and its performance for the treatment of model tannery effluents was compared with that of a SW30 reverse osmosis (RO) membrane in terms of species rejection efficiency and selectivity. The cs-PES MF022 membrane showed rejection coefficients <45% for most of the ionic species present, i.e., Na<sup>+</sup> , K<sup>+</sup> , Ca2+, Mg2+, Cl−, NH<sup>4</sup> + , and SO<sup>4</sup> <sup>2</sup>−. These rejection coefficients were significantly lower than those obtained with the SW30 membrane, which provided rejection values above 90% for all these ions. As a result, the cs-PES MF022 membrane was considered unsuitable to produce treated water with the characteristics required to allow the discharge of the effluents into the ecosystem. However, this membrane exhibited an outstanding Cr(III) rejection > 90%, and, thus, a Cr(III) removal selectivity much higher than that obtained with the SW30 membrane. These results evidenced the potential suitability of cs-PES MF022 membranes for the selective recovery of Cr(III) from tannery wastewaters, prompting us to investigate and suggest a two-step process for this purpose. This process consists of the integration of reverse osmosis (RO) (1st step), in which an SW30 membrane is used to obtain a permeate stream with a chemical composition which allows its direct discharge into the environment [12], followed by a diafiltration of the RO concentrate (2nd step) for a selective recovery of Cr(III), using the chitosan modified membrane (cs-PES MF022) mentioned above. Diafiltration involves a solvent (acidic water) consumption higher than that required by membrane processes when operated in conventional mode, and for this reason, it may be regarded as a disadvantageous separation process. However, the possibility to treat complex aqueous streams—such as tannery wastewaters—under conditions of reduced concentration polarization and fouling-related effects [32,33], made us anticipate the suitability of diafiltration for the enhanced removal of contaminants, i.e., other ionic species with a lower affinity to chitosan and low-molecular mass compounds, from these effluents, facilitating the isolation of Cr-containing compounds and, thus, allowing for its subsequent re-use in tannery processes. Therefore, this study aims at evaluating the performance of the cs-PES MF022 membranes for selective removal of Cr(III) from concentrates obtained by RO treatment of synthetic and real tannery effluents by diafiltration, and the possibility to recover Cr(III), by selective desorption, taking advantage of the pH-responsive ability of chitosan [34].

#### **2. Materials and Methods**

## *2.1. Materials*

The synthetic tannery effluent was prepared using magnesium chloride (MgCl2; purity 99%) provided by Alfa-Aesar (Kandel, Germany); calcium chloride 2-hydrate (CaCl2.2H2O; purity > 99%), ammonium sulphate ((NH4)2SO4; purity > 99%), and sodium chloride (NaCl; purity 99.8%) supplied by AppliChem PANREAC (Barcelona, Spain); and chromium(III) sulphate basic (Cr4(SO4)5OH2; 26% Cr2O3) obtained from Fluka Analytical (Buchs SG, Switzerland). The chitosan (cs) used for the preparation of functional chitosan membranes was provided from Sigma-Aldrich (St. Louis, MO, USA). Acetic acid (CH3COOH; purity 99.8%), potassium sulphate (K2SO4; purity > 99%), and sodium hydroxide (NaOH; purity > 93%) were supplied by Carlo Erba (Val de Reuil, France). Glutaraldehyde (GDA; 25% in H2O) and hydrochloric acid (HCl; purity 35–38%) were purchased from Sigma-Aldrich Chemicals (St. Louis, MO, USA).

#### *2.2. Methods*

2.2.1. Concentrates of Real and Synthetic Tannery Effluents Obtained from Reverse Osmosis (RO)

The concentrates of real tannery effluent was obtained by reverse osmosis (RO) treatment of a real tannery effluent supplied by TAMEG-Rouiba-SPA—a Leather Industry located in Rouiba, close to Algiers (Algeria), whereas the concentrate of a synthetic tannery effluent was obtained by RO treatment of a synthetic solution, prepared in the lab, mimicking the composition of the real effluent in terms of the inorganic content (inorganic salts). The treatment of these effluents were performed using an SW30 reverse osmosis (RO) membrane supplied from DOW Chemical Company (Midland, MI, USA). RO showed

Figure 1

Figure 2

**J (L/h.m2)**

1

rejections higher than 95% for all of the inorganic salts (99.2% rejection was obtained for Cr), allowing for the production of a treated water permeate [12] meeting the requirements for a direct discharge of the permeate stream into natural environments, according to the Algerian Legislation [35], and a concentrate enriched in the non-permeable components present in the effluents. The chemical composition of the concentrate of the synthetic and the real tannery effluents obtained by processing with the SW30 RO membrane are summarized in Tables 1 and 2, respectively.

**Table 1.** Chemical composition of the concentrate obtained by processing of a synthetic effluent with an RO SW30 membrane [12], at pH 3.6 and T = 20 ◦C.


<sup>1</sup>The electric charge of the elements determined by ICP-AES was omitted, as ICP-AES quantifies the total amount present in the solution independently of their electric charge.

**Table 2.** Chemical composition of the concentrate obtained by processing of a real effluent from TAMEG-Rouiba Tannery with an RO SW30 membrane [12], at pH 3.6 and T = 20 ◦C.


<sup>1</sup>The electric charge of the elements determined by ICP-AES was omitted, as ICP-AES quantifies the total amount present in the solution independently of their electric charge.

#### 2.2.2. Preparation of the Chitosan-Based Membranes (cs-PES MF022)

Functional chitosan membranes were prepared by coating a 2.5% (*w*/*w*) chitosan solution on a polyethersulfone microfiltration membrane (PES MF022, 0.22 µm) from Merck Millipore (Carrigtohill, Ireland) which was used as a support. The 2.5% (*w*/*w*) chitosan solution was prepared by dissolving chitosan in a 5% (*v*/*v*) acetic acid aqueous solution, at room temperature, and then casted on the top surface of the PES MF022 support. The casting was carried out using an Elcometer casting knife film applicator (E.U.), by setting an application air gap of 90 µm, to ensure the formation of a chitosan layer with an uniform and reproducible thickness of ca. 10 µm, as shown in Figure 1 [12]. The resulting membrane was kept and dried at room temperature in a fume hood until complete solvent evaporation.

**Figure 1.** Scanning Electronic Microscopy (SEM) image of the cross-section of a chitosan modified membrane, cs-PES MF022, obtained after filtration of the synthetic tannery effluent (adapted from [12]).

The membrane was then soaked in a 1 M NaOH solution for four hours, immersed in a glutaraldehyde solution (25% in H2O) overnight, and then washed exhaustively with

6 8 10 12 14 16 18 20 22

**Applied pressure,** Δ**P (bar)**

distilled water. The membrane washing efficiency was accessed by a periodic determination of the UV-Vis spectra (from 200 nm to 400 nm) using a spectrophotometer Thermo Scientific Evolution 201 (Madison, WI, USA), and by determination of the pH and conductivity of the washing solution (supernatant) for inspection of the complete release of the NaOH excess, and the possible release of loosely bound compounds, e.g., chitosan [12]. A pHmeter, type CRISON (Barcelona, Spain), was used to monitor the pH, whereas conductivity was measured by a Schott Lab960 conductivity meter (Mainz, Germany). The washing procedure was considered complete when the pH and conductivity registered for the washing solution were equal to the pH and conductivity values of the distilled water, and the absorbance of the washing was lower than 0.05 within the analysed wavelength range [26]. The membrane was kept in a closed petri dish with some drops of water to ensure the air humidity needed to prevent possible membrane structural alterations.

#### 2.2.3. Diafiltration

Before the permeation experiments, the chitosan modified membrane cs-PES MF022 was mounted in a pressurized stirred filtration cell (MetCell, Greenford, UK), which was then filled with demineralized water acidified with HCl to a pH = 3.6 (acidic water). In the first stage, the cs-PES MF022 membrane was operated in stepwise mode, increasing pressures up to 40 bar for membrane structural compaction, to avoid the possibility of membrane structural changes along the process. In a second stage, the permeation cell was re-filled with acidic water (pH 3.6), and the permeation was conducted by the subsequent increase and decrease of the pressure from 7 bar to 20 bar, in a stepwise way to confirm the structural stability of the membrane. Each pressure step was kept for 10–20 min.

The permeate solution obtained was collected in a reservoir and weighted for a long time using a balance connected to a PC for data acquisition, and then used for determination of the membrane hydraulic permeability.

Each diafiltration experiment was conducted using 35 mL of feed solutions (SW30 concentrate of the synthetic and real tannery effluents). The cell was operated at a constant pressure of 20 bar in a dead-end diafiltration mode, and under a constant stirring speed of 400 rpm. The diafiltration was conducted by the intermittent addition of water (acidified at pH 3.6 with HCl) during the process [36]. As illustrated in Scheme 1, intermittent diafiltration consisted in a series of successive concentration mode filtration steps, interrupted after each 20 mL of permeate collected, for the addition of an identical volume of fresh solvent (20 mL) to the remaining retentate solution in order to restore the initial feed volume in each step. The retentate was sampled (0.5 mL) each time before re-initiation of the diafiltration. This procedure was repeated 7 times during the diafiltration process, leading to a final number of 4 diafiltration volumes (diavolumes).

**Scheme 1.** Schematic representation of the intermittent diafiltration strategy used in the present work. V0 , V<sup>1</sup> and V<sup>N</sup> are the feed volumes at the beining of each diafiltration stage, V<sup>P</sup> is the permeate volume collected and V<sup>A</sup> is the volume of solvent added in each diafiltration stage. V<sup>S</sup> corresponds to the volume of the retentate sampled for analysis at each diafiltration stage.

After collecting the final retentate solution, the filtration cell was rinsed with distilled water and pressurized from 20 to 40 bar in order to check the membrane hydraulic permeability after the process.

## 2.2.4. Data Treatment

## Membrane Permeability

The permeate flux, *J,* L/(m<sup>2</sup> ·h) was calculated as:

$$J = \frac{Q\_P}{A} \tag{1}$$

where *Q<sup>P</sup>* is the volumetric permeate flow (L/h) and *A* is the surface area of the membrane (m<sup>2</sup> ). The membrane hydraulic permeability was calculated by registering the permeate flux and applying the Darcy equation:

$$J = L\_P \* \left(\Delta P - \Delta \pi\right) \tag{2}$$

where *L<sup>p</sup>* is the membrane hydraulic permeability, ∆*P* is the applied pressure, and ∆*π* is the osmotic pressure difference between the concentrate and the permeate side. The osmotic pressure difference (∆*π*) was also considered, but given the good permeability of the membrane to most of the species and the reduced amount of species remaining in the retentate along the process, an average ∆π < 1 bar was obtained, which was considered negligible compared to the applied operating pressure of 20 bar. Ionic speciation was accessed through the determination of chemical equilibrium diagrams obtained using Make Equilibrium Diagrams Using Sophisticated Algorithms (MEDUSA) software (version: Eq. calcs 32) [37].

Determination of the Compounds Retained in the Membrane and Cr Retention Selectivity

The amounts of each compound, *i*, retained in the membrane after diafiltration, were calculated by a global mass balance (*M<sup>i</sup>* ) to the process, using Equation (3):

$$M\_{m,i} = \mathbb{C}\_{F,i} \times V\_F - \sum \mathbb{C}\_{P,i} \times V\_P - \sum \mathbb{C}\_{S,i} \times V\_S - \mathbb{C}\_{R,i} \times V\_R \tag{3}$$

where *Mm,i* is the mass of compound *i* in the membrane; *CF,i*, *CP,i*, *CR.i*, and *CS,i* are the concentration of the compound *i* in the feed, permeate, retentate, and sampling solutions; and *VF*, *VP, VR,* and *V<sup>S</sup>* correspond to the volumes of feed, permeate, final retentate, and sampling volume.

The chromium retention selectivity (*SCR*) was calculated according to the equation:

$$S\_{\mathbb{C}r} = R\_{\mathbb{C}r} / R\_i \tag{4}$$

where *RCr* and *R<sup>i</sup>* are the mass of Cr and the mass of each compound *i* retained in the membrane, relative to that present in the feed, calculated according to Equation (5):

$$R\_{\bar{i}} = \frac{M\_{m,\bar{i}}}{M\_{\mathbb{F},\bar{i}}} \tag{5}$$

where *Mm,i* and *MF,i* correspond to the mass of each compound *i*, retained in the membrane at the end of the process (*N<sup>D</sup>* = 4) and that present in the feed solution at the beginning of the process, respectively.

#### 2.2.5. pH Induced Desorption

The cs-PES MF022 membrane used for the diafiltration of the SW30 concentrate from synthetic tannery effluent was cut into identical pieces with an area of 15.9 cm<sup>2</sup> . One piece was immerged in a beaker containing 10 mL of demineralized water (pH 5.8), and the other was immerged in an identical volume of demineralized water acidified with HCl to pH 2. The solutions were periodically sampled along 72 h, and the samples were analysed by ICP-AES as described below.
