A critical review on the formation, fate and degradation of the persistent organic pollutant hexachlorocyclohexane in water systems and waste streams.

The environmental impacts of persistent organic pollutants (POPs) is an increasingly prominent topic in the scientific community. POPs are stable chemicals that are accumulated in living beings and can act as endocrine disruptors or carcinogens on prolonged exposure. Although efforts have been taken to minimize or ban the use of certain POPs, their use is still widespread due to their importance in several industries. As a result, it is imperative that POPs in the ecosystem are degraded efficiently and safely in order to avoid long-lasting environmental damage. This review focuses on the degradation techniques of hexachlorocyclohexane (HCH), a pollutant that has strong adverse effects on a variety of organisms. Different technologies such as adsorption, bioremediation and advanced oxidation process have been critically analyzed in this study. All 3 techniques have exhibited near complete removal of HCH under ideal conditions, and the median removal efficiency values for adsorption, bioremediation and advanced oxidation process were found to be 80%, 93% and 82% respectively. However, it must be noted that there is no ideal HCH removal technique and the selection of removal method depends on several factors. Furthermore, the fates of HCH in the environment and challenges faced by HCH degradation have also been explained in this study. The future scope for research in this field has also received attention.

Technical insights into the production of green fuel from CO2 sequestered algal biomass: A conceptual review on green energy.

Algae a promising energy reserve due to its adaptability, cheap source, sustainability and it's growth ability in wastewater with efficient sequestration of industrial carbon dioxide. This review summarizes the pathways available for biofuel production from carbon sequestered algae biomass. In this regard, this review focuses on microalgae and its cultivation in wastewater with CO2 sequestration. Conversion of carbon sequestered biomass into bio-fuels via thermo-chemical routes and its engine emission properties. Energy perspective of green gaseous biofuels in near future. This review revealed that algae was the pre-dominant CO2 sequester than terrestrial plants in an eco-friendly and economical way with simultaneous wastewater remediation. Hydrothermal liquefaction of algae biomass was the most preferred mode for biofuel generation than pyrolysis due to high moisture content. The algae based fuels exhibit less greenhouse gases emission and higher energy value. This review helps the researchers, environmentalists and industrialists to evaluate the impact of algae based bio-energy towards green energy and environment.

Anaerobic digestate water for Chlorella pyrenoidosa cultivation and employed as co-substrate with cow dung and chicken manure for methane and hydrogen production: A closed loop approach.

In rural India, unpleasant atmosphere, anthropogenic gas emission, air and soil pollution are caused due to disposal of livestock's wastes (cow dung and chicken waste) in open environment. This study provides zero emission concept for waste disposal and value addition of these wastes for renewable green energy production. In this study, biogas production was carried out with varying proportion of cow dung to chicken waste (1:0, 0:1, 1:1, 2:1, 1:2, 3:1 and 1:3) for duration of 40 days. Chlorella pyrenoidosa was cultivated from digestate water and used as co-substrate in digester in varying proportions (2:1:1, 2:1:2 and 2:1:3) to study its role on biogas distribution. The effect of pH, feedstock ratio, time and C/N ratio for biogas production were evaluated. The maximum methane and hydrogen yield was 68% (30th day) and 29% (10th day) for 2:1:2 ratio respectively. The slurry possessed nitrogen (1.7%), phosphate (0.8%) and potassium (0.4%) respectively.

Present applications of titanium dioxide for the photocatalytic removal of pollutants from water: A review.

The evolution of modern technology and industrial processes has been accompanied by an increase in the utilization of chemicals to derive new products. Water bodies are frequently contaminated by the presence of conventional pollutants such as dyes and heavy metals, as well as microorganisms that are responsible for various diseases. A sharp rise has also been observed in the presence of new compounds heretofore excluded from the design and evaluation of wastewater treatment processes, categorized as "emerging pollutants". While some are harmless, certain emerging pollutants possess the ability to cause debilitating effects on a wide spectrum of living organisms. Photocatalytic degradation has emerged as an increasingly popular solution to the problem of water pollution due to its effectiveness and versatility. The primary objective of this study is to thoroughly scrutinize recent applications of titanium dioxide and its modified forms as photocatalytic materials in the removal and control of several classes of water pollutants as reported in literature. Different structural modifications are used to enhance the performance of the photocatalyst such as doping and formation of composites. The principles of these modifications have been scrutinized and evaluated in this review in order to present their advantages and drawbacks. The mechanisms involved in the removal of different pollutants through photocatalysis performed by TiO2 have been highlighted. The factors affecting the mechanism of photocatalysis and those affecting the performance of different TiO2-based photocatalysts have also been thoroughly discussed, thereby presenting a comprehensive view of all aspects involved in the application of TiO2 to remediate and control water pollution.

Hydrothermal liquefaction of Scenedesmus obliquus using a novel catalyst derived from clam shells: Solid residue as catalyst for hydrogen production.

This study explores the catalytic application of waste clam shell in hydrothermal liquefaction (HTL) of microalgae (Scenedesmus obliquus) for liquid hydrocarbons production. Novel catalyst (calcium hydroxide) was derived from clam shells. Catalytic HTL was performed at varying temperature of 240-320 °C for catalyst load (0.2-1 wt%) at a reaction time of 60 min. Bio-oil yield was maximum (39.6 wt%) at a temperature of 300 °C for catalyst load of 0.6 wt% at a reaction time of 60 min with calorific value of 35.01 MJ/kg. Compounds like phenols, aromatic hydrocarbons, acids and aldehydes were detected in bio-oil through Gas Chromatography Mass Spectrophotometry (GC-MS). Gasification of microalgae with waste solid residue obtained from HTL was carried out for hydrogen production. Valuable hydrogen gas production was maximum (37 wt%) at a temperature of 400 °C for 3 wt% of solid residue. Water-gas shift, methanation and steam reforming reactions favoured the hydrogen gas production.