Reply to the ‘Comment on “Shallow Shell SSTA63 resin: a rapid approach to remediation of hazardous nitrate”’ by K. H. Chu, Environ. Sci.: Water Res. Technol., 2026, 12, DOI: 10.1039/D4EW00976B

Abstract

Corrections are provided for “Shallow Shell SSTA63 resin: a rapid approach to remediation of hazardous nitrate” (Çendik et al., Environ. Sci.: Water Res. Technol., 2024, 10, 2765–2775, https://doi.org/10.1039/D4EW00584H) in response to the comment by K. H. Chu (Environ. Sci.: Water Res. Technol., 2026, 12, https://doi.org/10.1039/D4EW00976B).

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Water impact

Our study highlights Purolite Shallow Shell™ SSTA63 resin as a highly effective solution for mitigating nitrate ion (NO₃⁻) contamination in water. Utilizing advanced anion technology that strategically deactivates the gel bead center, the resin shows rapid and efficient removal within 10 minutes and robust regeneration capabilities with hydrochloric acid. Minimal interference from coexisting ions underscores its reliability in diverse environmental conditions, suggesting broad applicability in water treatment and environmental remediation efforts.

1. Introduction

In response to the comment by K. H. Chu (https://doi.org/10.1039/D4EW00976B), the authors would like to correct some errors in their original article (https://doi.org/10.1039/D4EW00584H).

The authors regret that there are a few errors in the equation of the Temkin and D–R isotherm models, as well as in calculating the thermodynamic parameters.

3.4. Sorption isotherms

In the initial version of our paper, the Temkin isotherm model was presented using the following equations:

q_e = B ln(A_TC_e) (1)

B = RT/b_T (2)

However, in these equations, the parameter q_max was inadvertently omitted. The correct form of the Temkin isotherm model¹ is shown in eqn (3):In the Temkin model, q_e (mg g⁻¹) represents the resin capacity at equilibrium and q_max (mg g⁻¹) is the maximum capacity of the resin, while R is the universal gas constant (J mol⁻¹ K⁻¹), T is the absolute temperature (K), and b_T (J mol⁻¹) is the Temkin constant related to the heat of adsorption.

The calculations for the Dubinin–Radushkevich (D–R) model were originally presented using the following equations:

q_e = q_max exp(−βε²) (4)

However, in eqn (5), the term (1 + 1/C_e) was incorrectly given. The correct expression should be C_s/C_e.

The correct form of the D–R model² is shown in eqn (7).

In this equation, C_s is the solute solubility of NaNO₃, which is 11.327 mol kg⁻¹ water at 30 °C.³ This value was converted to 699 476 mg NO₃⁻ per L by calculating mol L⁻¹ using the density of water at 30 °C (0.99565 g cm⁻³).⁴E (kJ mol⁻¹) is the mean free energy of adsorption per mole of solute.

The calculated q_max values from the Temkin model were significantly lower than the experimental findings (Table 1). This discrepancy may indicate that the Temkin model is not suitable for high solute concentration.¹ In contrast, the DR isotherm yielded a higher q_max value, which could be attributed to the high C_s value.

Table 1 Isotherm models with associated parameters and values for the sorption of NO₃⁻ on Purolite SSTA63 anion exchange resin

Isotherm model	Nonlinear equations	Parameters	Values
Langmuir		q max (mg g^−1)	53.65
		K L (L mg^−1)	0.069
		R ^2	0.99
Freundlich	q e = KFC^1/ne	K F (mg g^−1)(L mg^−1)^1/n	6.38
		n	2.38
		R ^2	0.95
Temkin		b T (J mol^−1)	8.88
		A T (L mg^−1)	1.26
		q max (mg g^−1)	0.03
		R ^2	0.92
D–R		E (kJ mol^−1)	12.05
		q max (mg g^−1)	205.08
		R ^2	0.96

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3.5. Sorption thermodynamics

In the initial version of the paper, the thermodynamic calculations were performed using the following eqn (8):

K_D = q_e/C_e (8)

However, in the corrected form, the distribution coefficient (K_D) has been replaced with the equilibrium constant (K⁰_e), as shown in eqn (9).

The nature and thermodynamic feasibility of the sorption process were assessed by analyzing the thermodynamic constants, including the standard free energy (ΔG°, kJ mol⁻¹), standard enthalpy (ΔH°, kJ mol⁻¹), and standard entropy (ΔS°, kJ mol⁻¹ K⁻¹), using eqn (9)–(12):⁵

ΔG° = −RT ln(K⁰_e) (10)

ΔG° = ΔH° − TΔS° (11)

K⁰_e is the thermodynamic equilibrium constant, γ is the coefficient of activity, [Adsorbate]⁰ is the standard concentration of the adsorbate (1 mol L⁻¹), K_g is the Langmuir equilibrium constant (obtained from sorption isotherm tests), and ΔG° is the Gibbs free energy change (kJ mol⁻¹). ΔH° (kJ mol⁻¹) and ΔS° (J mol⁻¹ K⁻¹) are the enthalpy change and entropy change, respectively. For this calculation, the adsorbate solution is significantly diluted; therefore, one can consider the activity coefficient as unitary.⁶

Fig. 1 is the plot of ln K⁰_evs. 1/T used to estimate the ΔH° and ΔS° values for the sorption of NO₃⁻ using Purolite SSTA63 resin from the slope and intercept, respectively. The summarized values of the parameters are presented in Table 2. The sorption process exhibits spontaneity, as evidenced by the negative ΔG° value at all studied temperatures. The negative ΔH° value (−15.3 kJ mol⁻¹) indicates an exothermic interaction between NO₃⁻ and the resin. Furthermore, the positive ΔS° value (+19.1 J mol⁻¹ K⁻¹) indicates the affinity of the sorbent for NO₃⁻ as randomness increased in the sorption process.

Fig. 1 ln K⁰_evs. 1/T plot for sorption thermodynamics of NO₃⁻ onto Purolite SSTA63 anion exchange resin.

Table 2 Thermodynamic parameters for the sorption of NO₃⁻ onto Purolite SSTA63 anion exchange resin

Temperature (°C)	ΔG° (kJmol^−1)	ΔH° (kJ mol^−1)	ΔS° (J mol^−1 K^−1)
15	−20.8	−15.3	+19.1
20	−20.9
30	−21.1

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The corrections made to the Temkin and Dubinin–Radushkevich isotherm models, as well as to the thermodynamic parameters, provide a more accurate description of the sorption process. Although the corrected equations lead to changes in the calculated parameter values, they do not affect the overall interpretation of the sorption behavior. The main conclusions regarding the efficiency of NO₃⁻ removal by Purolite Shallow Shell™ SSTA63 anion exchange resin and the underlying sorption mechanism remain valid.

Author contributions

Elif Çendik: funding acquisition, investigation. Mügenur Saygı: investigation. Yaşar Kemal Recepoğlu: writing – review & editing, conceptualization, visualization. Özgür Arar: writing – review &editing, supervision, resources, investigation.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be available upon request.

Acknowledgements

We thank Purolite Int. Co. (Türkiye) for providing ion-exchange resin samples.

References

1. K. H. Chu, Revisiting the Temkin isotherm: dimensional inconsistency and approximate forms, Ind. Eng. Chem. Res., 2021, 60, 13140–13147.
2. K. H. Chu, M. A. Hashim, G. Hayder and J.-C. Bollinger, Comparative Evaluation of the Dubinin–Radushkevich Isotherm and Its Variants, Ind. Eng. Chem. Res., 2024, 63, 15002–15011.
3. D. G. Archer, Thermodynamic properties of the NaNO₃+ H₂O system, J. Phys. Chem. Ref. Data, 2000, 29, 1141–1156.
4. W. M. Haynes, CRC handbook of chemistry and physics, CRC press, 2014.
5. E. C. Lima, A. Hosseini-Bandegharaei, J. C. Moreno-Piraján and I. Anastopoulos, A critical review of the estimation of the thermodynamic parameters on adsorption equilibria. Wrong use of equilibrium constant in the Van't Hoof equation for calculation of thermodynamic parameters of adsorption, J. Mol. Liq., 2019, 273, 425–434.
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