3.8.3 Magnetite

Synthetic magnetite nano particles was used to remove Hexavalent chromium (Cr(VI)) from synthetic wastewater containing 0 to 50 mg/L Cr(IV) at pH 3.0 and 30°C.Up to 72% of Cr was removed via adsorption and the isotherm fitted to Freundlich model [53]. In addition, rapid sorption of Cu(II), Zn(II), Cd(II), Pb(II) and Ni(II) on synthetic Amine-magnetic Fe-oxide is influenced by pH, ionic strength and the complexation of amino groups [101]. Maximum adsorption occur at pH 6, ionic strength of 1.0 mmol/L NaCl and duration of 60 min and are best described with Pseudo-second-order kinetic model, Langmuir model [101]. The adsorption affinity of the adsorbent follow this sequence Pb(II) > Cu(II) > Zn (II) > Cd(II) > Ni(II) [103].

### 3.9 Mixture of oxide, clay and/or other materials

Mixture of goethite, humic acid and kaolinite are good adsorbents for lead, cadmium, zinc, nickel and copper and the adsorption is better described with Langmuir and Freundlich isotherm model [104]. However, in a five metal ion system (Quinary), the adsorption of lead, cadmium and nickel are affected negatively by the presence of zinc and copper whereas the presence of lead, cadmium and nickel has synergistic effect on the sorption of zinc and copper [104]. Another study found that low-cost adsorbent Algeria clay that is composed of predominantly montmorillonite and kaolinite has the capacity of adsorbing Cu(II) at pH of 6.5 and 20°C with maximum adsorption capacity of 12.22 mg/g [105]. However, treated Algeria clay under similar condition as the untreated clay has adsorption capacity of 15.40 mg/g [105]. The process of adsorption was spontaneous and exothermic [105]. Sorption of copper (II) ion on palygorskite and sepiolite is enhanced at elevated temperature [106]. The adsorption reactions are endothermically driven but sepiolite sorbs copper spontaneously and has more copper retention capacity compared to palygorskite [106]. In another study, combination of montmorillonite

#### Sorption of Heavy Metals on Clay Minerals and Oxides: A Review DOI: http://dx.doi.org/10.5772/intechopen.80989

Ni(II)-birnessite suspension at pH 6.5 results to formation of edge-sharing Ni(II) complexes due to site competition [99]. However, at pH 7.5, the presence of Mn(II) results to transformation of birnessite into feitknechtite that encourages sorption and incorporation of Ni(II) from solution [99]. This suggests that alteration of birnessite can influence the solubility of nickel in anaerobic environment [98]. In another study, hexagonal birnessite (δ-MnO2) was used to sorb Cu(II) containing 143, 77 and 32 mg/L at pH 1–9 [100]. At pH 3, 100% adsorption results to about 5, 2.5 and 1% of copper in the adsorbate [100]. However, EXFAS characterisation of adsorbates at pH 4 reveals that Cu forms by inner-sphere complexation whereas at pH 8, it associates with birnessite via structurally incorporation [100]. Sorption of Zn onto synthetic δ-MnO2 from an initial solution containing 2000 mg/L of Zn occurs at pH 1 and at maximum pH 5 [101]. However, desorption reaction indicates the Zn sorption is reversible and EXFAS show that it form inner-sphere surface complexes at high pH [101]. Similarly, removal of Pb from solution containing 810, 1782, 2801 mg/L occur at pH 5.5 pH and equilibration time of 2 weeks, thus indication high sorption capacity of birnessite for Pb [101]. Also, desorption experiment show that the sorption is reversible at pH 1 [101]. In another study, sorption of Pb(II), Cu(II), Zn(II), Cd(II) onto hexagonal birnessite was carried out at pH 4.5 and characterised with powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The result of the study indicates that sorption capacity of biressinte for these metals follow this sequence: Pb2+ ≫ Cu2+ > Zn2+ > Cd2+ but Pb sorption is up to 3.9 times more than other metals [102].

Synthetic magnetite nano particles was used to remove Hexavalent chromium (Cr(VI)) from synthetic wastewater containing 0 to 50 mg/L Cr(IV) at pH 3.0 and 30°C.Up to 72% of Cr was removed via adsorption and the isotherm fitted to Freundlich model [53]. In addition, rapid sorption of Cu(II), Zn(II), Cd(II), Pb(II) and Ni(II) on synthetic Amine-magnetic Fe-oxide is influenced by pH, ionic strength and the complexation of amino groups [101]. Maximum adsorption occur at pH 6, ionic strength of 1.0 mmol/L NaCl and duration of 60 min and are best described with Pseudo-second-order kinetic model, Langmuir model [101]. The adsorption affinity of the adsorbent follow this sequence Pb(II) > Cu(II) > Zn

Mixture of goethite, humic acid and kaolinite are good adsorbents for lead, cadmium, zinc, nickel and copper and the adsorption is better described with Langmuir and Freundlich isotherm model [104]. However, in a five metal ion system (Quinary), the adsorption of lead, cadmium and nickel are affected negatively by the presence of zinc and copper whereas the presence of lead, cadmium and nickel has synergistic effect on the sorption of zinc and copper [104]. Another study found that low-cost adsorbent Algeria clay that is composed of predominantly montmorillonite and kaolinite has the capacity of adsorbing Cu(II) at pH of 6.5 and 20°C with maximum adsorption capacity of 12.22 mg/g [105]. However, treated Algeria clay under similar condition as the untreated clay has adsorption capacity of 15.40 mg/g [105]. The process of adsorption was spontaneous and exothermic [105]. Sorption of copper (II) ion on palygorskite and sepiolite is enhanced at elevated temperature [106]. The adsorption reactions are endothermically driven but sepiolite sorbs copper spontaneously and has more copper retention capacity compared to palygorskite [106]. In another study, combination of montmorillonite

3.8.3 Magnetite

138

(II) > Cd(II) > Ni(II) [103].

Advanced Sorption Process Applications

3.9 Mixture of oxide, clay and/or other materials

and ZnO show high capacity for adsorption of Pb and Cu from aqueous solution at wide range of pH [107]. The kinetic of the adsorption reaction follow pseudosecond-order whereas the adsorption isotherm is described by Langmuir isotherm [107]. Arsenic adsorption on goethite, amorphous Fe-hydroxide, and Ti(IV)-Fe (III)-Al(III)-pillared bentonite, clay pillared with titanium (IV), iron (III), and aluminium (III) reveal that amorphous Fe-hydroxide has highest adsorption capacity for arsenate and arsenite [108]. Arsenic is stable at pH 7 and is mobilised at pH 4 and 10 but mostly at acidic condition. Mn- oxides, amorphous iron oxides and clay minerals sequester up to 61% of arsenic [109].

Combination of montmorillonite and humic acid in the ratio of 100:3 is efficient for removal of cadmium at pH 8.5 and contact time of 24 h with adsorption capacity of 18.96 mg/g for cadmium [110]. Another study reveal that the adsorption and desorption of cadmium and copper from montmorillonite, allophane, kaolinite, halloysite reveal that sericite has the highest ability for Cd sorption, however, montmorillonite showed greatest retention for Cd [111]. In addition, all clay types has sorption ability for copper with pH of 50% metal sorbed lower than pH of 50% metal sorbed for cadmium sorption [111]. In another study, the removal of Cu and Zn from aqueous solution by Al-montmorillonite, goethite, kaolinite and their mixtures at room temperature and pH 4 reveal that adsorption is via inner and outer sphere complexation [112]. However, mixing of different mineral for sorption of Cu and Zn retards their removal and decreases the exchange of proton and acid/base potential of the reactive sites [112]. The sorption of Zn onto hydrous Mn-oxide (HMO)-coated clay reveals that the affinity the HMO-coated montmorillonite was greater than that of uncoated montmorillonite, and possess linear isotherm at pH 5–6 [113]. X-ray absorption spectroscopy (XAS) reveal reduction of first shell distance at surface loading of 10<sup>3</sup> mol and pH 5–7 due to higher electrostatic attraction [113]. In another study, sorption of copper, zinc and lead on soil composed of clay minerals (smectites and vermiculites), carbonates and Fe-oxide show that Copper and lead has higher sorption capacity and retention compared to Zn [114]. Clay minerals adsorbs more metals than other phases, however, for lead, similar capacity was obtained for Fe-oxides [114]. The presence of carbonate in alkaline condition increases the amount of metal uptake, and the mixture of clay minerals and Fe-oxide enhanced adsorption of the metals [114]. Competitive sorption and desorption of Cd, Cr, Cu, Ni, Pb and Zn by iron oxide, Mn oxides, kaolinite, vermiculite and mica from initial solution of 100 mg/L show that kaolinite and mica has strong affinity and retention capability for cd; vermiculite, Cu and Zn; iron oxide and Mn-oxide, Pb [115]. Kaolinite has low retention capability for Cu whereas vermiculite and Mn oxide has greatest retention capability of all the metal [115].

In another study, the effect of increase surface area of clay minerals (kaolinite, montmorillonite and illite) through coating with Fe-oxide, organic matter and Al-oxides for adsorption of heavy metals indicate that coating clay increases the surface area of clay minerals with expectation of Aluminium oxide coated montmorillonite and organic matter coated 2:1 phyllosilicates [116]. Another study found that, both amorphous hydrous manganese oxide (HMO) and HMO-coated montmorillonite sorbs Ni and Pb to form inner-sphere complexes with Ni coordinating to vacant site of Mn-oxide structure and Pb forming bidentate corner-sharing complexes [117]. In addition, another study reveal that montmorillonite clay coated with amorphous (hydrous manganese oxide (HMO), birnessite and pyrolusite has the same surface properties as the coated oxide, however, the surface area of the coated Montmorillonite increases whereas the while the pore size distribution decreased. The HMO- and birnessite-coated clay still retained their pH (point of net zero charge (pnzc)) of 2.8 and 3.1, respectively, [118].
