Factors affecting the sorption of perfluoroalkyl substances in biochars and other carbon-rich byproducts

Biochar is the solid byproduct obtained after the pyrolysis process of biomass waste, and it is usually applied in agricultural soils to increase soil organic carbon, and improve soil structure and water retention, among other benefits. As long as it has been demonstrated that biochar successfully sorbs either organic and inorganic pollutants, the investigation of other potential applications, such as water remediation, are of increasing interest (1). Similarly, other carbon-rich byproducts such as compost or coal fines are supposed to have similar properties than biochar. In this study, the sorption properties of biochars of different characteristics have been investigated and compared with those of other carbon-rich materials. As target compounds, we have used perfluoroalkyl substances (PFASs), which are pollutants of concern because of their high bioaccumulation, extreme persistence and toxic properties, and widespread in different environmental compartments, among them soils and waters (2). Sorption parameters of seven PFASs with different perfluorinated chain length have been determined in biochar, compost and coal fines, using batch experiments according to the OECD 106 guidelines. Biochars of different biomass wastes were obtained by slow pyrolysis at 350oC and a rate of 5oC min-1, achieving biochar yields between 40% and 90% depending on the feedstock (3). The effect of various experimental conditions (contact time, initial PFASs concentration, pH and composition of contact solution) in the sorption capacity of the materials was evaluated. Sorption was slightly faster for the long-chain PFASs, with more that 90% of sorption of PFNA in 24 h in all materials. For longer times, in almost all materials, a 100% of sorption was reached. Linear isotherms were obtained for all the PFASs in all materials in a wide range of concentrations, allowing the calculation of a solid-liquid distribution coefficient (Kd) and a solid-liquid distribution coefficient referred to the organic carbon of the material (Koc) for every PFAS-material combination. Similarly, in a previous work (4), linear isotherms were also obtained for the same PFASs in soils, using similar experimental conditions. The sorption process in all matrices was considerably irreversible, especially for the long chain PFASs. The obtained log Koc values for each PFAS in each material were successfully correlated with the number of CF2 units of the alkyl chain and the log Kow values of the PFASs, which suggest that hydrophobic interaction is the main mechanism controlling PFASs sorption in biochar and other carbon-rich materials. Moreover, the sorption of PFASs in the materials was negatively correlated with the dissolved organic carbon (DOC) of the aqueous solution, which was attributed to a competitive sorption process of PFASs with the dissolved organic matter. Therefore, materials with low DOC, as is the case of most biochars, are the best candidates for PFASs sorption. Low pH values and high cationic content of the aqueous solution also contribute to a decrease of DOC and, therefore, leads to optimum PFASs sorption in the material. (1) M. Ahmad et. al. Biochar as a sorbent for contaminant management in soil and water: A review. Chemosphere 99 (2014) 19 – 33. (2) K. Prevedouros, I.T. Cousins, R.C. Buck, S.H. Korzeniowski. Sources, fate and transport of perfluorocarboxylates. Environmental Science & Technology 40(1) (2006) 32 – 44. (3) M.E. Doumer et. al. Slow pyrolysis of different Brazilian waste biomasses as sources of soil conditioners and energy, and for environmental protection. Journal of Analytical and Applied Pyrolysis 113 (2015) 431 – 443. (4) J. Milinovic, S. Lacorte, M. Vidal, A. Rigol. Sorption behaviour of perfluoroalkyl substances in soils. Science of the Total Environment 511 (2015) 63 – 71

Effect of ageing on the availability of heavy metals in soils amended with compost and biochar: evaluation of changes in soil and amendment properties

Remediation strategies using soil amendments should consider the time dependence of metal availability to identify amendments that can sustainably reduce available pollutant concentrations over time. Drying-wetting cycles were applied on amendments, soils and soil + amendment mixtures, to mimic ageing at field level and investigate its effect on extractable Cd, Cu, Ni, Pb and Zn concentrations from three contaminated soils. The amendments investigated were municipal waste organic compost and biochars. The amendments, soils and mixtures were characterised by their physicochemical properties at different ageing times. The amendments were also characterised in terms of sorption capacity for Cd and Cu. The sorption capacity and the physicochemical properties of the amendments remained constant over the period examined. When mixed with the soils, amendments, especially the compost, immediately reduced the extractable metals in the soils with low pH and acid neutralisation capacity, due to the increase in pH and buffering capacity of the mixtures. The amendments had a relatively minor impact on the metal availability concentrations for the soil with substantially high acid neutralisation capacity. The most important changes in extractable metal concentrations were observed at the beginning of the experiments, ageing having a minor effect on metal concentrations when compared with the initial effect of amendments

Changes in heavy metal extractability from contaminated soils remediated with organic waste or biochar

The effect of the addition of organic waste or biochar on the extractability of heavy metals (Cd, Cu, Ni, Pb and Zn) was assessed in five heavy metal-contaminated soils. The amendments studied were: municipal organic waste compost (MOW), green waste (GW), biochar derived from tree bark (BF) and biochar derived from vine shoots (BS). The amendments were added to the soil at 10% dose. A pHstat leaching test was applied to the soils and soil + amendment mixtures to assess the effects of the amendments on the extractable metal concentration at the initial pH and in the 2-12 pH range. MOW increased the DOC content in the mixtures for most soils, whereas the rest of amendments only increased the DOC content for the soil with the lowest DOC value. Moreover, in the mixtures obtained from soils with a low buffering capacity, the amendments increased pH (up to 3 units) and the acid neutralization capacity, thus decreasing the extractability of heavy metals at the initial pH of the mixtures. In a few cases, the amendments further decreased the concentrations of extractable metal due to an increase in the sorption capacity of the mixture, even though the soil had high initial pH and ANC values. MOW and GW generally led to larger decreases in metal extractability in the resulting mixtures than biochar, due to their higher sorption and acid neutralization capacities

Testing biochar as sorbent to decrease samarium mobility in contaminated areas

Samarium (Sm), a rare earth element of lanthanides group, is an emerging contaminant that has been found in wastes resulting from agricultural and industrial activities, especially in China. There are many evidences of watersheds that, due to contamination by industrial wastes, specially mine tailings, contain Sm at high concentration levels, such as the case of Baotou Area containing up to 130 μg Sm L-1 (1). Besides, some works have also reported that the Sm concentration in lands treated with rare earth-rich fertilizers or irrigated by the abovementioned contaminated water sources is abnormally high, e.g., 492 mg kg-1 in Bayan Obo (2). This concomitant soil contamination is also of special concern since soils can act as sink of Sm that can be further incorporated in the food chain or reintroduced into water sources due to irrigation and raining processes. Biochars are increasingly being considered as sorbents for the removal of inorganic pollutants from contaminated waters (Doumer, 2016), but also as soil amendments as they may have a high sorption capacity for several inorganic pollutants, and they are also low-cost materials that may improve soil properties as water holding capacity, nutrient status and pH. However, secondary effects on soil solution composition, as the increase in the dissolved organic carbon (DOC), jeopardize their application for several pollutants, such as heavy metals forming soluble chelates with organic matter compounds. Moreover, biochar will decrease contaminant mobility in soils only when the sorption capacity of biochar exceed that of the soil in various orders of magnitude, considering the low dose (often lower than 10% w/w) with which they will be applied. Therefore, laboratory assays are required to assess the sorption properties of a biochar before using it at field level as soil amendment or as sorbent for water filtration. In the case of lanthanides, there are scarce data on their mobility in soils and on their sorption by biochars. In this work, we examine the capacity of a set of biochars of varying pH and DOC, to sorb Sm, as representative for lanthanide pollutants, in a wide range of Sm concentrations. Biochar concentration-dependent sorption capacities were compared with those of activated charcoal and of a by-product from the metallurgical industry (coal fines). Results indicated high sorption capacities at low Sm concentration range (up to distribution coefficients of 104 L/kg), with desorption rates lower than 1%. Distribution coefficients of Sm in biochar significantly decreased when increasing Sm concentration levels, indicating saturation of the sorption sites of the material with maximum sorption values very similar to the cation exchange capacity of the biochar and suggesting that the main interaction mechanism was cation exchange. The implications of the use of biochars in soil remediation actions as well as in the treatment of contaminated waters are also discussed. (1) He, J., Lü, C.W., Xue, H.X., Liang, Y., Bai, S., Sun, Y., Shen, L.L., Mi, N., Fan, Q.Y., 2010. Species and distribution of rare earth elements in the Baotou section of the Yellow River in China. Environ. Geochem. Health, 32, 45-58. (2) Jinxia, L., Mei, H., Xiuqin, Y., Jiliang, L., 2010. Effects of the accumulation of the rare earth elements on soil macrofauna community. J. Rare Earth 28 (6), 957-964. (3) Doumer, M.E., Rigol, A., Vidal, M., Mangrich, A.S., 2016. Removal of Cd, Cu, Pb, and Zn from aqueous solutions by biochars. Environ. Sci. Pollut. Res. 23, 2684-2692

Examining sorption of perfluoroalkyl substances (PFAS) in biochars and other carbon-rich materials

The use of carbon-rich sorbents to remove and/or immobilize perfluoroalkyl substances (PFAS) in contaminated environmental scenarios is attracting increasing interest. The identification of key sorbent properties responsible for PFAS sorption and the development of models that can predict the distribution coefficients (Kd) for PFAS sorption in these materials are crucial in the screening of candidate materials for environmental remediation. In this study, sorption kinetics, sorption isotherms, and the effects of pH, calcium concentration and dissolved organic carbon (DOC) content on PFAS sorption were evaluated in four representative carbon-rich materials: two biochars with contrasting properties, a compost, and charcoal fines rejected by the metallurgical industry. Subsequently, the sorption of seven PFAS with numbers of fluorinated carbons ranging from 4 to 11 was evaluated in a total of ten carbon-rich materials, including activated carbons, so as to build up a Kd prediction model. The sorption of PFAS increased with greater fluorinated chain length, suggesting that hydrophobic interactions play a major role in sorption and electrostatic interactions a minor one. These results were confirmed by a principal component analysis, which revealed that the CORG/O molar ratio and the specific surface area of the material were the two main sorbent properties affecting PFAS sorption. Furthermore, the DOC content in solution had a negative effect on PFAS sorption. Using this information, a simple Kd prediction model applicable to a wide range of materials and PFAS was developed, using only a few easily-derived physicochemical properties of sorbent (CORG/O molar ratio and SSA) and PFAS (number of CF2), and was externally validated with data gathered from the literature

Demonstrating sorption analogy of lanthanides in environmental matrices for effective decision-making: The case of carbon-rich materials, clay minerals, and soils

Examining the effect of lanthanide-contaminated wastes, which have the potential to impact to other environmental compartments, requires conducting interaction studies with soils, as feasible first receptors of lanthanide leachates, and, if necessary, with sorbent materials, such as clay minerals and carbon-rich materials, which can serve as natural barriers and immobilisation agents used in remediation strategies. In this context, it is relevant to have available and reliable data on solid–liquid distribution coefficients (Kd) to understand the lanthanide sorption in these environmental matrices. Moreover, confirming lanthanide sorption analogies permits filling data gaps and data extrapolation among different contaminated scenarios, and thus facilitate to have available input data for decision-making related to the impact of a contaminated site. In this study, we demonstrate for the first time an analogous sorption of La, Sm, and Lu in carbon-rich materials (i.e., biochar and activated charcoal), clay minerals and soils, through laboratory batch experiments. The obtained sorption Kd values revealed similar sorption patterns among the three lanthanides for each matrix tested, even at different initial lanthanide concentrations. In all matrices, the maximum Kd values exceeded 104 L kg−1, with a significant decrease when testing high lanthanide concentrations. The analogy was first confirmed by examining the Kd correlations for the La-Sm, Lu-Sm, and La-Lu pairs within each matrix, for which strong linear correlations were obtained in all cases. Data compilations were built with own and literature data, and derived cumulative distribution functions revealed statistically equal lanthanide distributions and Kd best estimates. In addition to this, Kd variability decreased when grouping the data according to significant material properties. For the first time, Kd (Ln) best-estimates for different scenarios and materials were proposed as input data for risk assessment models

Dependence of samarium-soil interaction on samarium concentration: Implications for environmental risk assessment

The sorption and desorption behaviour of samarium (Sm), an emerging contaminant, was examined in soil samples at varying Sm concentrations. The obtained sorption and desorption parameters revealed that soil possessed a high Sm retention capacity (sorption was higher than 99% and desorption lower than 2%) at low Sm concentrations, whereas at high Sm concentrations, the sorption-desorption behaviour varied among the soil samples tested. The fractionation of the Sm sorbed in soils, obtained by sequential extractions, allowed to suggest the soil properties (pH and organic matter solubility) and phases (organic matter, carbonates and clay minerals) governing the Sm-soil interaction. The sorption models constructed in the present work along with the sorption behaviour of Sm explained in terms of soil main characteristics will allow properly assessing the Sm-soil interaction depending on the contamination scenario under study. Moreover, the sorption and desorption K-d values of radiosamarium in soils were strongly correlated with those of stable Sm at low concentrations (r = 0.98); indicating that the mobility of Sm radioisotopes and, thus, the risk of radioactive Sm contamination can be predicted using data from low concentrations of stable Sm. (C) 2017 Elsevier Ltd. All rights reserved

Sorption of perfluoroalkyl substances in sewage sludge

The sorption behaviour of three perfluoroalkyl substances (PFASs) (perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA) and perfluorobutanesulfonic acid (PFBS)) was studied in sewage sludge samples. Sorption isotherms were obtained by varying initial concentrations of PFOS, PFOA and PFBS. The maximum values of the sorption solid-liquid distribution coefficients (Kd,max) varied by almost two orders of magnitude among the target PFASs: 140-281 mL g−1 for PFOS, 30-54 mL g−1 for PFOA and 9-18 mL g−1 for PFBS. Freundlich and linear fittings were appropriate for describing the sorption behaviour of PFASs in the sludge samples, and the derived KF and Kd,linear parameters correlated well. The hydrophobicity of the PFASs was the key parameter that influenced their sorption in sewage sludge. Sorption parameters and log(KOW) were correlated, and for PFOS (the most hydrophobic compound), pH and Ca + Mg status of the sludge controlled the variation in the sorption parameter values. Sorption reversibility was also tested from desorption isotherms, which were also linear. Desorption parameters were systematically higher than the corresponding sorption parameters (up to sixfold higher), thus indicating a significant degree of irreversible sorption, which decreased in the sequence PFOS > PFOA > PFB

Feasibility of using low-cost, byproduct materials as sorbents to remove heavy metals from aqueous solutions

This work investigates the sorption of heavy metals by low-cost, byproducts such as charcoal fines (CF), waste green sand, and rice husk ash, in order to examine the feasibility of their use as alternative filter materials for metal-contaminated waters. The sorption of Cd, Cu, Pb, and Zn was investigated in batch experiments and sorption isotherms were constructed. The three byproducts showed high metal removal efficiencies (>95%, regardless of the metal concentration tested). The highest metal sorption distribution coefficients were obtained for CF, with maximum values within the 105-106 L kg-1 range for all the target metals. The sorption isotherms were satisfactorily fitted using the Freundlich equation and a linear model, the latter only being valid for initial metal concentrations lower than 0.4 mmol L-1. Sorption reversibility was very low, with desorption yields lower than 2% and desorption distribution coefficients often higher than 106 L kg-1. The values of the sorption and desorption parameters indicated that the use of these materials, especially CF, could constitute a low-cost alternative for the remediation of contaminated waters

Suitability of various materials for porous filters in diffusion experiments

The suitability of different porous materials (stainless steel, VYCOR® glass, Al2O3 and PEEK) for use as confining filters in diffusion experiments was evaluated by measuring the effective diffusion coefficients (De) of neutral (HTO) and ionic solutes (Na+, Cs+, Sr2+, Cl-, SeO42−) in the materials in through-diffusion experiments. For stainless steel filters, the De values of the target solutes correlated satisfactorily with their bulk diffusion coefficient in water (Dw); thus, the diffusion process in the stainless steel filters was primarily controlled by the diffusivity of the solvated ions. For the remaining materials, the De and Dw values were also correlated for the target solutes, and the geometric factors were in the sequence: VYCOR® glass < Al2O3 < PEEK. Stainless steel and VYCOR® glass were the most appropriate materials because of their high De values, but a specific interaction of caesium with VYCOR® glass was hypothesised because the De values obtained for this solute were slightly higher than expecte