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Table 2

Characteristics of chemical modification

Methods	Mechanism	Pollutants	Advantages	Disadvantages	Reference
Acid	H⁺ replaces Ca²⁺ and Mg²⁺ in zeolite which have large diameter to increase specific surface area and microporosity	Ammonia, antimony(III), uranium (VI)	Simple and convenient, improving cations adsorption capacity	Low cation exchange capacity due to H⁺ competition and dissolution of Si-tetrahedra and free linkages	Zhang et al. (2014b), Matijasevic et al. (2016), Christidis et al. (2003)
Alkali	a. Dissolve silicon reduce Si/Al from zeolite; make uniform pore size b. Electrostatic interaction and form covalent bonds	Organics Ammonia, manganese (II), lead (II), copper (II), cadmium (II)	Simple and convenient, improving the removal rate of cations	Destruction of crystal structure by poor concentration control, reduction in absorption rate	Sancho et al. (2017), Guaya et al. (2015), Ali et al. (2014), Ates & Akgül (2016), Wen et al. (2016)
Salt	a. Exchanging with Ca²⁺ and Mg²⁺ in the zeolite, increasing the pore size b. Ligand-exchange and electrostatic interaction	Ammonia, iron (II), calcium (II), magnesium (II), manganese (II), copper (II), zinc (II), nickel (II), uranium (VI) Phosphorus, fluoride, chromium (VI), arsenic (III), arsenic (V), lead (II)	High pollutants removal efficiency	Expensive salt solutions	Huang et al. (2014), Jevti et al. (2014), Jiang et al. (2013), Ali et al. (2016), Ates (2014), Akigbe et al. (2016), Bakatula et al. (2011), Lofu et al. (2016), Awuah et al. (2016)
Cationic surfactant	a. Anions adsorption and associated compounds formation b. Cations exchange c. Organics adsorption by hydrophobic alkyl chain	Tungstate, antimonate phosphorus Ammonia, copper (II), nickel, iron Aniline, EDTA	Increase anion removal rate	More expensive than salt modification. Complicated modification process	Syafalni & Nair (2013), Pawaiya & Tomar (2014), Wingenfelder et al. (2016), Hussein et al. (2014), Khazaei et al. (2015)
Rare earth	a. Cations exchange b. Oxides and hydroxides formation on the surface of zeolite, electrostatic interaction, ligand-exchange reaction	Ammonia Phosphorus	Utilization of mineral resources and pollutants removal simultaneously	Expensive and secondary pollution	Zhang et al. (2012), Yang et al. (2014)

Methods	Mechanism	Pollutants	Advantages	Disadvantages	Reference
Acid	H⁺ replaces Ca²⁺ and Mg²⁺ in zeolite which have large diameter to increase specific surface area and microporosity	Ammonia, antimony(III), uranium (VI)	Simple and convenient, improving cations adsorption capacity	Low cation exchange capacity due to H⁺ competition and dissolution of Si-tetrahedra and free linkages	Zhang et al. (2014b), Matijasevic et al. (2016), Christidis et al. (2003)
Alkali	a. Dissolve silicon reduce Si/Al from zeolite; make uniform pore size b. Electrostatic interaction and form covalent bonds	Organics Ammonia, manganese (II), lead (II), copper (II), cadmium (II)	Simple and convenient, improving the removal rate of cations	Destruction of crystal structure by poor concentration control, reduction in absorption rate	Sancho et al. (2017), Guaya et al. (2015), Ali et al. (2014), Ates & Akgül (2016), Wen et al. (2016)
Salt	a. Exchanging with Ca²⁺ and Mg²⁺ in the zeolite, increasing the pore size b. Ligand-exchange and electrostatic interaction	Ammonia, iron (II), calcium (II), magnesium (II), manganese (II), copper (II), zinc (II), nickel (II), uranium (VI) Phosphorus, fluoride, chromium (VI), arsenic (III), arsenic (V), lead (II)	High pollutants removal efficiency	Expensive salt solutions	Huang et al. (2014), Jevti et al. (2014), Jiang et al. (2013), Ali et al. (2016), Ates (2014), Akigbe et al. (2016), Bakatula et al. (2011), Lofu et al. (2016), Awuah et al. (2016)
Cationic surfactant	a. Anions adsorption and associated compounds formation b. Cations exchange c. Organics adsorption by hydrophobic alkyl chain	Tungstate, antimonate phosphorus Ammonia, copper (II), nickel, iron Aniline, EDTA	Increase anion removal rate	More expensive than salt modification. Complicated modification process	Syafalni & Nair (2013), Pawaiya & Tomar (2014), Wingenfelder et al. (2016), Hussein et al. (2014), Khazaei et al. (2015)
Rare earth	a. Cations exchange b. Oxides and hydroxides formation on the surface of zeolite, electrostatic interaction, ligand-exchange reaction	Ammonia Phosphorus	Utilization of mineral resources and pollutants removal simultaneously	Expensive and secondary pollution	Zhang et al. (2012), Yang et al. (2014)