Table 7.

A summary of modification with surfactants of natural zeolites for heavy metals removal from environment.

Pollutant(s)	Environment	Zeolite/Treatment	Main results	References
As	Aqueous solution	•natural mordenite and natural clinoptilolite •treatment with HDTMA = hexadecyltrimethylammonium bromide	•both modified natural zeolites required about 20 and 10 h, respectively to reach a state of equilibrium •both modified natural zeolites reduced As(V) concentration below the WHO guideline values of 50 ppb As in drinking water	[110]
Cr	Aqueous solution	•natural zeolite with particle size of 0.12–0.18 mm •treatment with hexadecyltrimethyl ammonium bromide (HDTMA) and cetylpyridinium chloride (CPC) stirred at 60 °C for 5 h under water-cooled reflux condenser	•natural zeolite has little or no affinity to Cr(VI), while modification by HDTMA and CPC significantly enhanced the adsorption capacity	[139]
As	Wastewater	•natural zeolite pretreated with NaCl was treated with hexametheyl-1, 9-nonanediammonium dibromide at 150 rpm for 24 h	•As removal increased by increasing dosage of adsorbent and contact time •As removal efficiency from wastewaters was higher than 99 % after stirring for 180 min at pH = 6.9	[140]
Cr, Pb	Wastewater	•natural zeolite was placed in cetyltrimethylammonium bromide (CTMAB) and cetylpyridinium bromide (CPB), separately, and shaken at 35 °C for 48 h	•adsorption capacity of Cr(VI) by the modified zeolites was increased by 2–4 times relative to that by unmodified zeolite •the adsorption capacity of Pb folloed the order: unmodified zeolite > CTMAB-modified zeolite > CPB-modified zeolite, which is opposite to that of Cr(VI) adsorptio	[141]
As, Cd	Soil	•natural zeolit pretreated with NaOH was shaken with HDTMA-Br at 25 °C for 24 h	•sequential extraction results showed that the addition of HDTMA-modified zeolite not only increased the residual fraction of As (by 2.7–5.9 %), but also reduced the toxicity-related fraction (by 2.3–2.7 %) comparing to the unmodified zeolite and blank treatments •acid exchangeable Cd, the most mobile fraction of Cd, was significantly decreased from 10.06 mg kg−1 to below 4.60 mg kg−1	[142]