Environmental Engineering Reference
In-Depth Information
9.5
Batch Kinetics and Equilibrium Adsorption Studies
9.5.1 Materials and Methods
In these studies, all chemical stock solutions were prepared from reagent-grade
chemicals using ultrapure water (Milli-Q UV Plus, Millipore). For batch tests, stock
solutions of Cu(II), Ni(II) and Cr(VI), each 1000 mg/L, were first prepared by dissolving
the reagent-grade metal compounds with ultrapure water. In the preparation,
concentrated HNO
3
(70%) was added to lower the solution pH to less than 2.0 to prevent
metal precipitation during storage in polypropylene plastic containers at 4
o
C.
For electroplating plants, the composition of heavy metals in effluents depends
on the specific metal plating process such as chromium, copper and nickel. Na
+
, Cl
-
,
NO
3
-
ions are used to adjust conductivity while Ca
2+
, Mg
2+
, CN
-
, F
-
, SO
4
2-
, PO
4
3-
may be
present, depending on the process requirements. For the adsorption experiments, the
synthetic metal solutions were then freshly prepared for the adsorbates by diluting the
stock solutions (Cu
2+
+ Ni
2+
+ Cr
6+
or Cr
6+
only) to target concentrations with ultrapure
water. The metal solutions had a background electrolyte of 0.1 M NaNO
3
to maintain the
ionic strength. These conditions were chosen to mimic industrial effluents issued from
electroplating wastewater.
9.5.1.1 Batch Equilibrium Adsorption
Since Cr(VI), Cu(II) and Ni(II) exist commonly in the effluents of plating
factories, petroleum, electrolytic refining plants and acid mining industries, these three
heavy metals were chosen for batch adsorption studies. In particular, more batch studies
were performed for Cr(VI) due to its higher toxicity. For the batch adsorption tests, 0.1 g
adsorbent was placed in a conical flask, in which 20 mL solution of the metal ion with
the desired concentration was added, and the mixture was then shaken for 15 minutes to
reach equilibrium. All the adsorption experiments were conducted at a shaking rate of
200 rpm (for -Fe
2
O
3
) or 400 rpm (for MeFe
2
O
4
) and room temperature of 22.5
o
C,
unless otherwise noted. The metal solutions had a background electrolyte of 0.1 M
NaNO
3
to keep the ionic strength relatively constant. Since the typical Cr(VI)
concentrations in the electroplating wastewater range from 10 to 200 mg/L, the
adsorption isotherms were obtained for various initial Cr(VI) concentrations in this
range by changing pH or temperature, respectively. In addition, Cu(II) and Ni(II) single-
solute adsorption isotherms were obtained respectively by varying the initial
concentration of the metal from 10 to 200 mg/L at pH 6.5 and 8.5. The mixture was
separated via an external magnetic field and final concentrations of metal ions were
determined by inductively coupled plasma-atomic emission spectrometer (ICP). To
ensure the accuracy, reliability and reproducibility of the collected data, all the
experiments were conducted in duplicate and the average values were presented.
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