Sorption-Desorption of Pyrene for Colombia and New Mexico Soils

Mechanistic insight into pyrene removal by natural sepiolites

This paper investigates the sorption characteristics and mechanisms of pyrene onto two types of natural sepiolite-brown (B-Sep) and white (W-Sep). The effects of relevant properties such as clay content, surface area, pore diameter and volume, divalent cations, and organic carbon content were investigated by single component batch adsorption systems. The results suggest that pyrene has high affinity for both sepiolite and its sorption behavior could be mainly affected by exchangeable strongly hydrated cations such as Ca²⁺ and H₂O in the zeolite-like channels and by open channel defects (OCD) structures but no so much by the large number of Si-OH groups located on the sepiolite's basal surfaces. Mesoporosity rather than surface area largely controls the sorption capacity and intensity of both sepiolites. This is shown by the increase in pore volume that exhibited the greatest increase in BET surface area. Particle size and morphological changes of both sepiolites following pyrene adsorption determined by FE-SEM showed that the sepiolite fibers are much longer than their widths, which are only several laths (several nanometers). This is a result of growth, mostly along the c-axis, at the expense of the diffusion of pyrene molecules through aqueous solution. As a consequence, a significant fibrous morphology is produced following the adsorption of pyrene by both sepiolites.

Preliminary evaluation of PAH sorptive changes in soil by Soxhlet extraction

This study was conducted to evaluate the influence of sorbent modification by synthetic, chemical/thermal weathering on the sorptive behavior of polycyclic aromatic hydrocarbons (PAHs). A clean sandy-clay-loam soil was subjected to Soxhlet extraction and PAH sorptive phenomena were evaluated based on quantity and quality changes in soil organic matter (SOM) and clay minerals. Critical changes in sorption capacity were found to depend on the initial PAH concentrations. Above 7 mg/l, weathering increased the PAH in comparison to that of unmodified soil, whereas it decreased when applied below this concentration. Similarly, less PAH was desorbed from the altered soil when PAH was applied above 7 mg/l. Therefore, when PAH was applied below 7 mg/l, quantitative reduction of sorbent amount (i.e., SOM and clay minerals) by soil weathering governed PAH sorptive behavior. However, when the PAH was applied above the critical limit, qualitative modifications in the sorbents facilitated an opposite trend. Sorbent swelling, removal of competing compounds, and possible changes in surface characteristics by Soxhlet extraction, together with increased concentration gradient effects were factors that resulted in dissimilar PAH sorptive phenomena, pivoting at the critical concentration.

Preliminary exploration of the relationships between soil characteristics and PAH desorption and biodegradation

Desorption and biodegradation of pyrene (PYR) were investigated and their relationships to soil characteristics were addressed. The results indicated that maximum achievable desorption was 30.2, 10.4, and 1.0 mg/kg for soils that had 1.7, 2.2, and 4.4 wt.% of expandable clays (smectite and vermiculite), respectively. Neither dissolved organic matter (DOM) nor total clay amounts made a good prediction of the desorption trend. Subsequently, the ease of desorption facilitated a faster aqueous biodegradation rate. The slowest aqueous biodegradation rate, 0.02 l/h, was achieved for the soil system that had the greatest amount of expandable clays, whereas the soil containing 1.7% expandable clays only achieved 0.73 l/h. The soil with 2.2% expandable clays depicted 0.41 l/h of aqueous biodegradation rate. A good linear correlation was obtained between maximum achievable desorption and aqueous biodegradation rate (R(2)=0.92). Soil analysis revealed that the total (soil+water) biodegradation reached was 65%, 78.3%, and 81.8% of the initial concentration (100 mg/kg) for the sandy clay loam (Colombian), sandy loam (Ohio), and silty loam (New Mexico) soils, respectively. This biodegradation extent was also in good agreement of expandable clay amount. Although aqueous PYR bioavailability was limited due to the strong association with the expandable clays, microbial movement and adhesion to those clays seemed to result in a great extent of the soil-phase biodegradation.

Biodegradability of aged pyrene and phenanthrene in a natural soil

A study was conducted to evaluate the biodegradability of pyrene (PYR) and phenanthrene (PHE) aged in a natural soil. Both the single and binary systems were either biostimulated via a nutrient amendment or bioaugmented via an inoculation of the enriched bacteria and nutrients. Aging resulted in higher concentration of both compounds and smaller bacterial activity in the solution-phase. Surprisingly, the total biodegraded extent was greater in the aged soil system than in the freshly spiked system. As anticipated, biostimulation was not appropriate to attain an effective biodegradation in this study, and bioaugmentation achieved a substantial increase the total biodegradation extent. The above findings were attributed to indigenous Pseudomonas aeruginosa entering a stationary-phase during the 200-day aging and producing rhamnolipid biosurfactants. In addition, a different sampling technique (i.e., after vigorous hand-shaking) revealed a 15 times higher microbial population than the normal sampling from the stagnant solution. Therefore, PAH bioavailability in the aged soils can be underestimated when the microbial activity is determined only from the stagnant solution. Furthermore, cometabolism enhanced PYR degradation when PHE was present as a primary substrate.