An aldol condensation is a condensation reaction in organic chemistry in which two carbonyl moieties (of aldehydes or ketones) react to form a β-hydroxyaldehyde or β-hydroxyketone (an aldol reaction), and this is then followed by dehydration to give a conjugated enone. The overall reaction is as follows (where the Rs can be H):
sn sanyal organic chemistry pdf 93
Heavy metals are well-known environmental pollutants due to their toxicity, persistence in the environment, and bioaccumulative nature. Their natural sources include weathering of metal-bearing rocks and volcanic eruptions, while anthropogenic sources include mining and various industrial and agricultural activities. Mining and industrial processing for extraction of mineral resources and their subsequent applications for industrial, agricultural, and economic development has led to an increase in the mobilization of these elements in the environment and disturbance of their biogeochemical cycles. Contamination of aquatic and terrestrial ecosystems with toxic heavy metals is an environmental problem of public health concern. Being persistent pollutants, heavy metals accumulate in the environment and consequently contaminate the food chains. Accumulation of potentially toxic heavy metals in biota causes a potential health threat to their consumers including humans. This article comprehensively reviews the different aspects of heavy metals as hazardous materials with special focus on their environmental persistence, toxicity for living organisms, and bioaccumulative potential. The bioaccumulation of these elements and its implications for human health are discussed with a special coverage on fish, rice, and tobacco. The article will serve as a valuable educational resource for both undergraduate and graduate students and for researchers in environmental sciences. Environmentally relevant most hazardous heavy metals and metalloids include Cr, Ni, Cu, Zn, Cd, Pb, Hg, and As. The trophic transfer of these elements in aquatic and terrestrial food chains/webs has important implications for wildlife and human health. It is very important to assess and monitor the concentrations of potentially toxic heavy metals and metalloids in different environmental segments and in the resident biota. A comprehensive study of the environmental chemistry and ecotoxicology of hazardous heavy metals and metalloids shows that steps should be taken to minimize the impact of these elements on human health and the environment.
Contamination of sediments with heavy metals is an environmentally important issue with consequences for aquatic organisms and human health. Sediments act as the main pool of metals in the aquatic environment. Their quality can indicate the status of water pollution [47]. Sediments serve as both sink and source of heavy metals, releasing them into the water column [48]. Continuing deposition of heavy metals in sediments can also lead to contamination of groundwater with these pollutants [49]. The adsorption, desorption, and subsequent concentrations of heavy metals in sediments are affected by many physicochemical factors such as temperature, hydrodynamic conditions, redox state, content of organic matter and microbes, salinity, and particle size [50]. Distribution of heavy metals in sediments is affected by chemical composition of the sediments, grain size, and content of total organic matter (TOM) [51]. An important determinant of metal bioavailability in sediments is pH. A lowering in pH increases the competition between metal ions and H+ for binding sites in sediments and may result in dissolution of metal complexes, thereby releasing free metal ions into the water column [52]. Higher concentrations of toxic heavy metals in riverine sediments may pose ecological risk to benthos (bottom-dwelling organisms) [53].
Since heavy metals are persistent in the environment, they enter from the environment to the organisms and accumulate therein. As mentioned earlier, the uptake and bioaccumulation of heavy metals in biota depend on several factors. For example, the uptake of heavy metals in plants depends on bioavailability of the metal in soil, which in turn depends on several factors such as metal speciation, pH, and organic matter contents in soil. Metals which are more bioavailable in soil may be accumulated in plants more easily and thus will have more bioaccumulation potential. An assessment of bioaccumulation of heavy metals in plants may be used for an estimation of bioavailability of the metals in soil. Such an assessment may also be used for knowing the contamination status of the environment. It has been reported that plants seem to be more sensitive to environmental changes than soils [68]. Different plant species have been suggested as bioindicators of heavy metal pollution in the environment. Different animal species have also been suggested as bioindicators of heavy metal pollution. For example, the date mussel (Lithophaga lithophaga) has been suggested as a valid bioindicator of marine pollution [69].
Bioaccumulation of toxic heavy metals in cigarette tobacco is a human health concern because tobacco leaves are used for making cigarettes. Tobacco plants naturally accumulate relatively high concentrations of heavy metals in their leaves, and the metal bioaccumulation in tobacco leaves varies with the geographical origin of the tobacco plants [94]. Tobacco crop is grown with application of commercial inorganic fertilizers especially phosphate fertilizers, which contain considerable concentrations of some toxic heavy metals. During growth, uptake of heavy metals by tobacco roots is considerable and the same are translocated from soil to leaves. During cigarette smoking, a fraction of heavy metals is inhaled in the smoke and thus reaches the lungs of the smoker. Tobacco smoke, both mainstream and side stream, is an important source of environmental metal exposure. Passive smoking has an important contribution in the exposure of children to Pb [95]. Heavy metals inhaled during tobacco smoking are easily absorbed in the body from the lungs and reach the blood from where they may reach other parts of the body. Higher levels of toxic heavy metals have been reported in blood of cigarette smokers compared to that of nonsmokers.
Heavy metals and metalloids are ubiquitous environmental pollutants in both aquatic and terrestrial ecosystems. The hazard of an environmental chemical is a function of its environmental persistence, toxicity, and bioaccumulative potential. Toxic environmental chemicals which are persistent and bioaccumulative are more hazardous. Heavy metals are considered hazardous due to these three characteristics: persistence, bioaccumulation, and toxicity (PBT). Environmentally relevant most hazardous heavy metals and metalloids include Cr, Ni, Cu, Zn, Cd, Pb, Hg, and As. The trophic transfer of these elements in aquatic and terrestrial food chains/webs has important implications for wildlife and human health. It is very important to assess and monitor the concentrations of potentially toxic heavy metals and metalloids in different environmental segments as well as in the resident biota. A comprehensive study of the environmental chemistry and ecotoxicology of hazardous heavy metals and metalloids shows that steps should be taken to minimize the impact of these elements on human health and the environment. The following recommendations are made:(i)Background concentrations of heavy metals and metalloids should be documented in the different environmental media around the world, for later use as a reference.(ii)The levels of potentially toxic heavy metals and metalloids in water, sediments, soils, and the resident biota should be assessed and monitored regularly.(iii)Regular surveys should be conducted to record the per capita daily consumption of freshwater fish and other food items such as rice by the resident population around the world. Such data will be valuable for a more accurate and reliable human and ecological risk assessment.(iv)Efforts should be made to minimize heavy metal contamination in aquatic and terrestrial ecosystems to safeguard the biota and the health of their consumers.(v)The public should be educated about the harmful effects of toxic heavy metals on human health and the environment.(vi)Wastewaters from industries should be treated effectively before their discharge into the natural water bodies.(vii)Scientific research on environmental assessment of toxic chemicals including toxic heavy metals and metalloids should be encouraged and promoted through allocation of appropriate funds for protecting human health and the environment.
Originating from the desire to improve sustainability, producing fuels and chemicals from the conversion of biomass and waste plastic has become an important research topic in the twenty-first century. Although biomass is natural and plastic synthetic, the chemical nature of the two are not as distinct as they first appear. They share substantial structural similarities in terms of their polymeric nature and the types of bonds linking their monomeric units, resulting in close relationships between the two materials and their conversions. Previously, their transformations were mostly studied and reviewed separately in the literature. Here, we summarize the catalytic conversion of biomass and waste plastics, with a focus on bond activation chemistry and catalyst design. By tracking the historical and more recent developments, it becomes clear that biomass and plastic have not only evolved their unique conversion pathways but have also started to cross paths with each other, with each influencing the landscape of the other. As a result, this Review on the catalytic conversion of biomass and waste plastic in a unified angle offers improved insights into existing technologies, and more importantly, may enable new opportunities for future advances.
Esters are formed via condensation of alcohols and carboxylic acids, thus are prone to be cleaved following nucleophilic attack. Various nucleophilic reagents have been evaluated to break down the ester linkage. As these reagents are normally used in large excess and also act as the reaction solvent, this process is conventionally called solvolysis (Fig. 4d). Solvolysis of polyesters leads to high conversion into the constituent monomers, and therefore has attracted substantial industrial interest. In parallel, transesterification of triglyceride is one of the most practiced biomass utilization technologies. By using a simple alcohol (methanol or ethanol) in the presence of inorganic basic (for example, NaOH and KOH) or acidic (for example, H2SO4 and HCl) catalysts, biodiesel (fatty acid methyl or ethyl ester) can be generated in quantitative yield167,168.
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