Modeling the Effects of Salinity and pH on the Cadmium Bioabsorptive Properties of the MicroalgaeIsochrysis galbana(T-Iso) in Coastal Waters

Abstract

The changes in cadmium ion bioabsorptive properties were investigated for the brown microalgae Isochrysis galbana (T-Iso) in various saline solutions (0, 10, 20, 30, 40, and 50 parts per thousand, or ppt) and pH environments (pH 3, 4, 5, 6, 7, and 8) modeling those of coastal and intertidal waters. Optimal saline growth conditions for T-Iso were observed at 30 ppt. Under these optimal saline conditions, the effective concentration at 50% lethality for cadmium ion for T-Iso is 2.3 parts per million (ppm), with a maximum intracellular absorption of 8.6 fg/cell. T-Iso maximal surfacial cadmium binding was determined from Langmuir isotherm plots; Qmax = 98 mg/g (pH 6.00, 0 ppt), falling to 19 mg/g in higher salinities (pH 6.00, 50 ppt). The Freundlich constants n and Kf followed a similar cadmium binding trend: at 0 ppt, Kf = 16.6 and n = 2.83, while at higher salinities (50 ppt), the values dramatically decreased to Kf = 3.64 and n = 1.34. Direct relationships between ln Qmax (98, 70, 45, 28, 23, and 19) and ln Kf (16.6, 12.8, 8.36, 4.63, and 3.64) versus respective salinity (0, 10, 20, 30, 40, and 50 ppt) produced linear correlations. The pseudo-second-order binding kinetic rate in distilled water is 8.5 × 10−3 g/mg/min, while in saline conditions (30 ppt) the rate decreased to 4.4 × 10−3 g/mg/min. The percentage of adsorption loading capacity of the algae in the water column increases with initial cadmium exposure: the highest percentage of adsorbance (C0 = 25 ppm) in 0 ppt is 92%, while under equivalent conditions in saline water (50 ppt), adsorption falls to 31%.