Aging shapes Cr(VI) speciation in five different soils.

To make sound decisions regarding management of heavy metal contamination in soils, it is necessary to understand contaminant transformations over extended periods. In this study, sequential extraction methods were applied to quantify the changes of Cr fractions [available Cr(VI), immobile Cr(VI) and immobile Cr(III)] in five contrasting soils spiked with Cr(VI) over a 240-day incubation. Results showed that available Cr(VI) in soils continually decreased during aging, with a sharp decline occurring in the first 30 days. The best fit of available Cr(VI) data was obtained using an Elovich model for Brunisol and Anthrosol-1, a fractional power model for Anthrosol-2, and a pseudo first-order kinetic model for Luvisol-1 and Luvisol-2. After aging for 240 days, immobile Cr(VI) increased by 4.5-31% and immobile Cr(III) increased by 68-95% of total spiked Cr(VI) in Brunisol, Anthrosol-1 and Anthrosol-2. The two Luvisol soils had relatively high reduction rates with no Cr(VI) immobilized. A multireaction model was developed in MATLAB Simulink toolbox to describe transformation flow rates among soluble Cr(VI), adsorbed Cr(VI), immobilized Cr(VI) and immobilized Cr(III) in soils with aging. We conclude that (i) Cr(VI) reduction and immobilization were occurring concurrently in soils and competing for available Cr(VI) species; (ii) Cr(VI) reduction is favored by low soil pH and high organic carbon, while Cr(VI) immobilization occurs with cations (such as Ca2+) and Fe oxides.

Impact of pyrolysis temperature and activation on oily sludge-derived char for Pb(II) and Cd(II) removal from aqueous solution.

This study investigated the Pb(II) and Cd(II) sorption from aqueous solution by oily sludge-derived char (OSDC) prepared at different pyrolysis temperatures and chemical activation. The maximum Pb(II) sorption capacity for OSDC at pyrolysis temperature of 500 °C (OS500) was found as 351.48 mg/g, which was greater than that of OSDC produced at other temperatures. Post-sorption characterizations using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and sequential extraction test indicated the precipitation was the principal mechanism of sorption of Pb(II) onto OSDC prepared at low pyrolysis temperature (≤ 500 °C). The Pb(II) sorption capacity dramatically decreased as the pyrolysis increased from 500 to 900 °C. This is because the alkaline minerals decomposed at high temperature (≥ 700 °C), thereby the mineral precipitation with Pb(II) was inhibited. With increasing pyrolysis temperature from 500 to 900 °C, the contribution of precipitation to Pb(II) sorption decreased from 93.79 to 34.63%, while the contribution of complexation increased from 0 to 44.68%. The sorption capacity of Cd(II) was less than that of Pb(II), and no precipitation was formed during Cd(II) sorption by OSDC. Sorption results showed that OSDC prepared at pyrolysis temperature of 700 °C (OS700) had the best sorption capacity for Cd(II) (92.14 mg/g). The high sorption capacity of OS700 for Cd(II) was mainly attributed to the carboxyl/hydroxyl functional groups and complexation with mineral oxides. The hydrothermal treatment (8M NaOH solution) activated OS500 significantly with respect to surface area, cation exchange capacity (CEC), and total pore volume (PV). The substitution of Pb(II) with alkaline earth metals led to precipitation in the form of hydrocerussite (Pb3(CO3)2(OH)2) that was mainly responsible for Pb(II) sorption on activated OS500 (accounted for 93.79%). The activated OS500 showed a higher sorption capacity (90.06 mg/g) for Cd(II) than OS500 (23.95 mg/g) because the conversion of barite (BaSO4) to witherite (BaCO3) after chemical activation favored the precipitation of Cd carbonate. The contributions of precipitation to the total Cd(II) removal was 0% for OS500 but 76.12% for the activated OS500.

Formation and Immobilization of Cr(VI) Species in Long-Term Tannery Waste Contaminated Soils.

Chromium speciation in naturally contaminated soils appears more complex than spiked studies have shown. This study characterized Cr speciation (oxidation states; availability; molecular geometry) intended to highlight the genesis of immobile Cr(VI) species in long-term tannery waste-contaminated soils. In a series of samples obtained from Shuitou in China, chemical extraction methods showed that Cr(III) was dominant(>96.7% of total Cr), with Cr(VI) concentration up to 144 mg kg-1. Of the total Cr(VI) present, immobile Cr(VI) represents >90%. Synchrotron-based X-ray near-edge structure spectroscopy (XANES) showed the occurrence of Cr(VI), which was not removed by phosphate buffer extraction, confirming a significant amount of immobile Cr(VI) fractions in soils. X-ray fluorescence maps exhibited the heterogeneous distribution of Cr in soils associated with both Mn and Fe. Such a distribution suggests Cr(III) oxidation to Cr(VI) by Mn oxides and a possible immobilization of both Cr(III) and Cr(VI) onto Fe (hydr)oxides. Linear combination fitting of XANES spectra revealed that fractional weights (%) in samples were CrFeO3 (49.3-53.6), CrOOH (22.3-30.8), and CaCrO4 (13.2-25.3). Our results demonstrate that (i) Cr(VI) is immobilized in soils and (ii) mechanisms of Cr(VI) immobilization are CaCrO4 precipitation and recrystallization with Fe (hydr)oxides.

Co-pyrolysis of metal contaminated oily waste for oil recovery and heavy metal immobilization.

The treatment of metal-contaminated oily waste was investigated by using an approach of co-pyrolysis with hog fuel in a fixed bed reactor. Both oil recovery and heavy metal immobilization were evaluated. Three experimental factors, including pyrolysis temperature, reaction time, and hog fuel addition in the feedstock, were examined to find their effects on the treatment performance. Metal immobilization in the solid char obtained from co-pyrolysis was examined from its speciation characteristics. A higher pyrolysis temperature led to a decreased oil recovery but more non-bioavailable metal species in the char residue. This is also evident from the calculated risk index (RI) for eco-toxicity assessment, with RI of 34.6 and 117.1 at 600 °C and 400 °C, respectively. The addition of hog fuel into oily waste for co-pyrolysis also led to increased metal immobilization but a decreased overall oil recovery. A low temperature co-pyrolysis (400 °C) by adding 20% of hog fuel was found as the most effective oily waste disposal strategy with satisfactory oil recovery and an acceptable risk index of 54.1.

Elemental sulfur amendment decreases bio-available Cr-VI in soils impacted by leather tanneries.

This study investigated the potential use of elemental S (S(0)) to convert Cr-VI to Cr-III which should decrease the bio-availability hence, toxicity of Cr-VI in soils. The bio-available fraction of Cr in soil was measured by phosphate buffer extraction (PBE) and the results showed that the fraction is about 10% of the total Cr-VI and varied from 12.8 to 42.5 mg kg(-1). The addition of 4.0 mg g(-1) S(0) decreased PBE Cr-VI to <0.4 mg kg(-1) limit established for Cr-VI toxicity in soils. Synchrotron-based X-ray absorption near-edge structure (XANES), X-ray fluorescence (XRF) and micro-XRD revealed that Cr-III was the dominant species (99% of total Cr) and Cr was retained by hematite and goethite in soil. Fe-containing minerals may have provided sufficient protection to render the dominant Cr-III species biochemically inert to redox processes in soils. It is concluded that S(0)amendment is a promising approach to remediate Cr-VI contaminated soils.