Table 5.

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

Pollutant(s)	Environment	Zeolite/Treatment	Main results	References
Pb	Water	•natural zeolites (chabazite and clinoptilolite) •treatment with 12.1 M HCl solution and heating at 85 °C	•treatment with HCl eliminated Ca, Mg, Na, K, Al, Fe from the framework •Si framework remained intact	[132]
Cr	Aqueous solution	•natural zeolite •treatment with 0.5 M HCl solution using a 1:3 acid ratio for 1 h at 70 °C and drying at 105 °C and calcination at 350 °C	•increase in SiO2 and decrease in Al2O3 content •decreased cation exchange capacity due to Al reduction •reduction in exchangeable cations due to the removal of Na+, K+, Mg2+ and Ca2+ cations •the removal percentage of Cr(VI) using NaOH-activated natural zeolite (16 %) was higher compared to HCl-activated zeolite (10 %).	[133]
Cd, Cu, Ni, Pb	Synthetic aqueous solution	•clinoptilolite - natural zeolite of 0.71–1.0 mm •2 M HCl, 24 h •static conditions	•efficiency of metals removal was inversely proportional to the metal concentration •the adsorption efficiency was Pb2+ > Cu2+ > Cd2+ > Ni2+ •the maximum sorption capacity was 4.22 mg/g for Cd2+ at an initial concentration of 80 mg/L and 27.7, 25.76, and 13.03 mg/g for Pb2+, Cu2+ and Ni2+ at an initial concentration of 800 mg/L	[134]
Pb, Co, Cr and Zn	Aqueous solution	•washed zeolite mixed with acetic for 1 h and drying at 170 °C for 24 h	•a removal efficiency of 89.51 % (Cr), 81.07 % (Cr) and 78.24 % (Zn)	[135]