Coal Fly Ash Waste, a Low-Cost Adsorbent for the Removal of Mordant Orange Dye from Aqueous Media

Rosa, Teresa; Martins, Angela; Santos, Maria T; Trindade, Teodoro; Nunes, Nelson

doi:10.21577/0103-5053.20210116

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Article • J. Braz. Chem. Soc. 32 (12) • Dec 2021 • https://doi.org/10.21577/0103-5053.20210116 copy

Coal Fly Ash Waste, a Low-Cost Adsorbent for the Removal of Mordant Orange Dye from Aqueous Media

Authorship SCIMAGO INSTITUTIONS RANKINGS

In this study, a coal fly ash material generated in a Portuguese coal thermal powerplant was tested as a low-cost adsorbent to remove dye molecules. Pre-treatment of the coal fly ash samples was not performed in order to reduce end use cost. Physical and chemical characterization revealed their inert nature and low effects lixiviation in aqueous media. Preliminary adsorption studies include adsorbent quantity, and adsorption kinetics. The adsorption studies focused on Mordant Orange 1 (Mo1) dye, but two other molecules, Rhodamine B (RhB) and Methylene Blue (MeB) were also included for comparison reasons. The adsorption isotherms were tested with different models including Langmuir, Freundlich and modified Langmuir-Freundlich. The effect of temperature, pH and unburn carbon in the adsorption process were also studied. The results show that adsorption capacity of the coal fly ash occurs mainly due to electroestatic interactions between the adsorbent surface and the adsorbate, which depends on the pH of the aqueous media and the surface chemistry of the material, quantified by the point of zero charge, pH_pzc. These joint effects are responsible for the higher retention of Mo1 that is about 16 times higher, when compared to the other two molecules tested.

Keywords:
coal fly ash; waste valorization; Mordant Orange 1; dyes; adsorption; aqueous media

Proximate analysis / (% m/m)
Fixed carbon	2.39 ± 0.16
Volatiles	3.40 ± 0.12
Ash	94.13
Moisture	0.07 ± 0.03

Langmuir isotherm model
Adsorbate	Temperature / °C	q_m / (mg g^-1)	K_L / (L mg^-1)	R²	σ²	F
Mo1	20	119.2 ± 5.4	0.029 ± 0.002	0.969	1.840	3047
	30	113.2 ± 11.5	0.022 ± 0.004	0.983	2.732	1152
	40	101.1 ± 12.1	0.021 ± 0.004	0.982	2.389	553
RhB	20	5.1 ± 0.3	2.3 ± 0.5	0.952	0.356	179
	30	6.2 ± 0.2	2.4 ± 0.9	0.985	0.239	662
	40	6.6 ± 0.4	2.5 ± 0.8	0.935	0.524	116
MeB	20	4.0 ± 0.5	3.8 ± 2.0	0.895	0.484	42
	30	3.98 ± 0.02	3.1 ± 0.5	0.984	0.158	364
	40	3.95 ± 0.07	2.5 ± 1.1	0.935	0.389	43

Freundlich isotherm model
Adsorbate	Temperature / °C	K_F / (mg^1-1/n L^1/n g^-1)	n	R²	σ²	F
Mo1	20	5.5 ± 0.3	1.52 ± 0.03	0.995	1.633	3873
	30	3.8 ± 0.5	1.40 ± 0.07	0.976	3.223	822
	40	3.7 ± 0.4	1.47 ± 0.07	0.985	2.192	659
RhB	20	3.2 ± 0.2	3.5 ± 0.7	0.901	0.512	82
	30	3.9 ± 0.2	3.2 ± 0.4	0.945	0.460	172
	40	4.4 ± 0.3	4.0 ± 1.0	0.903	0.654	74
MeB	20	3.0 ± 0.2	3.8 ± 1.4	0.872	0.534	34
	30	2.8 ± 0.1	3.4 ± 0.6	0.958	0.254	138
	40	2.6 ± 0.2	2.8 ± 0.6	0.956	0.319	66

Modified Langmuir-Freundlich isotherm model
Adsorbate	Temperature / °C	q_m / (mg g^-1)	K_MLF / (L mg^-1)	n	R²	σ²	F
Mo1	20	222.9 ± 69.2	0.021 ± 0.005	1.26 ± 0.08	0.996	1.416	2581
	30	109.2 ± 27.4	0.023 ± 0.004	1.0 ± 0.1	0.983	2.802	547
	40	266 ± 386	0.012 ± 0.014	1.3 ± 0.3	0.985	2.267	308
RhB	20	4.5 ± 0.2	5.4 ± 2.6	0.6 ± 0.1	0.966	0.318	114
	30	6.2 ± 0.4	2.53± 0.7	1.0 ± 0.1	0.985	0.252	298
	40	6.0 ± 0.5	4.2 ± 3.1	0.7 ± 0.3	0.941	0.535	56
MeB	20	3.9 ± 1.4	4.4 ± 9.4	0.9 ± 0.9	0.895	0.541	17
	30	3.6 ± 0.2	5.9 ± 3.0	0.7 ± 0.2	0.988	0.150	204
	40	25.8 ± 593	0.1 ± 3.0	2.5 ± 6.3	0.956	0.395	21

Dye	Adsorption capacity / (mg g^-1)	Temperature / °C	Reference
MeB	15.04	30	⁶⁰60 Hammond, C.; Padovan, D.; Tarantino, G.; R. Soc. Open Sci. 2018, 5, 171315.
	0.07	30	⁵³53 Karaca, H.; Altıntığ, E.; Türker, D.; Teker, M.; J. Dispersion Sci. Technol. 2018, 39, 1800.
	2.94	30	⁶¹61 Khan, T.; Ali, I.; Singh, V.; Sharma, S.; J. Environ. Prot. Sci. 2009, 3, 11.
	5.57	30	⁶²62 Kumar, K. V.; Ramamurthi, V.; Sivanesan, S.; J. Colloid Interface Sci. 2005, 284, 14.
	4.89	22	⁴³43 Janoš, P.; Buchtová, H.; Rýznarová, M.; Water Res. 2003, 37, 4938.
	3.98	30	this work
RhB	1.87	30	⁶¹61 Khan, T.; Ali, I.; Singh, V.; Sharma, S.; J. Environ. Prot. Sci. 2009, 3, 11.
	5.03	22	⁴³43 Janoš, P.; Buchtová, H.; Rýznarová, M.; Water Res. 2003, 37, 4938.
	6.2	30	this work

Adsorbate	Temperature / °C	ΔG° / (kJ mol^-1)	ΔH° / (kJ mol^-1)	ΔS° / (kJ mol^-1 K^-1)
Mo1	20	-23.019
	30	-22.838	-12.835	0.030
	40	-22.113
RhB	20	-33.908
	30	-35.975	2.812	0.122
	40	-35.724
MeB	20	-34.161
	30	-34.791	-15.070	0.063
	40	-35.462

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