Phycoremediation of Chromium (VI) by Nitella and impact of calcium encrustation

Abstract

This article discusses the applicability of the Charophyte, Nitella pseudoflabellata in the remediation of Cr (VI) contaminated waters at different calcifying potentials. Its growth was found to be positively correlated with Ca in water (CaW), but marginally significant in the presence of Cr (VI) in water (CrW). High CaW resulted in calcite encrustation on the plant cell wall. CaW was found to be aiding Cr (VI) fixation in the long run, as this correlated positively with both CaW and CrW. However, Ca interfered with passive Cr (VI) accumulation in live plant matter at low CrW concentrations (≤0.2 mg/L). Biosorption by dead plant matter seemed to be the major mechanism as the dead plant organs contained >1 mg/g Cr dry weight of plant. Cr (VI) concentrations greater than 0.4 mg/L were too toxic, showing maximum quantum efficiency of PSII photochemistry (Fv/Fm) values < 0.63. The opposite was noticed (Fv/Fm > 0.76) when Cr (VI) was less than 0.2 mg/L. Elongation curve patterns based on shoot lengths showed similar scenarios. In all cases high CaW units with calcite encrustation found to be least affected by Cr (VI) toxicity. Optimum remediation was obtained using a combination of high Ca and Cr (VI) in the case of passive (short-term) operation and low Ca and Cr (VI) for active (long-term) operation. Under the passive scenario, plants accumulated above 1.2 mg/g Cr dry weight whereas in the active case, accumulation was 0.8 mg/g Cr dry weight. We conclude that Nitella-mediated Cr (VI) remediation is a promising technique within the range and conditions investigated.