Table 6.

A summary of alkaline treatment of natural zeolites for heavy metals removal from environment.

Pollutant(s)	Environment	Zeolite/Treatment	Main results	References
Cu	Aqueous solution	•natural zeolite •treatment with 1 M and 2 M NaOH solution, 200 rpm, for 4 h	•natural zeolite modified using alkaline treatment displayed higher Cu uptake compared to that modified using a 1 M acetic acid solution, under the same experimental conditions	[104]
Cr6+	Aqueous solution	•natural zeolite •mixed with a 1 N NaOH solution in a 1:3 ratio for 1 h at 70 °C, followed by drying at 105 °C	•the amount of Al increased slightly •cation exchange capacity increases due to higher amount of Na+ •improved the removal efficiency of Cr6+	[133]
Pb2+	Aqueous solution	•natural zeolite •mixed with a solution of NaOH (0.5–5.0 g/L) at 90 °C and dried at 120 °C	•NaOH 1 g/L offered by the highest Pb2+ a removal efficiency of 60.75 %, and adsorption capacity of 136.26 mg/g	[136]
Cd2+, Cu2+, and Cr3+	Aqueous solution	•natural clinoptilolite zeolite •pre-dried at 60 °C, 24 h and mixed with 0.1–4 M NaOH solutions	•the crystal structure of clinoptilolite not affected under low NaOH concentration (≤0.5 M); partially transformed into zeolite Na–P under a 1 M NaOH concentration, and entirely transformed into zeolite Na–P under 2–4 M NaOH concentration •the adsorption capacity of the NaOH-treated zeolite for Cd2+, Cu2+, and Cr3+ progressively improved (0.1–3 M NaOH) and then slightly reduced (4 M NaOH)	[137]
Cr and Mn	Aqueous solution	•natural clinoptilolite zeolite •pre-dried at 60 °C for 24 h and mixed with 2 M NaOH solution	•low SiO2/Al2O3 ratio, and high hydroxyl group content showed outstanding hydrophilicity and ion exchange properties •high adsorption capacity, i.e. 86.5 % of Cr and 67.4 % of Mn	[138]
Cd and Zn	Sediment	•natural clinoptilolite •treatment with a 2 N NaOH solution followed by calcination at 200, 300, 400, 500 and 600 °C, for 2 h	•NaOH modified clinoptilolite of 2–4 mm and 600 °C had the best effect on the removal of Cd and Zn in the aqueous solution, comparing to NaCl, LaCl3, ZrOCl2, and Ce(NO3)3 under the same experimental conditions	[86]