| Summary: | There is need to explore all possible agro-based inexpensive adsorbents and study their feasibility for heavy metal removal with the aim of finding which is a better alternative to activated carbon. The focus of the present study is on investigating the adsorption characteristics of Delonix regia for the sorption of Pb, Ni, Cu, Cd and Co from polluted water and stabilization of contaminated soils. The focus also included assessment of available metal concentrations, pool size and kinetic resupply using Diffusive Gradient in Thin film (DGT) technique and DGT- Induced Fluxes in Sediments (DIFS) model. Batch adsorption experiments were carried out using the adsorbent prepared from the pods and leaves of Delonix regia in order to determine the suitable operating parameters for the adsorption of the target heavy metals from synthetic metal solutions. Results showed significant uptake occurred at pH 2 which makes the adsorbent suitable for use on low pH heavy metal bearing effluents. An interaction time of 30 minutes was sufficient to achieve maximum uptake. The kinetics of adsorption best fitted to the pseudo second order kinetic model while Langmuir and Freundlich isotherms were used to interpret data obtained from initial concentration study. The experimental adsorption capacity of the pods is 31.2mgPb/g; 5.9mgNi/g; 9.1mgCu/g; 6.5mgCd/g and 5.8mgCo/g while the leaves powder has 10.3mgPb/g; 10.3mgNi/g; 9.2mgCu/g; 8.4mgCd/g and 10.3mgCo/g. The investigation on the sorption ability of Delonix regia continued by using the pods on tropical soil samples to study the effect of aging; pollutants concentration difference; soil pH and soil organic matter on the removal efficiency of Delonix regia pods and biochar for Pb, Cu and Cd during the 42 days aging time. Results showed that removal efficiency was dependent on aging and soil pH for both materials and all metals studied except for Cd where soil pH had no effect. Delonix regia could be used to remove these target metals from organic matter rich agricultural soil while the biochar with higher removal efficiency in most soil samples is better suitable for acidic and poor organic matter soils. Further investigation for availability and resupply kinetic assessment before and after remediation was done using the DGT technique and DIFS model. The use of Delonix.regia and biochar resulted in a reduction in Css and CDGT for all metals in the order of Biochar-treated soils < D. regia treated soils< control samples; an evidence of the good remediation that was achieved by each treatment. The labile pool size of metals, Kd, (Pb, Cu and Cd) that can be resupplied to soil solution from the soil solid phase follows the order of Bio-treated soils > Delonix.regia treated soils > control soil samples. The depletion time (Tc) and the desorption rate constant, k-1, values followed the order: control samples > Delonix.regia treated soil > biochar treated soils. The Kd, labile pool size is increasing with aging time. In control samples, the Tc decreased with aging; while in treated soils, Tc increased with aging and D.regia have the higher value. In the soils to which organic matter, OM, were added, Tc values were the same for each metal throughout the aging period. The values obtained were the same for individual soil with and without treatment but vary from metal to metal. In the OM soils, treatment types and aging have no effect on metal resupply. The resupply of Pb is the fastest, then Cd and the slowest was Cu. The R values in the control soils were reducing with aging but in the treated soils, it was increasing with the resupply slower in D.regia treated soils than biochar treated soils. This work has demonstrated that D.regia has the potential for simple, cheap and effective/efficient remediation of metal polluted waters and soils.
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