Effective stabilization of per- and polyfluoroalkyl substances (PFAS) precursors in wastewater treatment sludge by surfactant-modified clay.

The application of biosolids or treated sewage sludge containing per- and polyfluoroalkyl substances (PFAS) in agricultural lands and the disposal of sludge in landfills pose high risks to humans and the environment. Although PFAS precursors have not been regulated yet, their potential transformation to highly regulated perfluoroalkyl acids (PFAAs) may enable them to serve as a long-term source and make remediation of PFAAs a continuing task. Therefore, treating precursors in sewage sludge is even more, certainly not less, critical than treating or removing PFAAs. In this study, a green surfactant-modified clay sorbent was evaluated for its efficacy in stabilizing two representative PFAA precursors in sludge, e.g., N-ethyl perfluorooctane sulfonamido acetic acid (N-EtFOSAA) and 6:2 fluorotelomer sulfonic acid (6:2 FTSA), in comparison with unmodified clay and powdered activated carbon (PAC). Results showed N-EtFOSAA and 6:2 FTSA exhibited distinct adsorption behaviors in the sludge without sorbents due to their different physicochemical properties, such as hydrophobicity and functional groups. Among the three sorbents, the modified clay reduced the water leachability of N-EtFOSAA and 6:2 FTSA by 91.5% and 95.4%, respectively, compared to controls without amendments at the end of the experiment (47 days). Within the same duration, PAC decreased the water leachability of N-EtFOSAA and 6:2 FTSA by 60.6% and 37.3%, respectively. At the same time, the unmodified clay demonstrated a poor stabilization effect and even promoted the leaching of precursors. These findings suggested that the modified clay had the potential for stabilization of precursors, while negatively charged and/or hydrophilic sorbents, such as the unmodified clay, should be avoided in the stabilization process. These results could provide valuable information for developing effective amendments for stabilizing PFAS in sludge or biosolids. Future research should evaluate the long-term effect of the stabilization approach using actual sludge from wastewater treatment facilities.

Sweet sorghum bagasse and corn stover serving as substrates for producing sophorolipids.

To make the process of producing sophorolipids by Candida bombicola truly sustainable, we investigated production of these biosurfactants on biomass hydrolysates. This study revealed: (1) yield of sophorolipds on bagasse hydrolysate decreased from 0.56 to 0.54 and to 0.37 g/g carbon source when yellow grease was dosed at 10, 40 and 60 g/L, respectively. In the same order, concentration of sophorolipids was 35.9, 41.9, and 39.3 g/L; (2) under similar conditions, sophorolipid yield was 0.12, 0.05 and 0.04 g/g carbon source when corn stover hydrolysate was mixed with soybean oil at 10, 20 and 40 g/L. Sophorolipid concentration was 11.6, 4.9, and 3.9 g/L for the three oil doses from low to high; and (3) when corn stover hydrolysate and yellow grease served as the substrates for cultivating the yeast in a fermentor, sophorolipid concentration reached 52.1 g/L. Upon further optimization, sophorolipids production from ligocellulose will be indeed sustainable.

Sophorolipid production from biomass hydrolysates.

Although extensive research has been conducted on producing sophorolipids using Candida (Starmerella) bombicola from pure sugars and various oil sources, production of this biosurfactant has not been evaluated when cells are cultivated in lignocellulosic hydrolysates. Here, we report for the first time that C. bombicola is capable of producing sophorolipids on hydrolysates derived from sweet sorghum bagasse and corn fiber. Without oil supplementation, a sophorolipid concentration of 3.6 and 1.0 g/L was detected from cultures with bagasse and corn fiber hydrolysates, respectively. With the addition of soybean oil at 100 g/L, the yield of sophorolipids from these two hydrolysates in the same order was 84.6 and 15.6 g/L. Surprisingly, C. bombicola consumed all monomeric sugars and nonsugar compounds in the hydrolysates, and cultures with bagasse hydrolysates had higher yield of sophorolipids than those from a standard medium which contained pure glucose at the same concentration.

Converting crude glycerol derived from yellow grease to lipids through yeast fermentation.

Cryptococcus curvatus, an oleaginous yeast was observed to grow on crude glycerol derived from yellow grease. When cultured in a one-stage fed-batch process wherein crude glycerol and nitrogen source were fed intermittently for 12 days, the final biomass density and lipid content were 31.2 g/l and 44.2%, respectively. When cultured in a two-stage fed-batch operation wherein crude glycerol was supplemented at different time points while nitrogen source addition was discontinued at the middle of the experiment, the biomass density was 32.9 g/l and the lipid content was 52% at the end of 12 days. Compared with other oil feedstocks for biodiesel production, lipid accumulated by C. curvatus grown on glycerol has high concentration of monounsaturated fatty acid, which makes it an excellent source for biodiesel use.

Use of sweet sorghum juice for lipid production by Schizochytrium limacinum SR21.

Stalk juice from sweet sorghum grown in Southern Illinois, USA, was examined for lipid production through microalgal fermentation. Juice concentrations at 100%, 75%, 50%, and 25% led to different biomass, lipid, and docosahexaenoic acid (DHA) production by Schizochytrium limacinum SR21. Biomass dry weight as 9.4g/l at 50% juice concentration was similar to that from pure glucose (10.9g/l). But with a 73.4% lipid content, this dose resulted in higher lipid and DHA production than those from pure glucose. Major fatty acids in cells grown on juice were identical to those fed by other substrates. Among the three sugars - glucose, fructose, and sucrose in sorghum juice, only glucose was utilized for growth. Spent medium after algal removal may be further processed for white sugar production in a traditional way since sucrose content remained the same throughout the algal fermentation process. Algal cells or lipids harvested can be utilized as fish meal, human nutrition supplements, or for biodiesel purpose.