A Improved Kinetics Model of Biosorption and Its Application in Bio-Electro Tower Reactor

Nowadays, industrial generates large quantity of aqueous effluent containing high levels of heavy metal ions. It is necessary to develop harmless novel process for efficiently treating this sort of wastewater. The BETR (bio-electro tower reactor, Fig 1) that we invented has an anode in the center and cathode on the tower wall. It fills 3-dimensional electrode materials and made them electrified by outer electric field. On the surface of the 3DE some electrochemistry reactions take place, and biofilm can be obtained after culturing and acclimating microorganism under low direct current. BETR is a coupling technique; the mechanic of it is that outer electric field can enhance the mass transfer rate when the solute is metal ions.

1 DC power  2 air pump  3 porous plane  4 insulation layer  5 Plexiglas tower 6 filler  7 stainless steel anode  8 active carbon fiber cathode   9 porous plane   10 aeration nozzle  11sampling nozzle   12 liquid inlet  13 peristaltic pump

 Fig.1 Electro-biofilm tower reactor

A 2-D numerical kinetic model (1) considering flow velocity and adsorption is developed to simulate the bio-electro tower reactor (BETR):                                    (1)

It is assumed that:

1) The supporting particles on which the microbial biofilm is located can be treated as porous media;

2) The adsorption of metal ions on the biofilm is of Langmuir style;

3) The wall effect of the tower is neglected;

4) The effect of acceleration on the ions movement due to the DC electric field only affects

According to the previous study of our group, the adsorption model is of Langmuir.

                                                           (2)

The rate of adsorption is expressed as:               

                                                         (3)

New model considers q_e the adsorbed amount when equilibrium as a time-dependent variable which is superior to the old pseudo-first-order and the pseudo-second-order model which regards q_e as a constant. However, it is obviously logical contradiction because q_e actually complies with the adsorption isotherm equation. As q_e is a time-dependent variable, the pseudo-first-order and pseudo-second-order model are not correct.

We researched the intensifying effect of electric field upon heavy metal ions adsorption process. The experiment is conducted under following conditions: direct current 2.8 voltage, air supply 0.75m³.h^-1, electrode immerge 1000mm in depth, stack volume of pottery balls (3DE material) 3.5L, temperature 25¡æ and initial concentration of Zn²⁺ is 150 mg¡°èL^-1. The adsorption process apply interim circle method and 4L Zn²⁺ solution was infused into reactor, velocity of circle flow is of 35ml¡°èmin^-1. We measure concentration of Zn²⁺ at fixed time interval.

The model is calculated using ADI algorithm.The calculation result fits well with the experimental data (Fig. 2). Two kinds of carriers, pottery ball (PB) and 3-dimensional electrode (3DE), were used to support the biofilm layer in BETR. And organic wastewater containing Zn²⁺ is selected as a sample to validate the model. The 3DE carriers can be polarized by outer electric field, but PB cannot. It is found that Zn²⁺ transfer faster in 3DE carriers than that in PB and a intensifying coefficient ¦Ç=( K¦Á)_3de/( K¦Á)_pb is introduced to stand for this effect in BETR. Therefore, ¦Ç=1.44 (Fig. 3). This suggests that the ions transfer 1.44 times faster in the 3DE carrier than that in the pottery ball carrier

 

Fig.2 The experimental and simulated data of solution ions concentration at different times

Fig.3 The simulated data of ions concentration inside biofilm at different times

Keywords:  numerical model, biosorption, 3-dimensional electrode, mass transfer, BETR