Biochar Switch Flips Weed’s Cleanup Role in Oil-Contaminated Soil

A common tropical weed that most farmers would happily pull up and discard may be one of the most versatile tools available for cleaning soils poisoned by spent engine oil, according to new research from Nigeria. The study, published in BMC Environmental Science, shows that the humble addition of biochar, a charcoal-like material made by heating crop residues in low oxygen, can dramatically reshape how the Mexican sunflower, Tithonia diversifolia, handles heavy metals in contaminated ground. In some cases, the biochar turned the plant from a metal remover into a metal locker. In others, it did the exact opposite, unlocking a cleanup ability the plant did not previously display.

The research team, led by Olamide Omolafe Ogunremi of the University of Ilorin and North-West University, set out to answer a deceptively simple question: does adding biochar to oil-polluted soil help or hinder a plant’s natural capacity to deal with heavy metals? Spent engine oil is a widespread pollutant in many developing regions, released from garages, workshops and informal dumping sites. Beyond the hydrocarbons themselves, used oil carries a cocktail of toxic metals including copper, zinc, lead, cadmium and chromium, all of which can persist in soil for decades and find their way into food crops and groundwater.

Tithonia diversifolia, a shrubby member of the daisy family native to Mexico but now naturalized in more than 60 countries, was an intriguing candidate for the experiment. In Nigeria it grows abundantly on farms, roadsides and waste grounds, often on soils too poor for crops. Previous studies had flagged the species as a promising phytoextractor, meaning it can pull metals out of the soil and store them in its tissues. Its invasive vigor, rapid biomass production and extensive root system all suggested it could thrive where other plants would simply die. Crucially, because it is not a food crop, any metals it absorbs pose no direct risk to the human food chain.

The team also suspected that the plant’s medicinal chemistry might play a role. Tithonia diversifolia produces distinctive secondary metabolites known as targitinins, and other medicinal plants have been shown to tolerate heavy metals by chelating them, forming stable complexes that neutralize the metal ions. That built-in tolerance, the researchers reasoned, could make the species especially well suited to surviving the harsh chemistry of an oil-spiked soil.

To test the idea, the researchers ran a greenhouse experiment at the University of Ilorin’s Botanical Garden. Sandy loam soil was sieved, homogenized and spiked with spent engine oil at a rate of 50 milliliters per kilogram, roughly 5 percent by volume, then left to equilibrate for two weeks. Three biochars were then mixed in at four application rates: zero, 1, 2 and 3 percent by weight. Two of the biochars were produced locally, one from rice straw and one from sorghum straw, while the third was a standard reference biochar made from wheat straw at 700 degrees Celsius, supplied by the UK Biochar Research Centre. After a month of incubation, Tithonia seeds were sown in 30 pots and the plants were grown for six weeks under a completely randomized design with three replicates.

The growth results were striking. Rice straw biochar in particular acted as a growth booster. Plants receiving 3 percent rice biochar reached the greatest height recorded in the study, 13.36 centimeters, and also showed the thickest stems. Leaf counts and leaf areas were significantly higher in rice biochar treatments than in untreated controls, and plants given rice biochar produced significantly greater shoot and root fresh weights and shoot dry weight. Sorghum biochar at 2 percent produced the longest roots, measuring 9.73 centimeters on average. By contrast, the standard wheat straw biochar tended to suppress height and stem girth as its application rate increased, a reminder that biochars are not interchangeable commodities but chemically distinct materials shaped by what they are made from and how they are burned.

The heart of the study, however, lay in how the biochars changed the plant’s relationship with metals. The researchers quantified this using two standard indices. The bioconcentration factor, or BCF, compares the metal concentration in the plant’s roots with that in the soil; a value above 1 signals a hyperaccumulator. The translocation factor, or TF, compares metal levels in shoots with those in roots; values above 1 indicate the plant moves metals upward into its above-ground parts, which is the hallmark of phytoextraction, while values below 1 indicate the plant traps metals below ground, the signature of phytostabilization.

Those indices revealed a remarkable plasticity. In untreated contaminated soil, Tithonia behaved as a phytostabilizer for copper, keeping it locked in roots, but it readily extracted lead, cadmium and chromium, with translocation factors well above 1 for all three. When sorghum biochar was added, however, the plant’s copper behavior flipped entirely: at every sorghum biochar rate, translocation factors for copper exceeded 1, converting a copper-storing plant into a copper-extracting one. Meanwhile, the standard biochar at 1 and 2 percent achieved the opposite effect for zinc, producing bioconcentration factors of 1.23 and 1.30 alongside translocation factors of essentially zero, meaning zinc was absorbed by the roots but firmly prevented from reaching the shoots. The same standard biochar at 2 percent also curbed the plant’s uptake and transport of cadmium, lead and chromium, damping down what had been vigorous extraction in the untreated soil.

The mechanisms behind these shifts are rooted in the physical chemistry of biochar itself. Earlier characterization work by the same team showed that the rice and sorghum biochars possess high pH, high porosity, abundant fixed carbon and rich surface functional groups. Biochar immobilizes metals through several parallel processes: negatively charged surface sites attract positively charged metal ions electrostatically; oxygen-containing functional groups can reduce chromium to less mobile forms that then bind to the surface; alkaline biochar raises soil pH, causing metals such as lead to precipitate, often reacting with phosphate and carbonate groups in the char; and ion exchange and complexation sequester additional metal ions. The porous structure also creates habitat for soil microbes, whose metabolic activity and organic acid secretions further interact with metals, and the dark color of biochar alters soil thermal dynamics, aiding germination and early growth.

The authors conclude that the phytoextraction and phytostabilization capacities of Tithonia diversifolia depend on the specific features of the biochar applied, which are in turn governed by the feedstock, the pollutant and the soil type. That dependency cuts both ways: biochar can be chosen to suppress metal movement into edible vegetation, or to supercharge a plant’s extraction of a particular contaminant. The team calls for long-term field trials and monitoring strategies across varied biochar types and plant species, and stresses that producing standardized, quality-controlled biochar should be a priority if the approach is to move from greenhouse pots to real-world remediation. If those trials succeed, a weed that farmers spend seasons eradicating could become a deliberately planted partner in restoring some of the world’s most damaged soils.

Subject of Research: Biochar-assisted phytoremediation of heavy metals in spent oil-contaminated soil using Tithonia diversifolia

Article Title: Efficacy of biochar on the phytoremediation potential of Tithonia diversifolia on spent oil-contaminated soil

Article References: Ogunremi, O. O., Amubieya, O. F., Ogunkunle, C. O., & Fatoba, P. O. (2025). Efficacy of biochar on the phytoremediation potential of Tithonia diversifolia on spent oil-contaminated soil. BMC Environmental Science, 2(1), Article 12. https://doi.org/10.1186/s44329-025-00026-6

Image Credits: AI Generated

DOI: 10.1186/s44329-025-00026-6

Keywords: biochar, phytoremediation, Tithonia diversifolia, heavy metals, spent engine oil, soil contamination, phytoextraction, phytostabilization, rice straw biochar, sorghum straw biochar, soil remediation, greenhouse experiment