FATE OF ALKYL POLYGLUCOSIDES IN THE ENVIRONMENT 95 Incr ease in the alkyl chain length will lead to increase in hydrophobicity of the surfactant, causing higher cellular penetration rate and faster metabolism. However, cell death associated with toxicity may also increase, resulting in overall lower degradation rates. Furthermore, the initial concentration in the infl uent can lead to confounding outcomes because longer alkyl chain APGs have poorer aqueous solubility. Hence, a balance in cellular toxicity, concentration, and solubility needs to be achieved to draw an unbiased conclusion (17). iii. Size of hydrophilic head: The effect of hydrophilic headgroup on the degradation rate has been reported in limited cases. Increase in the saccharide residues resulted in lower biodegradation rates due to increased hydrophilicity and lesser cellular penetrability (10). Mine ralization studies are simulated in the laboratory as per the OECD 301 guideline under aerobic and anaerobic conditions (9). The eventual fate of an APG molecule is the conversion of primary degradation products, fatty alcohols, and glucose to carbon dioxide and water by the microbiome. Jurado et al. (18) have classifi ed these reactions as stage 2 of degradation, lasting from days 13 to 21. Coco glucoside yielded a 99.20% total organic carbon (TOC) removal for the 15 mg/L (lowest test concentration) and a 45.21% TOC removal for the 100 mg/L (highest test concentration) within the 21-d test period in aerobic environment. Measurement of total Colony Forming Units showed initial reduction suggesting an acclimatization period for the microbiome. For materials to be classifi ed as readily biodegradable, the OECD 301 guideline recom- mends minimum of 60% CO2 evolution or O2 uptake in a 28-d test period with 10% in a 10-d window (9). The study by Madsen et al. (15) showed that all glycosides showed more than 60% CO2 generation, except C12 monoester which failed to clear the 10-d window in aerobic conditions. Longer alkyl chain lengths and branching can lead to lower effective concentration for metabolism. In the anaerobic conditions, too, C12 mono- ester showed a higher degradation rate and can be compared with that of sodium benzo- ate, with 80% mineralization for the former compared with 86% of the latter in 28 d. Similar degradation time has been reported in other studies (12,19). Overall, APGs of chain lengths from C8 to C14 continue to qualify as readily biodegradable with greater than 60% clearance in 28 d. However, in limited cases, they may not clear the 10-d window period (12,15,19). DO A PGS SHOW TOXICITY IN MARINE ENVIRONMENT? Surf actants when dissolved in water are known to accumulate at interfaces. Although this property makes them active for formulations, it can also intensify their toxicity potential at cellular interface. Surfactants can dissolve the phospholipid cell membrane of bacteria, damage protein envelopes leading to cellular disruption, and death. Most laboratory ex- periments with activated sludge report biodegradation tests with a maximum of 40 mg/L of surfactant concentration to minimize cellular toxicity (1,2). In one study by Rios et al. (17), test concentrations of 100 mg/L have been reported. However, the researchers do not comment on the toxicity at such high concentrations. C8 g lucoside showed least toxicity in three species, a microalga (Raphidocelis subcapitata/ Kirchneriella subcapitata), a plankton (Daphnia magna), and the zebrafi sh (Danio rerio/ Brachydanio rerio). D. rerio showed the highest sensitivity to APGs with an LC50 (lethal con- centration at which 50% of the test organisms die) of 558 mg/L for C8 and ~5 mg/L for C12–14

JOURNAL OF COSMETIC SCIENCE 96 also reconfi rming the higher toxicity potential of longer alkyl chains as observed in acti- vated sludge experiments (16). EC50 in D. magna also followed a similar pattern, sug- gesting nonspecifi c toxicity profi le of APGs. Similar observations were reported by Garcia et al. (19) for D. magna. In luminescent bacteria (Photobacterium phosphoreum), toxicity decreased rapidly, and no toxic effects were detected after the third day correlating with the surfactant biodegradation rate. This study also showed that the toxic effects were mainly due to the parent molecule, and its elimination resulted in toxicity reduction. In a separate experiment, caprylyl and decyl glucoside (C8 and C10), lauryl glucoside (C12), and coco glucoside (C8–16), and toxicity in Gram-negative bacteria (Vibrio fi scheri) were found to be dependent on the initial concentration, chain length, and Hydrophilic- Lipophilic Balance (HLB). Shorter carbon chain and higher HLB in C6 and C10 glucosides led to lower toxicity with EC50 (effective concentration at which 50% of test organisms show a response) of 29.05 (27.04–29.07) mg/L versus 13.81 (13.78–13.82) mg/L for C8–10 in 15-min exposure (20). Biod egradation of APG has shown to reduce dissolved oxygen levels in closed sys- tems, suggesting that the process may impact water quality. In a fi rst of its kind of a study, Sutton and Cohen evaluated the impact of APG at 0.1% and 1% concentration in low dissolved oxygen conditions using water columns in a blackwater pond in Georgia, USA. Concentration of dissolved oxygen was signifi cantly reduced, and con- ductivity was higher than controls at both treatment levels. An overall decrease in all taxa was noted for the treatment, with predominant reduction in dominant species, Chironomidae and Oligochaeta in 14-d test period. Because APGs have shown 80% degradation in 21 d, it is possible that some undegraded surfactant might be present in the mesocosm. Whether the invertebrate species revived after the supposed APG degradation period or not was not captured as part of this study (21). In the second study, the research group has studied APG concentrations from 0.01 to 10 mg/L on plankton abundance and dissolved oxygen levels. APG concentrations of 2.5 and 5 mg/L showed 75% reduction in zooplanktons, especially copepods. No evidence of recovery was seen throughout the 1-mo study period. Dissolved oxygen levels were reduced in the fi rst week but were seen to normalize by second and third weeks, except for 10 mg/L APG treatment where dissolved oxygen levels remained low throughout the study period. This change in the plankton community profi le and overall distribution could have potential impact on the food cycle (22). In a laboratory study, Duff et al. (23) have shown the effect of APG treatment on alga, Chlorella, in the presence of nitrogen and phosphorus as nutrients. The cell density was seen to decrease in the presence of APG alone. The presence of nutrients could not only reverse the impact of the surfactant higher chlorophyll-a levels were noted in the APG + N treatment group, suggesting that the metabolites, glucose, and fatty alcohols were nontoxic as well as the increased algal biomass was a result of stress response to APG. Alth ough toxicity studies reported are limited and isolated, they do suggest that high con- centrations of APGs may affect certain species in the food cycle. Usually, the chemical concentration is 10–100 times less in the discharge as compared with the infl uent, and the overall increase in the initial concentration can cause high levels in wastewater treatment plants (5,21). Linear alkylbenzene sulfonates, an example of the conventional surfactants used in the detergent industry, have been found up to 30.2 g/kg dry weight in treatment sludges, 1.09 mg/L in wastewater effl uents, and 0.42 mg/L in discharge bodies (24–26).