題目:Membrane Distillation
姓名:葉家宏 學號:49940078
原理:
The removal of toxic metals from industrial wastewater is a
major concern. Hexavalent chromium receives particular
attention
because of its high toxicity and numerous industrial
applications
(electroplating, metal finishing, and corrosion inhibition).
Plating
baths in the electroplating process contain 100–200 g/L of
hexavalent
chromium, and they can be polluted by metallic cations (i.e.,
Fe3+, Cr3+, Ni2+; concentrations of which change between 1 and
10 g/L) and by anions (nitrates, chlorides, phosphates, and
sulfates).
Concentration of Cr (VI) in the rinsing waters used after
chromium
plating need to reduce from 100–500 to <1mg/L before discharge
to the environment .
The treatment of electroplating effluents generally consists
in a reduction of Cr (VI) to Cr (III) with a chemical reducing
agent such as ferrous sulfate, sulfur dioxide, or sodium
bisulfite,
and then precipitation of the trivalent chromium as hydrated
oxides. Other recycling processes involve anionic-exchange
resins
which are used to remove chromate or dichromate followed by
elution with NaOH solution, and reverse osmosis. These
techniques
are costly and produce additional sludges. The possibility
of recovering and concentrating of Cr (VI) for reuse makes very
attractive the techniques of solvent extraction and
liquidmembrane
Solvent extraction technology has been widely used for the
recovery and/or removal of heavy metals in hydrometallurgy. A
limitation in traditional solvent extraction is that a large
inventory
of solvent is required, especially when processing dilute
solutions
. ELM systems have now become an alternative metal separation
technique from dilute solutions.
This technique offers advantages over conventional solvent
extraction. Because, conventional solvent extraction requires
larger
volumes of solvent and associated equipment, and therefore
becomes inefficient when the metal ion concentration in the
effluent
stream is low Particularly, a ELM process has sufficient
ability to selectively separate metals from aqueous solutions
using
a double W/O/W emulsion stabilized by the use of suitable
surfactants,
with a reduced amount of organic solvent and greater
extraction. First study on the industrial applications of ELM
was
made by Li et al.. They are normally credited with this
invention.
Many investigators have studied the practical operation of ELM
and the mechanisms that regulate the transport of metals
through
them, a process thatwould be regulated by a diffusive
phenomenon
of mass transfer with a chemical reaction. The ELM method would
have the ability to remove and concentrate selectively or
collectively,
depending on the extractants chosen, the lowmetal contents
present in these residual aqueous solutions, in a continuous
and
fast process, using a thin liquid membrane that has a large
inter facial area and needs only a very small volume of organic
solvent
In the present work, in order to have a better understanding
of
the dynamics of ELM technique, the major parameters influencing
to selective extraction and concentration of chromium (VI) by
ELM
process from the acidic feed solutions containing Co, Ni, Cd,
Zn, and
Cu ions were experimentally studied and the optimum conditions
were determined.
應用:
The extraction reactions of Cr (VI) are quite complicated.
Because, the forms of Cr (VI) in aqueous solutions vary with
the
concentration of Cr (VI) and solution pH. Very fewresearchers
have
taken account of the various forms of Cr (VI) in the analysis
of
extraction mechanisms. Recently, Huang et al. [12] have
established
the equilibria for the solvent extraction of Cr (VI) between
aqueous
solution (pH 2–4) and kerosene containing TOPO (Fig. 1a),
considering
the presence of various forms of Cr (VI) species in aqueous
phase and the influence of solution ionic strength for
equilibrium
constants. The equilibria for the extraction of HCrO4
− and Cr2O7
2−
with TOPO could be expressed through the formation of the
species
H2CrO4·(TOPO)(org) and H2Cr2O7·(TOPO)3(org).
The chromate ions may exist in the aqueous phase in different
ionic forms (HCrO4
−, CrO4
2−, HCr2O7
−, Cr2O7
2−). Any of these
forms will predominate to other forms of chromium depending on
total amount of chromium and pH of the aqueous phase. CrO4
2−
anion prevails in basic or slightly acidic solution while Cr2O7
2−
anions dominate in acidic aqueous solution. Moreover, Cr2O7
2−
convert into HCrO4
− anions in acidic aqueous solution at a total Cr
(VI) concentration lower than (1.26–1.74)×10−2 mol/L [13].
Thus,
in this study, chromate ions will exist as HCrO4
− in the multicomponent
acidic solution at low initial concentration (100–500 mg/L)
of Cr (VI).
The reaction for the extraction of HCrO4
− with TOPO (Fig. 1b)
from aqueous solutions could be expressed by the following
equations
[2]:
HCrO4(aq)
− +H+
(aq) +TOPO(org)⇔ H2CrO4TOPO(org) (1)
The complex formed as above diffuses through the membrane
toward the stripping side, then in the presence of (NH4)2CO3
there,
the following reaction (Fig. 1b) is expected to take place at
the
membrane face on the stripping solution side:
H2CrO4TOPO(org) +(NH4)2CO3(aq)
⇔ TOPO(org) +(NH4)2CrO4(aq) +CO2(aq) +H2O(aq) (2)
參考文獻: