(579a) Competitive Adsorption of Ca2+, K+, Cd2+ and Pb2+

Authors: 
Wang, T., Tianjin Polytechnic University
Zhou, Y., University of Colorado, Boulder

There has been increasing concern and more stringent regulation standards pertaining to the discharge of heavy metals to the aquatic environment, due to their toxicity and detriment to living species including humans. Metals are non-degradable and can accumulate in living tissues, so they must be removed from polluted water. Nowadays, most of researches of adsorption to heavy metal ions by biofilm are focus on single kind of ions or neglect influence of non heavy metal ions which coexist with heavy metal ions.

Pearson classified metal ions into hard ions (such as K+, Mg2+, Ca2+), soft ions (such as Hg2+, Cd2+, Pb2+) and fringe ions (such as OH-, HPO42-, RCOO-) according to their capability of forming coordination key. In this work, the single and competitive adsorption between hard-hard ions, soft-soft ions and hard-soft ions was studied.

In experiment of single kind of metal ions adsorption, solution of metal ions had been prepared at certain concentration in series. Each solution was added into conical flask together with biofilm weight about 7g. Keep pH value of solution at 7.0. Put conical flasks into shaker then. Under the experiment conditions of 25¡æ and 150 rpm, The adsorption reaction time would be 30 min. Mixture in flasks would be centrifugal separate for 10 min at 3000 rpm. Balance Concentration of metal ions in solution was analysed by atomic absorption spectrophotometer (Type HITACHI 180-80, Japan).

Langmuir and Freundlich isothermal adsorption equation were used to fitting experiment data. Equation parameters and result can be seen in Tab 1 and Fig 1 to 4

Table 1 Adsorption model parameters of Ca2+, K+, Cd2+ and Pb2+

Langmuir model

Freundlich model

Q0

b

R2

a0

b0

R2

Ca2+

1.6435

0.0246

0.99773

0.06669

0.68502

0.99037

K+

0.04919

0.01412

0.99971

0.001135

0.74314

0.99552

Cd2+

2.0975

0.5658

0.99994

0.8679

0.31632

0.97088

Pb2+

16.897

2.61905

0.99106

14.72313

0.58902

0.99617



   

Fig.1 The adsorption isotherm of Ca2+

Fig. 3 The adsorption isotherm of K+

Fig.2 The adsorption isotherm of Cd2+

Fig.4 The adsorption isotherm of Pb2+



Balance adsorption amount in biofilm of Ca2+, K+, Cd2+ and Pb2+ is of 1.6425mg.g-1, 0.04919 mg.g-1, 2.0975 mg.g-1 and 16.897 mg.g-1, so Pb2+>Cd2+>Ca2+>K+

In experiment of competitive adsorption, we studied 3 kinds of reaction.

1)Hard-hard: Study adsorption amount of Ca2+ on biofilm with different concentration of K+

2)Soft-soft: Study adsorption amount of Cd2+ on biofilm with different concentration of Pb2+

3) Hard-soft: Study adsorption amount of Ca2+ on biofilm with different concentration of Pb2+

Solution of every two kinds of metal ions had been prepared at certain concentration in series. Other conditions are as same as experiment of single metal ions except adsorption reaction time would be 20 min. Freundlich competition adsorption model was used to fitting experiment data. Equation parameters and result can be see in Tab 2 to 4 and Fig 5 to 7

Fig 5 Competition adsorption isotherm of Ca2+ with K+

Fig 6 Competition adsorption isotherm of Cd2+ with Pb2



+

Fig 7 Competition adsorption isotherm of Ca2+ with Pb2+

Table 2 The binary Freundlich adsorption isotherm model parameters of Ca2+ and K+

    Binary Freundlich model

Ca2+

1-5#

0.66855

1897.96

-2.88406

0.99541

6-10#

0.66007

6383.61

-2.31729

0.99758

Table 3 The binary Freundlich adsorption isotherm model parameters of Cd2+ and Pb2+

    Binary Freundlich model

Cd2+

1-5#

0.36405

0.19718

-1.6693

0.96317

6-10#

0.93985

1.87212

1.67962

0.99993

Table 4 The binary Freundlich adsorption isotherm model parameters of Ca2+ and Pb2+

    Binary Freundlich model

Ca2+

1-5#

0.69271

477.43535

6.82804

0.98886

6-10#

0.69753

0.10694

-1.46138

0.99869

We draw conclusion that competitive adsorption obviously happened between the same kinds of hard or soft metal ions. To the metal ions which initial concentration was fixed, its adsorption amount in biofilm would be decreased with increasing of concentration of another kind of ions because same kind of ions find same coordination radicle, so the high concentration kinds of ions would gained more active coordination radicle when adsorption happened in biofilm. Competitive adsorption happened not obviously between different kinds of metal ions because different kind of ions needs different coordination radicle and different coordination method.

Soft ions could be better absorbed than hard ions in biofilm because what the hard ions need coordination radicle is always contain oxygen atom such as OH-, HPO42-, RCOO- etc which generally exist in anaerobic biofilm layer and what the soft ions need such as CN-, RS-, -SH- is always exist in aerobic biofilm layer. When liquid flow slowly (it means shear force is weak) and substrate enough, anaerobic biofilm layer always thicker than aerobic biofilm layer, so the coordination radicle which soft ions needed is more than coordination radicle hard ions needed. Another reason is that soft ions bond its coordination radicle by covalent bond. It is a far more firm bond method than hard ions join with its coordination radicle.

 

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