Phytoremediation and phycoremediation are cost-effective and environmentally sound technologies for the treatment of polluted streams and wastewaters contaminated with metals. Currently, the most commonly used parameter to assess the metal uptake of biomass is (q) expressed as mg metal g dry weight(-1). By contrast, the bioconcentration factor (BCF) is one of the most widely used factors to evaluate the metal uptake capacity of macrophytes. However, both parameters the metal uptake (q) and the BCF cannot be applied to differentiate between the ability of live plants or photosynthetic microorganisms to adsorb the metal onto their surface through passive mechanisms or to accumulate the contaminant at intracellular level through metabolically active mechanisms. This mini review has the objective of discussing the need to differentiate between bioadsorption and bioaccumulation of metals in live plants and photosynthetic microorganisms used in phytofiltration and phycoremediation processes, respectively. The use of two specific factors, the bioadsorption factor (BAF) and the intracellular accumulation factor (IAF) that have been previously reported in order to make a clear differentiation between these two metal removal mechanisms in Salvinia minima and Leptolyngbya crossbyana is highlighted. It is suggested that the BAF and the IAF can be used in phytofiltration wetlands and phycoremediation lagoons, where there is the need of specific information indicating the fate of the metal in order to gain information about possible removal mechanisms. These factors could also provide a tool to decide whether it is possible to harvest the biomass and to recover a fair amount of metal adsorbed onto the surface by means of desorbent agents. A critical assessment of the use of EDTA as desorbent agent is also included.

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The effects of environmental factors and nutrients on the various possible removal mechanisms (surface adsorption, intracellular accumulation and precipitation to sediments) and partitioning of lead among various compartments (plant biomass, water column and sediments) in Salvinia minima batch-operated lagoons, were evaluated. Surface adsorption was found to be the predominant mechanism for Pb(II) removal under all environmental conditions tested in the absence of nutrients (an average of 54.3%) and in a nutrient medium (modified Hutner 1/10 medium) free of EDTA and phosphates (54.41%) at "high" initial Pb(II) concentrations (in the range of 10.3+/-0.13 to 15.2+/-0.05 mg/L). Under these conditions, the bioconcentration factors (BCFs) were 2,431+/-276 and 2,065+/-35, respectively. Lead removal was very rapid during the first 4 h and reached 70% in the absence of nutrients at the "medium" light intensity and temperature (LIT) tested, 88% in nutrient medium free of EDTA and supplemented with synthetic wastewater (at the "lowest" LIT tested), and 85% in medium free of EDTA and phosphates. It was concluded that the mechanisms of lead removal by S. minima, and the compartmentalization of this metal in the microcosm of batch-operated lagoons, are primarily a function of the presence of certain nutrients and chelants, with secondary dependence on environmental conditions. In addition, the results indicate that the percentage of lead removed is only a gross parameter and that the complementary use of BCF and compartmentalization analysis is required to gain a full insight into the metal removal process.

Salvinia minima has been reported as a cadmium and lead hyperaccumulator being the adsorption and intracellular accumulation the main uptake mechanisms. However, its physicochemical properties, the effect of metal concentration and the presence of organic and inorganic compounds on its hyperaccumulating capacity are still unknown. Furthermore, the specific adsorption and accumulation mechanisms occurring in the plant are not clear yet. Thus, based on a compartmentalization analysis, a bioadsorption (BAF) and an intracellular accumulation factor (IAF) were calculated in order to differentiate and quantify these two mechanisms. The use of kinetic models allowed predicting the specific type of uptake mechanisms involved. Healthy plants were exposed to five lead concentrations ranging from 0.80± 0.0 to 28.40±0.22 mg Pb 2+ l −1 in batch systems. A synthetic wastewater, amended with propionic acid and magnesium sulfate, and deionized water were used as media. The BAF and IAF contributed to gain an indepth insight into the hyperaccumulating lead capacity of S. minima. It is clear that such capacity is mainly due to adsorption (BAF 780-1980) most likely due to its exceptional physico-chemical characteristics such as a very high surface area (264 m 2 g −1 ) and a high content of carboxylic groups (0.95 mmol H + g −1 dw). Chemisorption was predicted as the responsible mechanism according to the pseudo-second order adsorption model. Surprisingly, the ability of S. minima to accumulate the metal into the cells (IAF 57-1007) was not inhibited at concentrations as high as 28.40±0.22 mg Pb 2+ l −1 .

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Salvinia minima combines several advantages for being used in aquatic phytoremediation. The objectives of this work were to compare the growth kinetics and productivity of S. minima and Spirodela polyrrhiza in high-strength synthetic organic wastewater (HSWW) and to evaluate the growth characteristics of S. minima in various culture media, including anaerobic effluents from pig wastewater (PWAE). It was found that the Relative Growth Rate (RGR) of S. minima was significantly higher (p<0.05) compared to the RGR of S. polyrrhiza in Hutner Medium (HM) and in HSWW. Also, S. minima showed a 1.5 fold productivity and a 2.3 fold productivity, compared to S. polyrrhiza in HM and HSWW, respectively. Diauxic growth of S. minima was observed preferentially under pH control and there was a simultaneous consumption of two nitrogen sources. Productivity of S. minima was similar in pig waste anaerobic effluents (PWAE) and in HM without ammonium nitrate and amended with ammonium sulphate (MHM+AS), at an initial NH 4 concentration of 35 mg l −1 . Above this level, the productivity was found to decrease as the initial ammonium concentration increased, in both media. Growth was completely inhibited at 140 mg l −1 in the PWAE. In summary, S. minima is a better option than S. polyrrhiza for treating high-strength organic wastewater and lagoons should be operated at a maximum initial ammonium-nitrogen concentration of 70 mg l −1 and at a pH of 5.0 or 6.0. Likewise, the initial density should be maintained in the range of 7 to 15 g dw m −2 .

Microalgae have demonstrated a large potential in biotechnology as a source of various macromolecules (proteins, carbohydrates, and lipids) and high-added value products (pigments, poly-unsaturated fatty acids, peptides, exo-polysaccharides, etc.). The production of biomass at a large scale becomes more economically feasible when it is part of a biorefinery designed within the circular economy concept. Thus, the aim of this critical review is to highlight and discuss challenges and future trends related to the multi-product microalgae-based biorefineries, including both phototrophic and mixotrophic cultures treating wastewater and the recovery of biomass as a source of valuable macromolecules and high-added and low-value products (biofertilizers and biostimulants). The therapeutic properties of some microalgae-bioactive compounds are also discussed. Novel trends such as the screening of species for antimicrobial compounds, the production of bioplastics using wastewater, the circular economy strategy, and the need for more Life Cycle Assessment studies (LCA) are suggested as some of the future research lines.

An overview of the state of the art in phytofiltration of nutrients and heavy metals (HMs) from wastewaters using tropical and subtropical plants in constructed wetlands (CWs) and lagoons is presented. Various mechanisms to remove these pollutants are discussed, in regard to three different types of systems: surface flow constructed wetlands (SFCWs), subsurface flow constructed wetlands (SSFCWs), and lagoons with floating plants. Only recent reports at laboratory, pilot and full scale, especially in tropical regions, are discussed. Most of the experiences around the world have shown that these systems are efficient and high removal percentages have been reported for both, nutrients and metals. However, there are still several unsolved or partially understood issues. Long-term studies at the mesocosms or large scale, in order to gain a full insight of the various mechanisms occurring in each system, are required. The understanding of the fate or compartmentalization of the pollutants in these complex artificial ecosystems, especially in the case of HMs, will permit us to establish the frequency of harvesting and the advantages of the use of specific species. The huge bio-diversity that is commonly found in tropical and subtropical regions represents a challenge for finding new species with outstanding characteristics for tolerance to toxic and recalcitrant pollutants or to extreme environmental conditions, such as high temperature or salinity.