Phytoavailability and Mobility of Heavy Metals in Oil Refinery-Polluted Soil Amended with Olive Waste Biochar and Maize (Zea mays L.) Cultivation

The accumulation of heavy metals in the soil surrounding oil refineries is a major environmental and agricultural issue, demanding effective and environmentally friendly cleanup methods. This study was conducted to determine the effectiveness of olive waste biochar (OWB) in reducing the bioavailability of these heavy metals, ultimately assessing its potential as a sustainable remediation strategy for polluted environments, particularly in soils near an oil refinery. A 75-day pot experiment was conducted using maize (Zea mays L.) to evaluate heavy metal uptake, as this duration provides for adequate root development and translocation mechanisms, the experiment was conducted through a randomised complete block design after olive waste biochar preparation by pyrolysis at 500 to 600°C (Setting the pyrolysis temperature at 500-600 °C due to this range is widely recognized as optimal for producing high-quality biochar that is suitable for environmental remediation). In addition, OWB was added to soil at the rate 0% (control), 2.5%, 5%, 7.5%, and 10% (w/w). Heavy metal concentrations in both soil and maize tissues were quantified using inductively coupled plasma optical emission spectrometry (ICP-OES). The use of olive waste biochar at different application rates successfully (P < 0.05) affected soil phytoavailability and immobilization of all studied heavy metals. The results showed inverse relationships in terms of heavy metal accumulation in plant tissues. When Zn being most taken up, followed by Cr and Ni and finally Fe, with reductions of 62.68%, 59.1%, 46.15%, and 32.26%, respectively. Heavy metal immobilisation in soil exhibited different patterns, with a tendency of Zn > Cr > Ni > Fe. The optimal reduction in metal uptake and highest immobilization occurred at the 10% OWB application level. The findings have proven that OWB functions effectively as an environmental sustainability measure for handling heavy metal contamination in soil.