Tolerance and phytoremediation potential of four tropical grass species to land-applied drill cuttings

Petroleum hydrocarbons reduction by selected tropical grass species in oil-based drill cuttings contaminated soil

The potential of phytoremediation of oil-based drill cuttings (OBDCs) contaminated soil was assessed by measuring the level of petroleum hydrocarbons reduction. The contamination experiment was simulated in a randomized complete block design by factorial of 6 × 3 × 2 × 2 for grass species (Pennisitum purpureum, Panicum maximum, Andropogon gayanus, Heteropogon contortus, Axonopus compressus, and Chloris virgata), drill cuttings treatments (0%, 25%, and 50% OBDCs contamination), time (0 day of planting and 105 days of harvesting), and growth stage (young and mature). The parameters assessed were total petroleum hydrocarbons (TPHs) in soils, roots, and shoots; bioconcentration factor in roots and shoots; and translocation factor. The TPH reductions achieved in 25% treatment level were young A. compressus (58.01%), mature P. purpureum, young A. gayanus and C. virgata (44.24%), young P. purpureum (27.67%), mature A. compressus (25.29%), mature H. contortus (2.56%), mature P. maximum (4.01%), and unplanted soils (2.10%). Thus young A. compressus, A. gayanus, C. virgata, and mature P. purpureum are recommended for TPH reduction in 25% OBDCs contaminated soils. Young P. purpureum and mature A. compressus can be used to achieve 25% - 27% TPH reduction.

Low-temperature thermal desorption and secure landfill for oil-based drill cuttings management: Pollution control, human health risk, and probabilistic cost assessment

Oil-based drill cuttings (OBDCs) were managed in two scenarios including low-temperature thermal desorption (LTTD) and secure landfill through a case study. The removal of polycyclic aromatic hydrocarbons (PAHs) and heavy metals in OBDCs by LTTD under different conditions was investigated. Probabilistic human health risk assessment was performed to quantify the health risk posed to waste management workers under the two scenarios, while the associated costs were also analyzed. The results show that LTTD at 300 °C for >20 min could remove 96.27% of PAHs in OBDCs but its removal effect on heavy metals was not significant. It was found that cancer risks posed by PAHs in both securely landfilled and LTTD-treated OBDCs were not significant (<1e-06); however, significant cancer risks (7.95e-05-9.45e-05) were identified for exposure to toxic heavy metals. Increased health risk was observed as a result of exposure to LTTD treatment residues compared to securely landfilled OBDCs. Inhalation of chromium(VI) and oral ingestion of arsenic in OBDCs were critical exposure routes. Both cancer and non-cancer risks in the secure landfill scenario were negligible. The cost analysis results suggest that LTTD combined with stabilization/solidification could be more economically attractive than secure landfill for the handling of OBDCs.

Life cycle assessment of low-temperature thermal desorption-based technologies for drill cuttings treatment

The life cycle impacts of treatment of typical oil-based drill cuttings (OBDCs) using three low-temperature thermal desorption (LTTD)-based systems, including thermomechanical cuttings cleaner (TCC), screw-type dryer (STD), and rotary drum dryer (RDD), were explored with a case study in British Columbia, Canada. Two energy supply scenarios, including diesel generator-based onsite (scenario i) and hydropower-based offsite (scenario ii) treatments, were considered in the assessment. The results show that RDD generated the lowest life cycle impacts in terms of damages to human health, ecosystems, and resources in scenario i. TCC-scenario ii generated the lowest impacts among all assessed cases, suggesting that using renewable energy can greatly reduce the impacts of LTTD-based OBDCs treatment. Also, net environmental benefits could be achieved considering the reuse of recovered oil, and the highest net environmental benefits were obtained in TCC-scenario ii. The process contribution analysis found that thermal desorption process accounted for 80-95 % of impacts in almost all impact categories. Energy consumption contours and linear regression models were also developed to help drilling waste managers estimate the life cycle impacts of using hydropower-driven TCC to treat OBDCs with different water and oil contents.

Remediation of a metal-contaminated soil by chemical washing and repeated phytoextraction: a field experiment

Agricultural soil contaminated with potentially toxic metals poses great health risk to humans and it requires long-term remediation. Here, we investigate the remediation of metal-polluted agricultural soil by combining chemical washing with repeated phytoextraction. The polluted field was initially washed with 40 mmol L^-1 FeCl₃ (F) or 20 mmol L^-1 FeCl₃ + 40 mmol L^-1 citric acid (F + C). After the application of organic fertilizer (O), lime (L), and sepiolite (S), Sedum plumbizincicola was cultivated for three successive crops from 2017 to 2019. Results showed that the soil washed with FeCl₃ had high removal efficiencies of Cd (35.2%), Pb (24.3%), and Zn (26.6%). Although the shoot biomass and metal concentrations of S. plumbizincicola decreased significantly in the first crop, there were no significant differences in the subsequent two crops. Throughout the remediation process, the higher total removal efficiencies of Cd, Pb, and Zn were conducted in F + OLS treatment which observed in 71.0, 34.0, and 47.7%, respectively. The results, therefore, conclusively indicated that combining chemical washing with repeated phytoextraction showed considerable potential for the remediation of agricultural soils polluted with multiple metals. However, further studies are required to focus on the amelioration of the degraded soil quality and safe agricultural production.

Phytoaccumulation of heavy metals from municipal solid waste leachate using different grasses under hydroponic condition

Grasses have been used widely to remediate contaminants present in domestic wastewater, but leachate generated from municipal solid waste that usually contain some concentrations of heavy metals has never been reported to be treated with grasses, especially Rhodes grass. A series of experiments was performed to investigate the contaminant uptake from municipal solid waste leachate by Chloris gayana (Rhodes grass) grown in combination with two commonly available grass varieties namely Vetiveria zizanioides (Vetiver grass) and Pennisetum purpureum (Elephant grass). Leachate used for the experiments had high values for chemical oxygen demand (5 g/L), pH (8.5), electrical conductivity (9.0 mS/cm), nitrates (182.1 mg/L), phosphates 6.4 mg/L along with heavy metals i.e. copper, zinc and manganese. Different dilutions of leachate ranging from 0 to 100% were applied in batches and their result showed that collectively all the grasses reduced overall contaminant concentrations. These were reported for chemical oxygen demand, electrical conductivity, nitrates, and phosphates reduced up to 67, 94, 94, and 73%, respectively. Metals uptake by grasses also showed a significant decrease in applied dose i.e. zinc (97%), copper (89%), and manganese (89%). Plant analysis showed that all grasses showed preference to heavy metals uptake e.g. Rhodes grass favoured up taking zinc, Elephant grass for copper and Vetiver grass preferred manganese. Overall growth performance of Rhodes grass was better in dilute leachate, whereas in more concentrated leachate, Rhodes grass did not perform better and production of biomass decreased. In Vetiver grass, root and shoot lengths decreased with increasing leachate strength, but the biomass did not change significantly.

Remediation of oil-based drill cuttings using low-temperature thermal desorption: Performance and kinetics modeling

A bench-scale apparatus was used for the low-temperature thermal desorption (LTTD) treatment of oil-based drill cuttings (OBDCs). The effects of treatment temperature, treatment duration, sand/OBDCs mixing ratio, and initial oil content on the LTTD treatment performance were investigated. It was found that the petroleum hydrocarbons (PHCs) were barely left in the high-oil-content drill cuttings after LTTD (at 300 °C for 20 min), and thus the overall soil health was improved. The desorption kinetics models of PHCs under various conditions were established, and it was found that the LTTD of OBDCs followed nonlinear least-squares exponential kinetics (adjusted R² > 0.9). The energy consumption models of LTTD treatment under different temperatures were also developed. The modeling results are of practical guiding significance and useful for designing effective LTTD treatment systems of OBDCs.