Integrated electrochemical and spectroscopic assessment of heavy metal bioaccumulation in mangrove plants: A sustainable strategy for environmental monitoring and risk mitigation

Abstract

• Development of an ANE/rGO/BiNPs needle sensor for real-time, in-situ metal detection. • Baseline assessment of heavy metal contamination in AHMR water and sediment. • Root accumulation in mangroves with highest levels of In, Ag, and Cd. • Seedling phytostabilization of Cd and Pb, and phytoextraction of Zn. • Sensor performance beyond ICP-OES with non-destructive monitoring of bioavailable metals. Mangrove forests play a vital role in coastal protection, biodiversity, and pollution mitigation, but they are increasingly threatened by heavy metal uptake associated with industrialization. However, real-time, in-situ, and non-destructive methods for monitoring metal bioavailability within mangrove plants remain unavailable. This study develops and tests an acupuncture Needle Electrode modified with reduced graphene oxide and bismuth nanoparticles (ANE/rGO/BiNPs) for real-time heavy metal detection in Alhamriyah Mangrove Reserve (AHMR). To establish baseline contamination levels for a range of elements, water and sediment samples were collected from three sites within the reserve and analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). The chemical oxygen demand (COD) was high (> 11.0 mg/L), and the sludge was quite polluted with Cd, In, Ag, and Bi as the contamination factor (C f > 10). Analysis of a mature mangrove tree showed highest metal accumulation in roots, with contamination factors: In (870.460) > Ag (636.386) > Cd (416.588) > Bi (248.918) > Tl (93.144), suggesting industrial sources and highlight the need for monitoring non-traditional metals like Cd. Mangrove seedlings were irrigated with varying levels of Cd, Pb, and Zn, showing dose-dependent metal accumulation. Roots primarily accumulated Cd and Pb, indicating phytostabilization potential, while Zn's higher mobility suggests suitability for phytoextraction. The ANE/rGO/BiNPs sensor was inserted into living mangrove tissues to detect metal uptake using Square Wave Voltammetry (SWV). After four weeks, electrochemical readings showed high accumulation levels (Cd: 5182.29 µM, Pb: 5076.25 µM, Zn: 41,998.50 µM) under high treatment. Compared to much lower ICP-OES values (Cd < 150, Zn < 3, Pb < 5), electrochemical contamination factors were significantly higher, demonstrating the sensor’s superior sensitivity and effectiveness for non-destructive, real-time monitoring of bioavailable metal uptake in mangroves. This novel approach highlights the potential of needle-based sensors as practical tools for in-situ biomonitoring and environmental risk assessment in vulnerable coastal ecosystems.