• Energy Research

The removal of arsenic from water by adsorption is currently hindered by the elevated cost of conventional adsorbent materials. To overcome this limit, an innovative iron-coated adsorbent was produced by hydrothermal carbonization (170 °C, 30 min) of olive pomace, an inexpensive byproduct of the olive oil production. Hydrothermal carbonization experiments were performed starting from olive pomace dispersions in solutions with acidic, neutral and alkaline pH, in presence and absence of FeCl3. Acidic conditions improved the carbonization, ensuring reduced H/C and O/C ratios, and increased the adsorbent stability. However, acidic pH yielded unsatisfactory iron coating, with only 32% of the iron dissolved in the initial solution transferred to the produced hydrochar. Under alkaline pH, 96% of the iron in the feedwater was, in contrast, stably dispersed over the hydrochar surface, giving the highest maximum arsenic adsorption capacity (4.1 mg/g). However, alkaline pH promoted biomass hydrolysis, causing the loss of 60% and 87% of the total C and N, respectively, and reducing the stability of the produced hydrochar. A two-stage process was tested to overcome these issues, including hydrothermal carbonization under acidic pH with FeCl3, followed by the addition of NaOH. This process prevented biomass hydrolysis yielding a stable hydrochar. However, as compared to the one-stage alkaline synthesis, the two-stage process produced an hydrochar with reduced arsenic adsorption capacity (1.4 mg/g), indicating that biomass hydrolysis could positively influence hydrochar adsorption characteristics, possibly by increasing the specific surface area. Indications are then provided on how to optimize the two-stage process in order to produce a hydrochar with both satisfactory stability and arsenic adsorption capacity.

The microbial ability to accumulate biomolecules is fundamental for different biotechnological applications aiming at the production of biofuels, food and bioplastics. However, high accumulation is a selective advantage only under certain stressful conditions, such as nutrient depletion, characterized by lower growth rate. Conventional bioprocesses maintain an optimal and stable environment for large part of the cultivation, that doesn't reward cells for their accumulation ability, raising the risk of selection of contaminant strains with higher growth rate, but lower accumulation of products. Here in this work the physiological responses of different microorganisms (microalgae, bacteria, yeasts) under N-starvation and energy starvation are reviewed, with the aim to furnish relevant insights exploitable to develop tailored bioprocesses to select specific strains for their higher accumulation ability. Microorganism responses to starvation are reviewed focusing on cell cycle, biomass production and variations in biochemical composition. Then, the work describes different innovative bioprocess configurations exploiting uncoupled nutrient feeding strategies (feast-famine), tailored to maintain a selective pressure to reward the strains with higher accumulation ability in mixed microbial populations. Finally, the main models developed in recent studies to describe and predict microbial growth and intracellular accumulation upon N-starvation and feast-famine conditions have been reviewed.

The aviation industry is a “hard to electrify” sector with limited possibilities for alternative fuels such as green hydrogen. Therefore, there is a call for sustainable fuel alternatives which is driving intensive research into bio-jet fuels (BJF) that could be used in aviation, without revamping the aircraft fleets and the aviation infrastructures. This review evaluates the potential of photosynthetic microorganisms, focusing on the most challenging downstream processes, i.e. bio-oil production and its catalytic upgrade. The main bottlenecks of such processes are identified, and the most recent solutions, offered to overcome them, are critically reviewed. Finally, a statistical analysis of the key characteristics of current BJF derived from microalgae and cyanobacteria is presented, along with a discussion on their suitability for the aviation industry and the primary areas for improvement.

Much of our current knowledge of microbial growth is obtained from studies at a population level. Driven by the realization that processes that operate within a population might influence a population's behavior, we sought to better understand Tetradesmus obliquus (formerly Scenedesmus obliquus) physiology at the cellular level. In this work, an accurate pretreatment method to quantitatively obtain single cells of T. obliquus, a coenobia‐forming alga, is described. These single cells were examined by flow cytometry for triacylglycerol (TAG), chlorophyll, and protein content, and their cell sizes were recorded by coulter counter. We quantified heterogeneity of size and TAG content at single‐cell level for a population of T. obliquus during a controlled standard batch cultivation. Unexpectedly, variability of TAG content per cell within the population increased throughout the batch run, up to 400 times in the final stage of the batch run, with values ranging from 0.25 to 99 pg · cell−1. Two subpopulations, classified as having low or high TAG content per cell, were identified. Cell size also increased during batch growth with average values from 36 to 70 μm3 · cell−1; yet cell size variability increased only up to 16 times. Cell size and cellular TAG content were not correlated at the single‐cell level. Our data show clearly that TAG production is affected by cell‐to‐cell variation, which suggests that its control and better understanding of the underlying processes may improve the productivity of T. obliquus for industrial processes such as biodiesel production.

The influence of Ca2+ concentration on the growth of the microalga Scenedesmus sp. in heterotrophic and photoautotrophic cultivations was investigated. Heterotrophic growth was induced by the addition of olive mill wastewaters (9% v·v-1) to the culture. Variations in the calcium concentration affected differently biomass production depending on whether microalgae were cultivated under heterotrophic or photoautotrophic regime. In photoautotrophic regime, increasing the calcium concentration from 20 to 230mg⋅L-1 decreased maximum cell concentration and growth rate. In heterotrophic cultivation, cell concentration and growth rate decreased with Ca2+ concentration increasing from 20 to 80mg⋅L-1 but then increased with Ca2+ concentration increasing to 230mg⋅L-1. Increasing calcium concentration invariably promoted cell aggregation. The precipitation of calcium phosphates can explain the decreasing growth rate and cell concentration attained with increasing calcium concentration, while the influence of Ca2+ concentration on the adsorption of phenols on suspended solids can explain the enhanced growth attained at large Ca2+ concentration under heterotrophic regime. Implications of the illustrated results for industrial scale application of microalgae are thoroughly discussed.

Hydrothermal carbonization (HTC) is a promising technology for producing char material (hydrochar) from waste biomass. In the present work, a two-stage process was applied and optimized to obtain a composite Fe-loaded hydrochar effective in removing arsenic from water. The first stage of carbonization of the biomass in acid conditions was followed by loading Fe3O4 in the second stage into the hydrochar by alkaline co-precipitation. The effect on the kinetics and on the final yield of HTC induced by a variation of the initial acid pH (5.6, 2.0, and 0.5) was tested. Biomass hydrolysis initially decreased the hydrochar yield and released soluble organic species, responsible for the observed pH variation. This effect was more remarkable at the lower initial pH tested. Soluble organic compounds eventually underwent polymerization, with secondary char formation, an inversion of the pH trend and an increase of hydrochar yield and C%. The final pH attained was linearly related to the hydrochar C%, O/C ratio, and initial pH. Better carbonization performances were achieved at pH = 0.5, 200 °C, and 30 min reaction time, which resulted in 53% mass yield and 72 C%. This value is larger than those previously reported for processes conducted at higher temperatures, and it shows how the addition of acid allows working at lower operative temperatures. Fe loading gave better yields at lower hydrochar concentrations, producing an adsorbent with up to 74% Fe3O4, which adsorbed 2.67 mg/g arsenic. Its adsorption capacity was remarkably affected by the stirring method used, indicating that particle-to-particle interactions considerably influence the process. This effect should be better studied for improved applications in fixed-bed columns.

Olive oil is one excellence of the Italian food industry: around 300 kt yr−1 are produced, creating roughly the same amount of olive mill wastewater (OMW) to be disposed of. The present work describes a process to exploit OMW by converting its organic compounds to valuable gaseous biofuel. A sample OMW was characterized (COD, TOC, solids, and polyphenols) and submitted to membrane filtration tests to concentrate the organic compounds. Based on the results of the experiments, a treatment process was outlined: the retentate streams from microfiltration and ultrafiltration steps were fed to a cracking and a steam reforming reactor, respectively; the obtained syngas streams were then mixed and sent to a methanation reactor. The process was simulated with Aspen Plus (AspenTech©) software, assessing operating conditions and streams compositions: the final biofuel is around 81 mol.% methane, 4 mol.% hydrogen, and 11 mol.% carbon dioxide. The permeate stream cannot be directly disposed of, but both its amount and its polluting charge are greatly reduced. The heat needed by the process, mainly due to the endothermic reactions, can be obtained by burning an amount of olive pomaces, roughly corresponding to one-third of the amount left by olive treatments giving rise to the processed OMW feed.