The effect of ultrasound on the conventional mechanical soil-washing process was investigated. To determine the optimal frequency for maximum efficiency, tests were conducted with aluminum foils under four frequencies including 35, 72, 110, and 170 kHz. It is known that the physical effects generated during acoustic cavitation damage the foil by causing pits and holes. The sonication at 35 kHz resulted in maximum damage to the aluminum foil as compared to that observed at other frequencies. Based on these results, 35 kHz was selected for the ultrasonic soil-washing processes in this study. The optimal washing time was found to be 1 min, because there was no significant increase in the removal efficiency over 1 min for the three processes, mechanical, ultrasonic, and combined ultrasonic-mechanical. It was also found that the combined process enhanced the performance of the soil-washing process significantly as compared to other two processes in terms of (i) diesel removal efficiency, (ii) process time, (iii) consumption of electric energy, and (iv) production of washing leachate. The efficiency of washing under ultrasonic processing conditions was similar to that observed with mechanical washing in the presence of small amounts of sodium dodecyl sulfate (SDS), suggesting that the ultrasonic washing process does not require external chemicals and can be considered as a "green" process.
This study examines the adsorption isotherms, kinetics and mechanisms of Pb²(+) sorption onto waste cow bone powder (WCBP) surfaces. The concentrations of Pb²(+) in the study range from 10 to 90 mg/L. Although the sorption data follow the Langmuir and Freundlich isotherm, a detailed examination reveals that surface sorption or complexation and co-precipitation are the most important mechanisms, along with possibly ion exchange and solid diffusion also contributing to the overall sorption process. The co-precipitation of Pb²(+) with the calcium hydroxyapatite (Ca-HAP) is implied by significant changes in Ca²(+) and PO₄³⁻ concentrations during the metal sorption processes. The Pb²(+) sorption onto the WCBP surface by metal complexation with surface functional groups such as ≡ POH. The major metal surface species are likely to be ≡ POPb(+). The sorption isotherm results indicated that Pb²(+) sorption onto the Langmuir and Freundlich constant q(max) and K( F ) is 9.52 and 8.18 mg g⁻¹, respectively. Sorption kinetics results indicated that Pb²(+) sorption onto WCBP was pseudo-second-order rate constants K₂ was 1.12 g mg⁻¹ h⁻¹. The main mechanism is adsorption or surface complexation (≡POPb(+): 61.6%), co-precipitation or ion exchange [Ca₃(.)₉₃ Pb₁(.)₀₇ (PO₄)₃ (OH): 21.4%] and other precipitation [Pb 50 mg L⁻¹ and natural pH: 17%). Sorption isotherms showed that WCBP has a much higher Pb²(+) removal rate in an aqueous solution; the greater capability of WCBP to remove aqueous Pb²(+) indicates its potential as another promising way to remediate Pb²(+)-contaminated media.
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