TOXICOLOGICAL EVALUATION OF LEAD SIMULATION IN SOIL AND MITIGATION BY TITHONIA DIVERSIFOLIA BIOCHAR (TITHOCHAR)

Lead (Pb) is classified as a major pollutant in the environment arising from its natural occurrence aggravated by weathering process, exhaust from automobiles, battery waste, industrialization and agrochemical use. It is also the most reported heavy metal in the food chain. Intensive and indiscriminate use of agrochemicals (chemical fertilizers and pesticides) among local farmers therefore calls for an investigation into the fate of soil impacted by lead. The objectives of this work were to assess the concentrations and toxicity of selected heavy metal in lead simulated soil, leachate quality, earthworm and okra performance and determine the heavy metal mitigation potentials of two tithonia biochar types in lead impacted soil. The biochar studied were prepared from Tithonia diversifolia (TD) with (tagged TDBMB) or without bone meal (tagged TDB). The experimental soil was from the upper arable section of the Teaching and Research Farm LAUTECH The experiment was a factorial combination of two soil conditions (with heavy metal (WHM) and no heavy metal (NHM)), and two biochar types (TDB and TDBMB) each applied at 2.5 t/ha (equivalent to 3.13 g) and 5.0 t/ha (equivalent to 6.25 g) per 2.5 kg soil/column. The experiment was laid out in completely randomized design with three replications. The soil was moistened to field capacity using 530 ml of tap water. Lead stock solution (800 ppm) from Lead II acetate trihydrate ((CH3COO)2Pb.3H2O) was prepared by dissolving 0.73 g of Pb salt in 500 ml of distilled water in 500 ml volumetric flask. The lead stock solution was applied at 20 ml/column into all the WHM columns and basal NPK 15:15:15 was applied into all columns. One-time leaching simulation was thereafter done with 200 ml of tap water followed by okra sowing and nurturing for 3 weeks. Earthworms were collected from their natural habitat and acclimatized for 4 weeks. Prior to the evaluation, three earthworms were introduced into each column after their initial weights have been taken. The set-up was kept moist and covered with foil paper for three weeks. After the expiration of three weeks, the final earthworms’ weight were taken to adjudge their survival performance in the Pb impacted soil. Data were collected on soil (pH, Pb, Cd, available phosphorus and organic carbon), water (pH, EC, Pb and Cd contents), plant (fresh biomass weight, Pb and Cd contents) and earthworm (initial and final weight, percentage weight gain/loss and number of mortality). All the data collected were subjected to two-way analysis of variance using Genstat statistical tool and significant means were separated using LSD at 5% probability level. There was exceptionally high C, N, K and EC contents in the TDB. Lead simulation significantly increased Pb content by 50%, reduced available P and okra biomass weight by 5.8 and 83.3% respectively, and intensified earthworm weight loss by 26.6% compared to AC. Biochar pretreatment in Pb impacted soil however reduced lead soil content by 33.3%, increased SOC by 150 (in NHM by TDB) and 40% (in WHM by TDB), increased available P by 309% (in NHM by TDBMB) and 302% (in WHM by TDBMB), increased okra biomass weight by 464% (in NHM by TDB) and 650% (in WHM by TDBM) and reduced earthworm weight loss by 57.2% (in NHM by TDBMB) and 26.6% (in WHM by TDB).