Ternary Mixture Toxicity of 2,4-Dichlorophenoxyacetic Acid and Heavy Metals on Alkaline Phosphatase Activity in Desmodesmus abundans

Abstract

This study investigated the toxic effects of unary and ternary mixtures of heavy metals with 2,4-dichlorophenoxyacetic acid (2,4-D) on alkaline phosphatase activity in the freshwater microalga Desmodesmus abundans. The heavy metals assessed include Copper (Cu²⁺), Zinc (Zn²⁺), Lead (Pb²⁺), Chromium (Cr²⁺), and Cadmium (Cd²⁺). Toxicity assays were conducted using fixed ternary mixture ratios of 20:40:40 and 40:30:30 to evaluate the combinatory interactions. Results demonstrated that all individual metal ions exerted significant inhibitory effects on phosphatase activity in a concentration-dependent manner. Among the ternary mixtures at the 20:40:40 ratio, combinations containing 2,4-D/Pb²⁺/Cr²⁺, 2,4-D/Pb²⁺/Cd²⁺, and 2,4-D/Cr²⁺/Cd²⁺ exhibited the highest toxicity, with other mixtures following a descending toxicity pattern. For the 40:30:30 ratio, the most toxic mixtures were 2,4-D/Cr²⁺/Cd²⁺, 2,4-D/Pb²⁺/Cr²⁺, 2,4-D/Cu²⁺/Cr²⁺, and 2,4-D/Cu²⁺/Zn²⁺. Toxicity rankings further reflected varying intensities among the mixtures, with some displaying synergistic effects (2,4-D/Pb²⁺/Cr²⁺, 2,4-D/Cr²⁺/Cd²⁺), others additive (2,4-D/Cu²⁺/Cr²⁺, 2,4-D/Cu²⁺/Zn²⁺), and the majority antagonistic interactions. The toxic index analysis revealed predominately antagonistic behaviour in 20:40:40 ratio mixtures, except for specified synergistic and additive exceptions. Overall, increasing proportions of heavy metals in the mixtures intensified the toxic impact of 2,4-D. These findings highlight that mixtures of heavy metal ions with relatively low-toxicity pesticides such as 2,4-D can pose substantial ecotoxicological risks to freshwater algae, demonstrating complex interaction patterns dependent on mixture composition.