Multicomponent Separations in Wastewater

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GAC, the concentration of adsorbate in liquid will decrease with time until equilibrium achieve, where net uptake of adsorbate is zero. During that process, adsorbate concentration on surface will increase to , which is achieved in the same time as . Due to more number of active sites on the surface of adsorbent, the uptakes at initial stage are rapid. After that, adsorption becomes slower because of intra-particle diffusion. The uptakes of adsorbate against time curves are continuous leading to saturation, suggesting the possible monolayer coverage of adsorbate on the surface of the adsorbent.

The equilibrium concentration of adsorbate in solid can be calculated as below:

Where dose represents the amount of GAC (g) added in CMBR per litre solution.

Figure: GAC adsorption in CMBR

Figure: Concentration of adsorbate uptake in liquid phase and solid phase.

 Influence of GAC dosage in CMBR on adsorption

Like Figure demonstrates, decreases as adsorbent dosed more. It is readily understood that more adsorption sites are available due to adsorbent dosage increases and therefore leads to an increase of adsorption capacity. Additionally, the kinetics is affected. The kinetic mechanism needs detailed evaluated by pseudo-first-order or pseudo-second-order model to decide how dosage affects. However, in general as the amount of GAC increases, the mass flux into surface decreases and the adsorption sites would be reached more slowly, therefore increasing the time to equilibrium.

Figure: Equilibrium adsorbate uptake in liquid phase in CMBR as adsorbent dosage changes.

 Influence of Temperature in CMBR on adsorption

It is expected that physical adsorption are always exothermic because of heat releasing after bond formation between adsorbate and adsorbent. Hence, increasing temperature will reduce adsorption capacity and prolong process.

Figure: Equilibrium adsorbate uptake in liquid phase in CMBR as Temperature changes.

 Adsorption Isotherm

Adsorption isotherm demonstrates the distribution of adsorbate molecules between liquid and solid phases when adsorption heads to equilibrium. This equilibrium relationship is established by calculating equilibrium concentration of both solid and liquid phase that runs under different adsorbent dosage. The isotherm analysis allows adsorption capacity to be found at any given concentration of liquid phase and so that find the suitable model that can be used for design process.

In the Langmuir model, it is assumed only monolayer on the surface at maximum coverage, which means that no stacking of adsorbate. The Freundlich does not have this restriction and is a power function, works best. The linear form of Freundlich is as below:

where Freundlich adsorption capacity parameter

and Freundlich adsorption intensity parameter (unitless)

indicates the isotherm type. When >1, the adsorption is unfavourable, when =1, the adsorption is irreversible and when 0< <1, the adsorption is favourable.

As figure shown, when is constant the larger , the more adsorbates be adsorbed on GAC. Hence, can be used to compare the adsorption capacity of a given GAC when is relatively constant. Whereas if among different isotherms are significantly different, the plots will converge at certain concentration, in that case both and are needed to compare capacities.

Figure: Effect of and on log-log Freundlich isotherm

2. EXPERIMENTAL

2.1 EXPERIMENTAL DESIGN

The experiment is designed to focus on finding the effect of adsorbent dosage, temperature and contact time on adsorption. Other parameters that influence performance of adsorption include solution pH, stir speed and initial concentration. However, in theory an identical concentration of wastewater is provided and no substances are available for adjusting pH. On the other hand, the stir speed could only be judged by eyes. Hence, only the effect of dosage, temperature and time can be investigated by the available and precise equipment, for example, balances, hot plates and second chronograph.

2.2 METHODLOGY

I. Produce Calibration Curve

 PCT at 243nm and E124 at both 243nm and 332nm with 4000mg/L pure solution

a. To find the critical concentration whose absorbance within the tolerance of UV-Vis spectrophotometry, firstly take 10mL provided PCT solution into 100mL volumetric flask and then add water to obtain 400mg/L solution.

b. Take some obtained solution into sample glass filter by pasteur pipette.

c. After making zero by distilled water sample, take the absorbance reading of sample at 243nm (which gives nothing).

d. Dilute 400mg/L solution by taking 25mL into a 100mL flask and then adding water to obtain 100mg/L PCT solution. Repeat step b&c, the absorbance of which still exceeds the tolerance of reading.

e. Take 50mL of 100mg/L PCT solution into 100mL flask to make solution 50mg/L and measure absorbance like previous steps. Record abs for 50mg/L PCT solution.

f. Since 50mg/L gives abs reading, prepare 45, 40, 30, 25, 20, 15, 10, 5mg/L PCT solution successively by adjusting dilution rate of existing solutions with known concentrations. In that case, there is enough data for exploring the linearity of calibration.

g. Repeat step b&c and record abs of each solution samples at different concentration.

h. Plot calibration curve of PCT with concentration as x-axis and absorbance as y-axis.

i. Make 50, 45, 40, 35, 30, 25, 20, 10mg/L of E124 solution by diluting provided solution in different dilution rates.

j. After making zero by distilled water sample, take the absorbance reading of 50mg/L at 243nm and then put water sample back in.

k. Adjust wavelength to 332nm