“Kinetics of Arsenate Adsorption – Desorption in Solis”

Sangyul Kang

Review

• There have been two major types of arsenic in nature; As (Ⅴ) under aerobic conditions, As (Ⅲ) under anaerobic conditions.

• Fe and Al oxides hydroxides have a high affinity to arsenic.

• The objectives

1) To quantify the kinetics of adsorption and desorption of arsenate in three soils having different physiochemical properties

2) To assess multi-reaction (equilibrium-kinetic) modeling for its capabil-ity of describing the retention as well as the desorption or release behavior of As(V) in different soils.

Introduction

Materials and Methods

• Materials

1)Surface soils from the Ap horizon (0-10 cm) of Olivier loam 2)Sharkey clay 3)Windsor sand

• Method

- Batch experiments were initiated by mixing 3 g of air-dry soil with 30 mL of As(V) solution in a 40-mL Teflon tube.

- Test were performed in triplicate.

- shake mixture at 150 rpm on a reciprocal shaker centrifuged for 10 min at 4000 rpm after each specified reaction time.

- A 1-mL aliquot was sampled from the supernatant at reaction times of 2, 6, 12, 24, 72, 168, 336,and 504 h.

Results and Discussion

Nonlinear sorption Isotherms

• Nonlinear behavior• Nonlinear or concentration – dependent adsorption behavior is• characterized by the low values of the Freundlich parameter N.• N is a measure of the extent of heterogeneity of the sorption sites

having different affinities for solute retention by matrix surfaces.• The N values for all three soils did not vary significantly after 24 h of

reaction.• Low N values may be indicatives of extensive heterogeneity of sorp-

tion sites in these soils.

Adsorption kinetics

• Rapid --> Gradual or slow reactions --> continued As(V) retention with steady decrease of As(V) concentrations with reaction time

• Results concur with the nonlinear forms of the isotherms.

• These findings are in agreement with the biphasic arsenic adsorp-tion behavior observed on several soil minerals

Desorption Hysteresis and Binding Phases.

• Distinct discrepancies between adsorption and successive desorp-tion isotherms clearly indicate considerable hysteresis for As(V) re-lease.

• This observed desorption hysteresis might be due to kinetic reten-tion behavior( 동역학적 체류현상 ), such as slow diffusion (21) and ir-reversible retention

Multireation Modeling

• A major implication of this study is that contamination of soils with high concentrations of arsenic could result in slow release for ex-tended period of time, that is, weeks or months

• A secondary implication is that describing retention (adsorption) and subsequent release or leaching (desorption) by use of a mul-tiple reaction model of the equilibrium-kinetic-irreversible nonlin-ear MRM type provided good overall predictions of the fate of ar-senic in the soils examined in this study.

Thank you