NZVI and Fe3O4 Based Composites for the Treatment of Dairy Wastewater and Heavy Metal Removal by Adsorption using Digested Sludge

Abstract

Developmental and industrial activities have caused extensive environmental impacts worldwide. Many proactive ‘clean-environment’ strategies have been proposed and implemented with the support of UNEP. The peer pressure on the government has assured more stringent regulations to reduce the burgeoning effect of atmospheric pollution and improve environmental performance. To comply with environmental regulations, the stakeholders may need to depend on more efficient and competitive waste treatment methodologies. Researches on sustainable wastewater treatment technologies are on an increasing trend. Studies help to develop processes with eco-friendly reactants, less energy requirement, easy residue management, and high efficiency. Incorporating nanosystems into wastewater treatment methodologies has been observed to be highly efficient due to the superior physicochemical properties of nanoparticles. Iron nanoparticles (INPs) are preferred among the studied nanoparticles because iron is highly abundant, low cost, has no secondary pollution, and has easy separation from aqueous media. But, INPs show a high tendency for aggregate formation and scavenging effects when used alone. Composites of INPs have been employed to alleviate these problems. This study analyzed the impact of composites of nano zerovalent iron (NZVI) and nano Fe3O4 for wastewater treatment. A composite was developed with NZVI and reduced graphene oxide (RGO) and studied the effect on the anaerobic digestion of dairy wastewater. There was 86.27 ± 2.8% more CH4 production and 47.37 ± 1.3% improved COD removal. Comparing different ratios of RGO-NZVI revealed that a proportion of 2:1 was beneficial for maximum CH4 generation. Further, the higher concentrations of the conductive additives could be fatal for microbial metabolism. The metagenomic analysis showed that the diversification of the microbial community and switching to direct interspecies electron transfer caused higher CH4 generation. The effect of pretreatment on dairy wastewater digestion was also evaluated. Ammonium persulfate (APS) assisted photocatalytic pretreatment using RGO-NZVI catalyst and anaerobic digestion were integrated and assessed at different operational parameters. The maximum solubilization was found at an initial pH 5 until 4 h of pretreatment with an increase of 38.77±0.85% SCOD and 39.05±1.3% dissolved organic carbon (DOC). Digestion with pretreatment and fresh composite addition showed 81.01±3.24% more CH4 production and 72.99±2.84% more SCOD removal. The anaerobic digestion of pretreated wastewater containing spent catalyst was also producing a better volume of CH4. Another experiment incorporated the composite made of NZVI, polypyrrole (PPy), and carbon black (CB) in biogas production. A dosage of 0-0.8 g L-1 of Ppy-CB was chosen. The maximum cumulative biogas production and SCOD removal efficiency were 2185 ± 76 mL and 74.69%, respectively. A Ppy-CB-NZVI dosage of 0.4 g L-1 (D3) caused 43.27% more biogas than the control digester. Similarly, the total CH4 production in D3 was about 1.79 times higher. The results of residual VFA analysis and corresponding CH4 generation illustrated that the ternary additive significantly influenced hydrolysis, fatty acid metabolism and acetogenesis, leading to higher gas production. After anaerobic digestion, the active functionality of sludge was utilized to develop a low-cost adsorbent by magnetic modification of pretreated biogas slurry solids (BSS) to remove heavy metals such as Cu2+, Cd2+ and Pb2+. The temperature (423 K) and time (1.5 h) of pretreatment, BSS to KOH ratio (1:10 w/v) and the ratio of magnetic iron nanoparticle (MIN) to pretreated BSS (PSS) (1:2 w/w) were optimized for the preparation of adsorbent. The optimum conditions for the adsorption of heavy metals were obtained from response surface methodology (RSM) incorporating Central Composite Design (CCD). Model validation experiments for optimization of the adsorption process showed comparable results with predicted values. The adsorption capacity at optimum conditions from RSM analysis was 29.721 mg L-1, 28.551 mg L-1 and 28.601 mg L-1 for Cu2+, Cd2+and Pb2+, respectively. The adsorption kinetics followed a pseudo-second-order model with an R2 value above 0.9 for all metals with a well-approaching equilibrium pattern. The excellent fit of experimental data by the Langmuir isotherm model implied monolayer adsorption. By this, the effluents containing heavy metal, which causes contamination of water resources, may be remediated effectively.