Cadmium passivation by four Passivators: Isotherm adsorption, desorption and mechanisms

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In order to select passivator that can effectively passivate bivalence cadmium (Cd2+) in combined soil, this study will firstly operate an outdoor research and preliminary experiment which have already selected steel slag, phosphate rock, palygorskite and charcoal as passivators. This paper will operate indoor isothermal experiments of adsorption and desorption, the specific surface area - pore radius distribution instrument to measure the phase composition, specific surface and the characteristics of pore radius of passivators. The main result shows that: (1) The results of indoor isothermal experiments of adsorption and desorption show that steel slag, palygorskite and phosphate rock all have a good adsorption of heavy metal ion bivalent cadmium; the adsorption isotherm is conforming to L equation and F equation. These four passivators’ adsorption on Cd2+ tend to increase as the initial concentration increases. Under the condition of same initial concentration, these four passivators have different adsorption capacities on Cd2+. Under the condition of same equilibrium concentration, these four passivators’ adsorptions on Cd2+ is steel slag > phosphate rock≈palygorskite > charcoal and under the condition of low concentration. (2) Experiments of adsorption and desorption show that steel slag has a relatively small desorption on heavy metal ion Cd2+, ground phosphate rock, charcoal and palygorskite have an increasing desorption on this heavy metal ion as the adsorptions increase. Specifically, when the adsorption reaches its peak, desorption on Cd2+ is charcoal > palygorskite > phosphate rock > steel slag. After the comparisons we could find that among these four passivators, charcoal has a relatively huge desorption on Cd2+. (3) Among these four passivators, charcoal and palygorskite have relatively huge specific surfaces and pore volumes. The passivation mechanism of heavy metal is mainly physical adsorption and ion exchange adsorption. Though steel slag and phosphate rock have relatively smaller specific surfaces and pore volumes, they have higher pH values. The passivation mechanism of heavy metal is mainly chemical precipitation reaction.