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

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^-, HPO₄^2-, 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

Binary Freundlich model
Ca2+
1-5#	0.66855	1897.96	-2.88406	0.99541
6-10#	0.66007	6383.61	-2.31729	0.99758

Binary Freundlich model
Cd2+
1-5#	0.36405	0.19718	-1.6693	0.96317
6-10#	0.93985	1.87212	1.67962	0.99993

Binary Freundlich model
Ca2+
1-5#	0.69271	477.43535	6.82804	0.98886
6-10#	0.69753	0.10694	-1.46138	0.99869