Environmental Engineering Reference
In-Depth Information
surface area [34, 35]. h e widely studied NMnOs for environmental appli-
cations include hydrous manganese oxide (HMO) and nanoporous/nano-
tunnel manganese oxides [36].
Parida
et al.
prepared HMO by adding MnSO
4
·H
2
O into NaClO solu-
tion (containing active chlorine). h e precipitate was washed with HCl to
remove excessive alkali, followed by rinsing with double-deionized water.
h e BET surface area is around 100.5 m
2
/g [37]. h e BET surface of the
resultant HMOs is 359 m
2
/g. Heavy metal sorption onto HMOs, including
Pb(II), Cd(II), and Zn(II), usually results in the inner-sphere complex for-
mation, and it can be described by an ion-exchange process. Divalent met-
als on HMO consist of two similar steps as that of HFO: rapid adsorption of
metal ions to the external surface followed by a slow intraparticle dif usion
along the micropore walls [38]. h e adsorption can be represented by the
Freundlich model more reasonably than the Langmuir model, implying
that the active sites of HMO surface are heterogeneous for metal sorption.
h e HMO prefers metal sorption in the order of Pb
2+
> Cd
2+
> Zn
2+
, which
might rest on the dif erent sot ness of these metals [6]. Dyer
et al.
[39]
explored the sorption behavior of radio nuclides on crystalline synthetic
tunnel manganese oxides. Trace strontium (89 Sr) and cesium (137 Cs)
ions were removed through ion-exchange mechanism. Selectivity coei -
cients were estimated as K
Cs/K
= 0.6 and K
Sr/K
= 1.0 for OMS-2 and K
Cs/Mg
=
7550, K
Sr/Mg
= 50 and K
Sr/Ca
=10 for OMS-1. It was concluded that OMS-2
was particularly ef ective for the separation of trace silver ions. h e ai n-
ity trend for magnesium and calcium ion-extracted OMS-1 in HNO
3
was
137Cs > 59Fe > 51Cr ≈ 57Co ≈ 241Am > 54Mn > 63Ni > 65Zn > 236Pu >
89Sr. As reported by Pakarinen
et al.
[40], OMS materials exhibited selec-
tive adsorption of Cu
2+
, Ni
2+
and Cd
2+
in the presence of Ca
2+
and Mg
2+
. h e
exchange rates were reasonably high due to the small particle dimensions.
OMS materials are stable and their maximum Cu
2+
uptake capacity was
0.9-1.3 mmol/g. Koivula
et al.
[41] found that hydrometallurgical waste-
water (containing Al, Ca, Fe, Mg, Mn and Na) rich in manganese can be
easily used as a manganese precursor for OMS synthesis. A synthetic rai -
nate solution, which contained 4500, 490, 300, 150, 200 and 3500 mg/L of
Mn, Mg, Fe, Al, Ca and Na, respectively, could be used as the manganese
source for preparing OMS-2.
9.2.3
Nano Titanium Oxides (NTOs)
Both bulk and nanoparticle TiO
2
s show dif erent chemical behavior, cata-
lytic reactivity, and surface acidity based on their dif erent structure [42,
43]. Engates
et al.
reported that specii c surface area of the nanosized and
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