Effects of nano cerium (IV) oxide and zinc oxide particles on biogas production
Graphical abstract
Introduction
With the rapid growth of nanotechnology in the past decade, nanoparticles (NPs) are now widely being used in commercialized products. With the unique physicochemical characteristics (e.g. magnetic, optical and electrical features) the use of NPs are deemed ideal in manufacture industries. Cerium oxide (CeO2) and zinc oxide (ZnO) nanoparticles have become popular in recent years and are currently being used in a variety of fields. CeO2 NPs are being used as fuel catalyst to reduce harmful emission from engine combustion. Moreover, they are also used as fuel cell electrolyte, antioxidant, semiconductor, UV absorber, coating and polishing chemical (Laberty-Robert et al., 2006). ZnO NPs have been widely commercialized in consumer products, such as anti-bactericidal coating, sunscreen, various industrial, medical and military applications (Moezzi et al., 2012). With the increasing industrial and commercial applications, the release of these compounds to the environment is inevitable. NPs can be easily enter the wastewater systems through their released either from the production process and/or during product consumption. Recently, fate and transport of NPs in wastewater treatment plants (WWTPs) and their toxicological effects on biological activities at various stages of wastewater treatment processes have been investigated (Brar et al., 2010). Researchers (Liu et al., 2011, García et al., 2012) have reported adverse effects of NP's in wastewater treatment and activated sludge with respect to biodegradation and nitrification processes. Agglomeration and adsorption of nanoparticles play a major role in the sedimentation of NPs in the sewage sludge, which resulted in low concentration of NPs in effluent (Brar et al., 2010). Thus indicating higher removal efficiency of NPs in biological treatment processes leads to an elevated concentration of NP in the sludge. Therefore, NPs can potentially accumulate to a very high concentration in the waste sludge. As a result, effects of NPs on sludge treatment processes can pose a severe threat and need to be investigated.
There are various mechanisms that may cause damage to exposed organism, such as damage cell membrane, protein, DNA; interrupt electron transportation; release toxic components; or release reactive oxygen species (ROS) (Klaine et al., 2008, Xia et al., 2008). Possible toxicity mechanisms of nanoparticles are illustrated in Fig. 1. The primary mechanism of nanoparticle toxicity involves reactive oxygen species (ROS) production. ROS consist of free radicals, such as superoxide anion (O2−) and hydroxyl radical (OH). These are strong oxidants that can oxidize DNA, protein, cell membrane, and other cell components. These ROS are also normally formed in metabolism process that cause cell aging. However, when present at high concentration, they cause oxidative stress, inflammation, and cell death. High reactivity of nanoparticles release a large amount of ROS. Some nanoparticles, like TiO2 and ZnO are photoactive and generate higher amount of ROS when exposed to sunlight, in the reaction called photocatalysis (Rincon and Pulgarin, 2004, Brunner et al., 2006). There are many factors that affect the nanoparticles toxicity, such as size, surface characteristic, dissolution, etc., with interaction between each other and with environmental factors (Chang et al., 2012). Nanoparticles at smaller size showed higher toxicity then their bulk sized particles due to the higher solubility and reactivity of small sized particles (Liu et al., 2011, Luna-delRisco et al., 2011). Moreover, the behavior of nanoparticles in real environment is affected by various environmental factors. Natural organic matters (NOM) and ionic strength were pointed out to have great impact on size and morphology of silver nanoparticles (Delay et al., 2011) and solubility of ZnO nanoparticles was greatly affected by chemical characteristics of different matrices (Reed et al., 2012).
There are many contrasting view about the effect of cerium oxide nanoparticles with biological system. Some researchers have found that cerium oxide have the protective effect for mammalian cell, since it can act like free radical scavenger to reduce ROS (Xia et al., 2008, Amin et al., 2011). In contrast, engineered cerium oxide nanoparticles have been reported to be toxic for bacteria (Thill et al., 2006, Pelletier et al., 2010) and inhibit activated sludge (García et al., 2012, Gomez-Rivera et al., 2012). Toxic mechanism of cerium oxide nanoparticles is suggested due to the oxidative stress when the nanoparticles attaches to the bacterial membrane surface (Thill et al., 2006). In the case of zinc oxide, toxicity results from dissolution of particles that create soluble Zn (i.e. Zn2+ ion) and from production of ROS while exposing to UV lighting. This was confirmed in the study on effect of ZnO on activated sludge activities, where at same concentration, Zn2+ is the most toxic, followed by nano-sized and then bulk-sized ZnO (Liu et al., 2011). However, a single nanoparticle can affect the cell by multiple mechanisms at the same time. For example, Ag NPs interrupt the permeability of cell membrane, cease the membrane respiration, release toxic Ag+ ions, and penetrate inside the cell to cause damage to DNA (Sondi and Salopek-Sondi, 2004).
Thus this research was designed to study possible impacts of nanoparticles on gas production in anaerobic environment at lower concentrations (10,100 mg/L) to spike concentration (500, 1000 mg/L) in sludge, as it could accumulate to a high concentration in the sludge. CeO2 and ZnO NPs were selected for this study since both are representative nanoparticles, which are being manufactured at large amount with a variety of applications (OECD, 2010). Additionally researchers have also presented at select concentrations they pose inhibition but the studies are scarce. Bacterial toxicity tests of these nanoparticles were included to explain possible mechanism of the inhibition on anaerobic digestion of sludge.
Section snippets
Materials and methods
In this section, steps to prepare experimental materials (i.e. nanoparticle dispersion, inoculum, and substrate) are presented, followed by methods for the biogas inhibition test and the bacterial toxicity test.
Effects of CeO2 and ZnO nanoparticles on biogas production
CeO2 and ZnO nanoparticle at concentration of 10, 100, 500, and 1000 mg/L were applied on anaerobic batch reactor to analyze inhibition effects on the system. The cumulative biogas production for all the samples (40 days) are presented in Fig. 2. Every sample observed in this study had biogas production less than the blank sample, except the sample with CeO2 exposure at 10 mg/L. When CeO2 nanoparticle dispersion was added at low concentration of 10 mg/L, it stimulated the biogas production
Conclusions
The result from this study present that inhibitory effects of ZnO and CeO2 nanoparticles on biogas production in sludge digestion are minimal for biogas production except at spike concentrations. ZnO NPs were more toxic to bacteria and caused greater inhibition to biogas production than CeO2 NPs. At tolerable exposure concentration (ZnO at 100 and 500 mg/L), the inhibition on biogas production could be overcome after 14 days. The recovery could come from environmental factors or bacterial
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