. 2023 Mar 26;13(4):379. doi: 10.3390/membranes13040379

Table 5.

Recent studies on factors affecting reverse solute flux (RSF) for the FO process.

Groups	Draw Solution	Reverse Solute Flux	Findings	Reference
Different membrane properties
Tortuosity (1.07–2.5)	2 M NaCl	0.155–0.1 mol/m²h	High tortuosity leads to declining both water flux and RSF since lengthens the mass transfer path and reduces the mass transfer coefficient, which would amplify ICP.	[169]
Porosity (0.15–0.95)	2 M NaCl	0.065–0.18 mol/m²h	High porosity (ε) leads to increasing both water flux and RSF since enhances the concentration gradient and reduces the resistance to solute diffusion (i.e., dilutive ICP). When ε > 0.8, the enhancement of water flux becomes less significant but RSF enhancement is still significant. Thus, higher ε does not always mean better performance.	[169]
Pore size (0.025–0.45 nm)	1.5 M NaCl	0.93–8.30 g/m²h	The pore size of about 0.2 μm promoted both high water flux and low RSF due to its open, highly porous structure and reduced tortuosity creating less resistance to water transport and solute diffusion (i.e., lower S value = 1220 ± 380). It also helped the selective layer to avoid defects, resulting in a higher cross-linking degree and hence higher selectivity.	[170]
Different salt solutions with the same ion
Na⁺	0.6 M NaCl 0.72 M NaHCO₃	8.17 g/m²h 3.22 g/m²h	NaCl is higher in water flux and 2.5 times larger than NaHCO₃ in reverse diffusion. Although identical in the osmotic pressure (28 bar) and the presence of ${Na}^{+}$ in both solutions, the size of the hydrated anion is what causes this difference, i.e., ${HCO}_{3}^{- 1}$ (0.45 nm) > ${Cl}^{-}$ (0.3 nm).	[106]
Mg²⁺	1 M MgSO₄ 1 M Mg(NO₃)₂	1.32 g/m²h 24.18 g/m²h	${Mg}^{2 +}$ is completely soluble in water as Mg(NO₃)₂ produces the highest osmotic pressure (84 bar at 1 M) and the highest diffusion (3.31 × 10⁻⁶ m²/h) (i.e., reducing dilute ICP) this will ensure three times higher water flux compared to MgSO₄ (1.7 × 10⁻⁶ m²/h). Thus, RSF typically increases as water flux increases.	[110]
Different ions	22 g/L NH₄Cl 22 g/L KCl 22 g/L (NH₄)₂SO₄ 22 g/L NaH₂PO₄	3.71 g/m²h 1.98 g/m²h 0.82 g/m²h 0.12 g/m²h	${NH}_{4}^{+}$ showed the highest RSF, followed by $K^{+}$ , ${SO}_{4}^{2 -}$ and ${PO}_{4}^{3 -}$ . It has been noted that cations paired with ${Cl}^{-}$ anion have high RSF than those that pair with the sulfate group. While, multivalent negatively charged anions, such as ${SO}_{4}^{2 -}$ and ${PO}_{4}^{3 -}$ , have RSF lower than that of monovalent anions because of higher electrostatic repulsion via the negatively charged CTA membrane. Then, KCl exhibited the highest water flux followed by NH₄Cl, (NH₄)₂SO₄, and NaH₂PO₄.	[112]
Different operating conditions
Concentration	0.5–3 M CaCl₂	2.55–11.45 g/m²h	Increased viscosity and osmotic pressure, and low diffusion coefficient are all effects of higher DS concentration, which also increases water flux and RSF, but it will not be beneficial as it may cause FS contamination.	[139]
Flow rate	1 M NaCl at 0.2–1 L/min	4.3–2.8 g/m²h	Decreased concentrative ECP and increased dilutive ICP are all effects of a higher DS flow rate, which results in decreases in the water flux and RSF since it reduced the residence time of liquid in the FO unit.	[140]
Temperature	3 M NaCl at 20–50 °C	0.21–0.3 mol/m²h	Reduced viscosity (1.3408–0.7574 mPa.s), CP, increased osmotic pressure (162.95–173.61 bar), diffusion coefficient (1.067–2.063 nm²/s), and water permeability are all effects of higher temperature, which also increases the water flux and RSF. However, it may raise the risk of membrane fouling brought on by an increase in ion permeability and membrane clogging (i.e., larger hydrated ion size).	[142]
Different nanoparticles (NPs)
SiO₂ (negative), TiO₂ (neutral) and ZnO (positive) in feed solution	0.5 M NaCl	16.8 mol/m²h 16.5 mol/m²h 15.7 mol/m²h	ZnO (29.7 mV) and TiO₂ (0.6 mV) showed higher RSF because they carried a positive charge opposite to the membrane charge (−12 mV), which forms a fouling layer on the surface that attracts ions in the DS and impedes water flux. Whereas SiO₂ (−20.2 mV) formed a relatively thin film of fouling, which facilitates water transport. After the aggregation of NPs with NaCl for 30 min, a size increase in less than 20% was observed for SiO₂ (42–49 nm) and ZnO (41–50 nm). While it increases by 40% for TiO₂ (38–54 nm). Thus, the aggregation of NPs may not significantly impact FO performance.	[171]
TiO₂ and Al₂O₃ in the support layer	1 M NaCl	7. 1 g/m²h 5.4 g/m²h	1% TiO₂ in the support layer leads to high water flux and lower RSF due to increased porosity and hydrophilicity (80.72%, 61.85°) compared to the CA membrane (71.81, 67.86°). However, we notice a further decrease in RSF by adding 0.1% Al₂O₃ to the TiO₂-modified membrane (80.96%, 56.7°). However, a further increase in NPs loading can lead to lower water flux and higher RSF due to NPs aggregation in the sublayer.	[172]
GO in the active layer	1 M NaCl	2.6 g/m²h	0.1% GO in the active layer leads to high water flux and lower RSF due to increased roughness and hydrophilicity (54.1 nm, 64°) compared to the control membrane (31.7 nm, 82°). However, a further increase in GO loading leads to agglomeration of the nanostructure, which limits the formation of the ideal thin film of the polyamide layer and consequently to lower water flux and higher RSF.	[173]