As the most recognizable natural secondary carotenoid astaxanthin producer, the green microalga Haematococcus pluvialis cultivation is performed via a two-stage process. The first is dedicated to biomass accumulation under growth-favoring conditions (green stage), and the second stage is for astaxanthin evolution under various stress conditions (red stage). This mini-review discusses the further improvement made on astaxanthin production by providing an overview of recent works on H. pluvialis, including the valuable ideas for bioprocess optimization on cell growth, and the current stress-exerting strategies for astaxanthin pigment production. The effects of nutrient constituents, especially nitrogen and carbon sources, and illumination intensity are emphasized during the green stage. On the other hand, the significance of the nitrogen depletion strategy and other exogenous factors comprising salinity, illumination, and temperature are considered for the astaxanthin inducement during the red stage. In short, any factor that interferes with the cellular processes that limit the growth or photosynthesis in the green stage could trigger the encystment process and astaxanthin formation during the red stage. This review provides an insight regarding the parameters involved in bioprocess optimization for high-value astaxanthin biosynthesis from H. pluvialis.

TiO₂ is a semiconductor material with high chemical stability and low toxicity. It is widely used in the fields of catalysis, sensing, hydrogen production, optics and optoelectronics. However, TiO₂ photocatalyst is sensitive to ultraviolet (UV) light; this is why its photocatalytic activity and quantum efficiency are reduced. To enhance the photocatalytic efficiency in the visible light range as well as to increase the number of the active sites on the crystal surface or inhibit the recombination rate of photogenerated electron–hole pairs electrons, various metal ions were used to modify TiO₂. This review paper comprehensively summarizes the latest progress on the modification of TiO₂ photocatalyst by a variety of metal ions. Lastly, the future prospects of the modification of TiO₂ as a photocatalyst are proposed.

In recent years, simulation studies have emerged as valuable tools for understanding processes. In particular, molecular dynamic simulations hold great significance when it comes to the adsorption process. However, comprehensive studies on molecular simulations of adsorption processes using different adsorbents are scarcely available for wastewater treatment covering different contaminants and pollutants. Hence, in this review, we organized the available information on various aspects of the adsorption phenomenon that were realized using molecular simulations for a broad range of potentially effective adsorbents applied in the removal of contaminants from wastewater. This review was compiled for adsorbents under five major categories: (1) carbon-based, (2) oxides and hydroxides, (3) zeolites, (4) metal–organic frameworks and (5) clay. From the review, it was found that simulation studies help us understand various parameters such as binding energy, Gibbs free energy, electrostatic field, ultrasound waves and binding ability for adsorption. Moreover, from the review of recent simulation studies, the effect of ultrasound waves and the electrostatic field was elucidated, which promoted the adsorption capacity. This review can assist in the screening of classified adsorbents for wastewater treatment using a fast and cheap approach while helping us understand the adsorption process from an atomistic perspective.