Effect of carbon nanospheres on algal growth: the influence of physicochemical properties and environmental chemistry

Abstract

Algae (Pseudokirchnierella subcapitata) were exposed to pristine (p-) and oxidised (o-) forms of carbon nanospheres (CNSs) at concentrations of 0.1, 0.18, 0.32, 0.46 and 20 mg/L. At each concentration, the CNSs were sonicated for 1-h in three types of water (artificial soft, moderately hard, and hard water) for 72-h at 24-h intervals: (i) in the absence (0 mg/L) of humic acid, and (ii) presence of 5 mg/L humic acid at neutral to slightly alkaline pH. The study findings suggested that, algal growth in the presence of humic acid was visible to the human eye, however, in the absence of the humid acid; the growth could only detected using a light microscope. Additionally, in the absence and presence of humic acid in all concentrations of o-CNSs algal growth was established in soft water, moderately hard water, and hard water as highest, moderate, and inhibited, correspondingly. Similar trend was observed for p-CNSs with a distinctive difference in that the growth inhibition was concentration (dose) dependent. The exposure suspensions were then monitored for CNSs’ aggregate size and surface charge changes over the same exposure period under similar environmental conditions. We observed that the p-CNSs dispersed with difficulties unlike the oxidised forms that were easily dispersed. Our findings suggested that the presence of humic acid and water hardness were factors that influenced CNSs dispersion in water. Moreover, the high surface charge (= +30 mV) was also established as an additional factor in enhancing the ease of CNSs dispersion in the aquatic environment although for the pristine form re-aggregation was observed as a function of time. In summary, we established that the algal growth inhibition and/or stimulation by CNSs is largely dependent on the presence or absence of humic acid, degree of water hardness as well as the CNSs’ concentration, and surface functionalization.