Phytoremediation potential of duckweed (Lemna minor L.) for hexavalent chromium removal in synthetic wastewater: unveiling physiological response and defense mechanisms against excessive heavy metal uptake

Heavy metals pose a significant threat as environmental pollutants, known for their high toxicity. Traditional physiochemical methods for their removal are costly and energy-intensive, making bioremediation a promising alternative. This study aims to assess the potential of duckweed (Lemna minor L.) in removing hexavalent Chromium ((Cr (VI)) from wastewater and examines the impact of Cr (VI) toxicity on the plant's metabolism in aquatic conditions. The experiments were conducted at least three replicates in completely randomized blocks. Various parameters, including Cr concentration (0-100 mg L-1), pH levels (5, 7 and 9), and treatment duration (24-168 h) were investigated to optimize phytoremediation conditions. Physiological and biochemical parameters such as content of photosynthetic pigments, MDA, total phenolics and flavonoid compounds, reducing sugar and polysaccharides, proline and lignin were measured. Additionally, chromium accumulation in plant tissues and residual levels in the culture media were determined using inductively coupled plasma (ICP). After 24 and 168 h, chromium adsorption by L. minor resulted in decreased levels of Cr (VI) in the media. Determination of bioaccumulation factor (BCF) after 168 h indicate higher efficiency of the plant at moderate Cr (VI) concentrations. At higher concentrations of Cr, salinity, EC and TDS increased in the media across all pH condition. The finding revealed exposure to Cr (VI) led to a notable increase in the content of H2O2 (1.2- to 1.7-fold) and MDA (2.2- to 5-fold) in L. minor tissues, particularly evident at pH 9. The contents of total phenolics, flavonoids, lignin and proline (at pH 5 and 7), reducing sugars (at pH 7 and 9) increased; while polysaccharide content was fluctuated. Also, content of photosynthetic pigments decreased by approximately 50% under treatment with 50 and 100 mg L-1 Cr (VI) at pH 5 condition. These responses indicating ROS accumulation as a signal for plant defense mechanism activation. Our results showed that L. minor is tolerant up to 50 mg L-1 Cr (VI), but is susceptible to higher concentrations of this heavy metal. Despite its susceptibility, L. minor proves effective in absorbing Cr (VI) from contaminated waters, highlighting its potential for phytoremediation, albeit requiring careful management for optimal results.