2.2.6. Analytical Methods Quantification of the Compounds

An inductively coupled plasma atomic emission spectrophotometer, ICP–AES (Ultima model, Horiba Jobin-Yvon, France)—equipped with a radio-frequency (RF) generator of 40.68 MHz—a Czerny–Turner type monochromator with 1.00 m (sequential), an AS500 autosampler, and data acquisition software were employed to determine the concentration of target elements, i.e., Na, K, Ca, Mg, Cr, and S in the feed, permeate, and retentate solutions. Kits (LCK11) from Hach Lange GmbH (Düsseldorf, Germany) were used for chloride determination. The NH<sup>4</sup> + content was determined colorimetrically using a Skalar SAN++ segmented flow analyser, Skalar Analytical B.V. (AA Breda, The Netherlands).

Characterization of Membrane Structure and Chemical Characterization

The structural characteristics of the cs-PES MF022 membranes were examined using a Field Emission Scanning Electronic Microscopy, FEG-SEM (Jeol JSM–7001F, Tokyo, Japan), upon coating the membrane with an AU/Pd film of 20 nm thickness using a sputter coater from Quorum Technologies (model Q150TES, West Sussex, UK), which were discussed in a previous paper [12]. An energy dispersive X-ray spectrometer (EDS) detector was also used for inspecting the presence of compounds at the membrane surface.

## **3. Results and Discussion**

## *3.1. Diafiltration of the SW30 Concentrate Solution—Synthetic Tannery Effluent*

The concentrate of a tannery synthetic effluent obtained by reverse osmosis (RO) was processed by diafiltration using the chitosan modified membranes (cs-PES MF022), aiming at a selective recovery of chromium.

Prior to filtration of the SW30 concentrate, the structural stability of the membrane was inferred, after its compaction, based on the determination of the membrane hydraulic permeability at increasing and decreasing pressures in a range from 7 bar to 20 bar. As shown in Figure 2, identical permeate fluxes were obtained at the same ∆P values during ascending and descending volumetric flux profiles, resulting in a practically identical hydraulic permeability, *Lp*, of about 0.5 L/(h·m<sup>2</sup> ·bar). This evidences that the cs-PES MF022 membrane has good structural stability, with an ability to support pressures within the considered ∆P range.

**Figure 2.** Permeate fluxes, *J,* obtained during filtration of acidic water (pH 3.6) at increasing (squares) and decreasing (circles) pressures using the cs-PES MF022 membrane before diafiltration. *Lp* represents the hydraulic permeability of the membrane determined at increasing and decreasing pressures.

*Water* **2021**, *13*, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/water

The SW30 concentrate obtained from the RO treatment of a synthetic effluent with the chemical composition indicated in Table 1 was diafiltrated through the cs-PES MF022 membrane, at a ∆P of 20 bar, using a solvent (acidic water) stepwise addition strategy. The permeate flux profile observed during the diafiltration, depicted in Figure 3, shows a constant oscillating profile with an average value of 10.5 L/(h·m<sup>2</sup> ). A time dependent decline of the permeate flux was not observed, suggesting the absence of significant concentration polarization and fouling phenomena during the process. This may be explained by the attenuation of solutes' accumulation related effects at the membrane surface due to the intermittent addition of solvent and the fluid dynamic conditions established in the filtration cell.

**Figure 3.** Permeate flux, *J*, obtained during diafiltration of the SW30 concentrate of the synthetic tannery effluent, using the cs-PES MF022 membrane at an applied pressure of 20 bar.

The efficiency of the diafiltration process was evaluated based on the membrane ability to selectively remove Cr(III) (the target compound) in reference to the other ionic species present in the SW30 concentrate, namely Na, K, Ca, Mg, S, Cl− containing ionic species, and NH<sup>4</sup> + .

As shown in Figure 4, the removal of the different compounds from the SW30 concentrate by diafiltration shows an exponential decay reaching minimum values for most of the species, after permeation of at least 2.5 diavolumes. The mass removal of all the compounds tested fit a negative exponential function, as represented in Equation (6):

$$\frac{M\_{\text{Ret, i}}}{M\_{\text{F, i}}} = e^{-aN\_{\text{D}}} \tag{6}$$

where *MRet,i* corresponds to the mass of a given compound, *i*, in the diafiltration retentate at a specific diavolume number, *ND*, whereas *MF,i* is the mass of compound *i* in the feed solution (*N<sup>D</sup>* = 0). The exponential term *a* reflects the intensity of the mass decay of each compound *i* in the retentate solution during diafiltration, which is commonly interpreted as the sieving coefficient of a given molecular species at each time, t, of the process [38]. The *MRet,i*/*MF,i* reflects the apparent mass rejection of the target species, thus, it only accounts for the mass of each species in the retentate solution (excluding that retained in the membrane) at a given *ND*. The fitting and exponential parameters obtained for the different compounds are listed in Table 3.

**Figure 4.** Mass ratio of each compound in retentate during diafiltration with a cs-PES MF022 membrane at 20 bar. Mass ratio was calculated in reference to the total mass of the same compound, *Mi,x*, in the SW30 concentrate of a synthetic tannery effluent before diafiltration (diafiltration feed solution). Note that the electric charge of the elements quantified by ICP-AES was omitted, as this technique quantifies the total amount of the element in the solution independently of its electric charge.

**Table 3.** Fitting parameters to the exponential decay of the mass removal for different compounds in the diafiltration retentate: *a* corresponds to the exponential term, whereas the fitting quality is expressed by the respective R<sup>2</sup> .


<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.

A comparative analysis of the exponential function parameters shows that the removal of all compounds tested fit adequately to Equation (6) with R<sup>2</sup> <sup>≥</sup> 0.982. The exponential term *a* showed similar values for most ionic species, being slightly higher for Cr and Na, reflecting a faster depletion of these elements (or the respective ion-containing species) from the retentate. On the other hand, a lower value was observed for NH<sup>4</sup> + , indicating a slower removal of this ionic compound from the retentate solution.

The depletion of these compounds from the retentate solution may be explained either by their permeation through cs-PES MF022 membrane or by their adsorption to the membrane, which depends on the chemical affinity of their ionic-containing species to the membrane material.

To elicit the removal mechanism of each compound, a comparative analysis of their permeation and adsorption profiles through/to the membrane was performed.

Figure 5

Figure 5

The permeation profiles (Figure 5A) clearly show that the permeability of the Cr(III) containing species is significantly lower than that observed for the other species. The mass ratio of Cr(III) in the permeate solution reached only 19.2% after permeation of four diavolumes (*N<sup>D</sup>* = 4), whereas mass ratios higher than 50% were observed for all of the other species tested. The highest mass ratios in the permeate were observed for Mg and K, showing mass ratios of 96.5% and 98.0%, respectively, whereas a mass ratio of 62.6% was observed for Ca, after *N<sup>D</sup>* = 4. (**A**)

**Diavolumes (ND) Figure 5.** (**A**) Mass ratio of each compound permeated and (**B**) mass ratio of each compound retained in the cs-PES MF022 membrane during diafiltration with the cs-PES MF022 membrane at 20 bar. Mass ratio of each compound was calculated in reference to the total mass of the same compound, *MF,I*, in the SW30 concentrate of a synthetic tannery effluent before diafiltration (diafiltration feed solution). Note that the electric charge of the elements quantified by ICP-AES was omitted, as this technique quantifies the total amount of the element in the solution independently of its electric charge.

(**A**)

2

2

The high transmission of these compounds to the permeate resulted in their low accumulation in the membrane, which was found to be inferior to 35% of their total mass, with K and Mg registering residual values close to zero, whereas the mass ratio of Ca in the membrane was equal to 30.7%, at *N<sup>D</sup>* = 4. In contrast, the loss of Cr(III) from the retentate solution is mainly explained by the effective retention of Cr-containing compounds in the membrane. As shown in Figure 5 B, a maximum Cr(III) mass ratio of 80% was found in the membrane after permeation of two diavolumes (*N<sup>D</sup>* = 2).

In a study developed by Vinodhini et al. [39], UF acetate cellulose and nanochitosan membranes showed Cr rejections varying between 78.5% (initial rejection) and 30.33% (rejection after 90 min of operation) when using feed solutions with 200 ppm of Cr(VI) at pH 5, which were explained by the adsorption of Cr to the amine groups of chitosan. Efficient Cr(VI) and Cr(III) rejections were also reported by Zhang et al. [40] using ultrafiltration membranes decorated with positively charged UiO-66-NH<sup>2</sup> compounds, from feed solutions containing 10.4 mg/L of these ions. The removal of Cr(VI) was explained by the electrostatic attraction to the positive charges in the membrane, whereas concentrations of Cr(III) lower than 1.5 mg/L in the effluent (stable during the operation time) were obtained with MOF@PVDF-0.02 and PEI/MOF@PVDF-0.02 membranes. However, and in contrast to that observed in the present work, the rejection of Cr(III) by these membranes was attributed to the electrostatic repulsion by the positively charged membrane.

The preferential accumulation of Cr-containing compounds at the surface of the chitosan modifying layer upon filtration of the tannery effluent was confirmed in the present study by SEM-EDS analysis. The SEM-EDS results obtained after analysis of chitosan modified membranes (Figure S1, in Supplementary Materials) after permeation of a synthetic tannery effluent (non-processed by reverse osmosis) in concentration mode, showed a significant relative intensity of Cr, in reference to elements such as C, O, and S, which are components of the membrane matrix, and to Au and Pd, which were used to coat the membrane surface, as required for the SEM-EDS analysis.

Despite the high Cr(III) rejection obtained through diafiltration, it was lower than that obtained with the same membrane operated in concentration mode (>99%) or with BW30 and SW30 reverse osmosis (RO) membranes, which led to Cr rejection values of 90% and 99.6%, respectively. However, diafiltration is advantageous if selective removal of Cr is required. Diafiltration produces a higher "cleaning effect", which results in a more efficient removal of compounds non-size excluded by the membrane, in the presence of insignificant fouling effects which prevents the permeate flux decline [12].

Analysis of the Cr(III) retention selectivity in reference to the other compounds tested (Figure 6) shows a retention selectivity of Cr(III)-containing compounds of 2.6 and 4.2 in reference to Na and Ca-containing compounds, respectively; a retention selectivity of 5.5, 7.1, and 7.6 in reference to S-containing compounds, NH<sup>4</sup> <sup>+</sup> and Cl−, respectively; and a selectivity >100 in reference of K-containing compounds, whereas only a vestigial accumulation of Mg-containing compounds in the membrane was detected. The selective retention of Cr(III) in the cs-PES MF022 membrane may be explained by the high chemical affinity of chitosan to heavy metals, via coordination with the amine (NH2) groups of chitosan [8]. The chemical characterization of cs-PES MF022 membranes by FTIR-ATR was performed and discussed in our previous paper [12]. The presence of chitosan was confirmed based on the detection of the NH<sup>2</sup> stretching band at 1586 cm−<sup>1</sup> , whereas chitosan crosslinking was confirmed by the decreased intensity of the C=N signal at 1680–1620 cm−<sup>1</sup> [12]. In particular, the preferential adsorption of Cr(III) to chitosan, in detriment of the other ionic species present in the SW30 concentrate, can be interpreted based on the ability of the NH<sup>2</sup> groups to coordinate heavy metals with *f* unsaturated orbitals, and its inability to form complexes with alkali and alkali earth metals, such as Na<sup>+</sup> , K<sup>+</sup> , or Mg2+ .

The selective adsorption of Cr(III) to the cs-PES MF022 membrane (as evidenced in Figure 5B) led to our hypothesis that it is possible to selectively recover the Cr(III) contained in the effluent by membrane desorption after diafiltration, taking advantage of the pH-responsiveness behaviour of chitosan.

#### *3.2. Diafiltration of the SW30 Concentrate Solution—Real Tannery Effluent*

The concentrate obtained by RO treatment of a real effluent from the TAMEG-Rouiba tannery with an RO SW30 membrane was also processed through a cs-PES MF022 membrane in a diafiltration mode. Identically to that observed for the diafiltration of the SW30 concentrate from a synthetic effluent, a relatively constant oscillating permeate flux was obtained through the whole process (Figure 7). These results are compatible with the absence of significant concentration polarization and fouling events, revealing that, contrarily to what might be initially expected, the organic content of this effluent (the chemical oxygen demand, COD, of the TAMEG-Rouiba tannery effluent is 92 mg O2/L after pre-filtration with a microfiltration membrane with an average pore size of 0.45 µm [12]), did not affect the performance of the diafiltration process, at least considering the investigated operation time window.

**Figure 7.** Permeate flux, *J*, obtained during the diafiltration of the SW30 concentrate of the real tannery effluent with the cs-PES MF022 membrane at an applied pressure of 20 bar.

As shown in Figure 8, the diafiltration process led to an exponential loss pattern of all the species studied. However, this exponential decay behaviour was clearly more accentuated for Cr(III), indicating a faster removal of Cr-containing compounds from the diafiltration retentate solutions. Similarly to that observed for the diafiltration of the SW30 concentrate from a synthetic effluent, the removal profiles fit very acceptably to the exponential function represented in Equation (6), as it can be confirmed by the respective R <sup>2</sup> values (Table 4).

**Figure 8.** Mass ratio of each compound in the retentate solution during diafiltration with a cs-PES MF022 membrane at 20 bar. Mass ratios were calculated in reference to the total mass of the same compound, *MF,i,* in the SW30 concentrate of the real tannery effluent before diafiltration (diafiltration feed solution).

A comparative analysis of the exponential term, *a*, obtained for the diafiltration of SW30 concentrates from synthetic and real tannery effluents, seems to indicate an identical removal efficiency of most of the compounds studied, with the exception of Cr(III)-containing compounds. The exponential term, *a*, corresponding to the loss of Cr(III) containing compounds from the SW30 concentrate of the real tannery effluent was equal to 5.85, thus being much higher than that corresponding to the diafiltration of the SW30 concentrate from a synthetic effluent, which was equal to 1.69.


**Table 4.** Fitting parameters to the exponential decay of the different compounds in diafiltration retentate: *a* is the exponential term, whereas the fitting quality is expressed by the respective R<sup>2</sup> .

<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.

Figure 9

Such an increase in the exponential term reflects an improved removal of Cr(III) species from the SW30 concentrate from a real tannery effluent, potentially associated to the adsorptive capacity of the organic matter present in real effluent, as discussed in detail below.

In order to understand the removal mechanisms associated with the depletion of each species from the diafiltration retentate, the mass ratio of each compound permeating across the cs-PES MF022 membrane, and that retained in the membrane during diafiltration were determined by respective mass balance calculations.

The profiles depicted in Figure 9A,B show that the cs-PES MF022 membrane exhibits a satisfactory permeability to most of the studied species. Mass ratios higher than 74% were obtained, with Mg and S-containing compounds showing the highest permeability and mass ratios in the permeate of 98% and 99%, respectively, for *N<sup>D</sup>* = 4. Oppositely, the mass ratio profiles in the permeate indicate that the Cr-containing compounds are almost completely retained by the cs-PES MF022 membrane, with the Cr(III) mass ratio in the membrane reaching ca. 94% after the permeation of only 0.6 diavolumes. The Cr(III) retention efficiency obtained with the SW30 concentrate of a real tannery effluent was thus much higher than the 80% retention of Cr-containing species obtained by diafiltration of the SW30 concentrate of the synthetic effluent. These results suggest that organic matter present in real tannery effluents contributes positively to the retention of Cr-containing compounds in the membrane. Although the reason for this behaviour is not yet clearly understood, it suggests that the overall adsorption of Cr(III) results from a combined effect of the ability of chitosan to selectively adsorb Cr(III), as previously discussed, and the membrane ability to reject/adsorb organic matter, thus rejecting/adsorbing simultaneously the Cr(III) potentially bound to it.

 Cr Na K Ca Mg S Cl- NH<sup>+</sup> 4

**Figure 9.** *Cont.*

(**B**)

0.0

0.2

0.4

0.6

**Mass ratio in the membrane, MMem,iI/MF,i**

0.8

1.0

01234

**Diavolume (ND)**

3

Figure 9

01234

**Diavolume (ND)**

 Cr Na K Ca Mg S Cl- NH<sup>+</sup> 4

0.0

0.2

0.4

0.6

**Mass ratio in the permeate, MP,i/MF,I**

(**A**)

0.8

1.0

**Figure 9.** (**A**) Mass ratio of each compound *i* permeated and (**B**) mass ratio of each compound *i* retained in the cs-PES MF022 membrane during diafiltration at 20 bar. Mass ratio for each compound was calculated in reference to the total mass of the same compound, *MF,I,* in the SW30 concentrate of a real tannery effluent before diafiltration (diafiltration feed solution).

3 The mass ratios of other compounds in the membrane showed maximum values of 18.7% and 22.4% for Cl<sup>−</sup> and NH<sup>4</sup> + , respectively, whereas Mg, K, S, and Ca compounds showed mass ratios in the membrane inferior to 7.5%. These differences led to significantly high Cr(III) retention selectivity in the membrane, of 4.2 and 5 in respect to NH<sup>4</sup> <sup>+</sup> and Cl−; 12.9 and 14.6 in reference to K and Na-containing compounds, respectively; and higher than 45 in respect to Mg, Ca, and S -containing compounds, as shown in Figure 10. As shown in Figure S2, the Cr(III) retention selectivity relative to the divalent ions was higher during the diafiltration of the real tannery effluent than that found during the diafiltration of synthetic tannery wastewaters.

**Figure 10.** Selective retention of Cr(III) in the cs-PES MF022 membrane during the diafiltration of the SW30 concentrate of a real tannery effluent at 20 bar.

These results suggest the possibility of recovering Cr(III) from tannery effluents by desorption, taking advantage of the pH-responsiveness of chitosan, and thus allowing for Cr(III) reuse in the tannery industry. Moreover, a permeate solution with a residual amount of Cr(III) (only 3% of the Cr(III) mass content of the SW30 concentrate from a real tannery effluent permeate in the cs-PES MF022 membrane) and a retentate solution with a content in ionic species lower than that of the permeate solution are generated by the diafiltration process. Therefore, considering the overall treatment of the tannery effluent through an RO-diafiltration integrated process, illustrated in Scheme 2, the processing costs associated with the disposal of the effluent in the diafiltration could be reduced by recycling the permeate solution to the inlet of the reverse osmosis process.

**Scheme 2.** Schematic representation of the integrated RO-diafiltration process for purification of a real tannery effluent from TAMEG-Rouiba-SPA and selective recovery of Cr(III).

The retentate solution may also be recirculated to the RO inlet for water recovery; however, the low content of ionic species in this processing stream allows for its direct discharge into the ecosystem via Reghaia Lake, respecting the limits for discharge of liquid industrial effluents imposed by the norm No. 06-141 of Rabie El Aouel 1427 (19 April 2006) [35].

## *3.3. pH Induced Desorption of Cr from the cs-PES MF022 Membrane*

The membranes used in the diafiltration of the SW30 concentrate from a synthetic effluent were then immersed in water solutions with pH 2 and pH 5.8 in order to demonstrate, as a proof of concept, the possibility to recover Cr(III) by desorption.

The desorption profiles obtained for the different compounds studied are shown in Figure 11.

As shown, a poor desorption of most of the compounds was observed at pH 5.8 (Figure 11A). The desorption of Cr(III) was negligible, while a higher desorption was found for chloride ions (Cl−). In contrast, as shown in Figure 11B, the desorption of Cr(III) significantly increased at pH 2 in reference to most of the other compounds, except for Cl− and Na<sup>+</sup> ions (considering that the Na content is all present in the form of Na<sup>+</sup> ). The mass fraction of Cr(III) equal to 10.7% in the membrane before desorption increased to 20.6% in the solution (leaching solution). Note that the mass fractions in the membrane that were determined only considered the studied compounds (Ca, Cr, Na, K, Mg, S, and Cl−).

The adsorption of Cr(III) to chitosan is explained by the ability of amine groups (NH2) to coordinate Cr(III), and also by possible interactions to the hydroxyl groups (OH−). Amine and hydroxyl groups are mainly neutral at pH 6 (pKa = 6.3 for R-NH<sup>3</sup> + ) and protonated at pH 2 (R-NH<sup>3</sup> <sup>+</sup> and R-OH<sup>2</sup> + ). Therefore, the adsorption ability of chitosan is ruled by pH induced protonation/deprotonation of amine and hydroxyl groups, promoting the desorption of cationic species at lower pH values. It particularly favours those with higher electronic valence, cationic Cr(III) species, such as Cr(OH)2+ and Cr(OH)<sup>2</sup> + , which are the prevalent Cr(III)-containing species in an aqueous solution at pH 3.6 [6].

Figure 11

**Figure 11.** Mass of the compounds desorbed from the cs-PES MF 022 membrane at (**A**) pH 5.8 and (**B**) pH 2, after diafiltration of the SW30 concentrate obtained from filtration of a synthetic tannery effluent.

The high amount of Cl<sup>−</sup> and Na<sup>+</sup> ions found in the leaching solution is due to their much higher amount compared to that of Cr(III) in the tannery effluents. Therefore, despite the much lower percentage retention of Na<sup>+</sup> and Cl<sup>−</sup> in the membrane, this results in final amounts in the membrane of 6.17 mg and 7.91 mg for Na<sup>+</sup> and Cl−, respectively, which were higher than the mass of Cr(III) retained in the membrane, equal to 2.91 mg.

4 As a preliminary proof of concept, the desorption of the target ions was evaluated. A moderate desorption extent (<25%) was obtained for three compounds, Cr(III), Na<sup>+</sup> , and Cl−, in these preliminary desorption assays. Optimization of the desorption process is thus required by the suitable adjustment of desorption parameters, such as temperature, stirring speed, and volume of the leaching solvent for enhanced desorption of these elements. Although desorption optimization is out of the scope of this work, a comparative analysis of the desorption profiles obtained at pH 2 and 5.8 on one hand evidences that Cr(III) desorption selectivity may be improved by conducting the process through successive desorption steps at variable pH values, i.e., a significant Cl<sup>−</sup> and Na<sup>+</sup> removal might

be attained in a first desorption step at pH 5.8, with good maintenance of the adsorbed Cr(III). This is then followed by a second desorption step at pH 2, for the selective removal of Cr(III) with minimal interference of Na<sup>+</sup> and Cl−. On the other hand, the desorption profiles obtained for Cr(III), Na<sup>+</sup> , and Cl−, at pH 5.8, show that the Na<sup>+</sup> desorption was faster during the first 5 h of the process, tending to plateau after this period. In contrast, the Cr(III) desorption kinetics showed a more constant behaviour, crossing the Na<sup>+</sup> desorption curve after 72 h, suggesting that process optimization may render a final leaching solution with a more significant Cr(III) content relative to the amount of Na<sup>+</sup> .

Nevertheless, if a complete separation of Cr(III) and Na<sup>+</sup> is required. This can be achieved either by an additional nanofiltration operation benefiting from the different size and valence of Cr(III)-containing compounds and Na+, or by using selective cationexchange resins. The desorption of chloride ions is not actually regarded as a significant problem, as it may lead to the recovery of Cr(III) in the form of chromium chloride.

#### **4. Conclusions**

This work evaluates the performance of chitosan modified membranes (cs-PES MF022) for selective recovery of Cr(III) from synthetic and real tannery effluent concentrates obtained by reverse osmosis (RO) with an SW30 membrane, through a diafiltration process. The cs-PES MF022 membrane shows a high ability to retain Cr-containing compounds and a satisfactory permeability to the other compounds studied, which results in a selective removal of Cr-containing compounds of 81% and 97%, in synthetic and real tannery effluents, respectively. The removal of Cr(III)-containing species was mostly explained by Cr(III) retention in the membrane, as only an insignificant amount (<2% of the total amount of Cr in the feed solution) was found in the retentate solution after diafiltration of four diavolumes. The high retention of Cr(III) in the membrane was attributed to the chitosan capacity to adsorb heavy metals with unsaturated *f* orbitals, such as Cr(III), by coordination to the amine groups of chitosan.

It was noteworthy that the permeability of Cr(III)-containing species through the cs-PES MF022 membrane changed significantly during the filtration of real and synthetic tannery effluents. The mass transmission of Cr(III) contained in the real tannery effluent was significantly lower than that obtained by filtration of the synthetic tannery effluents, resulting in a higher and more selective membrane retention of Cr(III)-containing compounds in the former case. This demonstrates the role of the organic matter present in real effluents in the retention of metals.

Moreover, 2.5 diavolumes were found to be the optimal condition for achieving the maximum transmission of most of the compounds studied and the highest retention of Cr(III), without significant variation of permeate flux. This demonstrates the feasibility of a diafiltration strategy for attenuating concentration polarization and membrane fouling effects.

Proof of concept experiments were carried out to demonstrate the possibility to recover Cr(III) by its selective desorption based on the pH-responsive behaviour of chitosan. While a low desorption was obtained for most of the compounds, Cr(III) shows an increased desorption at more acidic pH values (pH = 2), confirming that Cr(III) adsorption to chitosan is mainly due to the coordination to the amine groups. A leaching solution enriched in Cr(III), but still with considerable amounts of Na<sup>+</sup> and Cl– , was obtained. These results showed that the diafiltration of RO concentrates of tannery effluent with the cs-PES MF022 membrane followed by acidic desorption did not allow for a complete isolation of Cr(III) containing species. Instead, it rendered a final solution of 92.5% enriched in Cr(III), relative to the tannery effluent RO concentrate (feed solution). Improved Cr(III) purification requires an optimization of the desorption process, or additional treatment of the effluent for separation of Cr(III) from Na<sup>+</sup> , e.g., by nanofiltration or by selective Cr(III) adsorption by cation exchange resins. However, this decision depends on the required degree of Cr(III) purity, and should be taken upon a careful analysis of the economic viability of the integrated process.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/w13182598/s1, Figure S1: SEM-EDS analysis of the cs-PES MF022 membranes before (top spectrum) and after (bottom spectrum) filtration of the tannery effluent at 20 bar, in concentration mode. Figure S2: Comparative analysis between the selective retention of Cr(III), in reference to other species, by the cs-PES MF022 membrane, during diafiltration of the SW30 concentrate of a synthetic and real tannery effluent at 20 bar.

**Author Contributions:** Conceptualization, C.A.M.P., J.G.C., and S.V.; methodology, C.A.M.P., J.G.C., and S.V.; data curation, A.Z. and C.A.M.P.; writing—original draft preparation, A.Z. and C.A.M.P.; writing—review and editing, A.Z, F.S., C.A.M.P., J.G.C., and S.V.; supervision, C.A.M.P., J.G.C., and S.V.; funding acquisition, C.A.M.P., J.G.C., and S.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Associate Laboratory for Green Chemistry—LAQV which is financed by national funds from FCT/MCTES (UIDB/50006/2020 and UIDP/50006/2020).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data available on request.

**Acknowledgments:** This work was supported by the Associate Laboratory for Green Chemistry— LAQV—which is financed by national funds from FCT/MCTES (UIDB/50006/2020 and UIDP/50006/ 2020). Asmaa Zakmout gratefully acknowledges the Erasmus and International Credit Mobility Program for her internship grant at LAQV, Faculty of Science and Technology.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

