Formation Of Secondary Pollutants Research Articles

Nanozeolites from thermoelectric waste for application to wastewater treatment: review

Along the years, nanozeolites have been proved extremely efficient in wastewater treatment. These nanostructured materials have a series of applications such as nanoadsorption, nanocatalysts to heterogenous photocatalyst, membrane separation processes and Fenton-like, showing a good removal of inorganic and organic pollutants. However, conventional synthesis of nanozeolite have some limitations, such as the formation of secondary pollutants by using organic supports or prolonged reaction times. In this context, the present work aims to provide an overview about the application of nanozeolites from coal fly ash in wastewater treatment. For this purpose, Science Direct platform was used, with the descriptors “Zeolite” AND “fly ash” AND “organic pollutants” AND “wastewater”, according to Boolean logic, and limiting to the last 5 years (from January 2015 to September 2020). Then, 22 articles were found being 70% of these studies corresponding to the nanoadsorption technology, 15% to Fenton-like treatment, 10% membrane separation processes, and only 5% to application in heterogenous photocatalysis, like supported catalysts. Therefore, it is possible to identify a wide application of these materials for application in wastewater treatment, being a potential alternative to green nanotechnology with sustainable development.

Nitrogen Oxide Soil Emission Measurements Using Passive Samplers and Static Chamber Method

Nitrogen oxides play a major role in atmospheric chemistry, like primary pollutants, in the formation of secondary air pollutants or greenhouse gases (GHGs). This research study was conducted in the Western Pannonian sub-region of Croatia with the aim to determine the suitability of our internally developed passive sampler and static chamber method for N-NO2 concentration measurement. The aim was also to determine the impact of mineral soil fertilization on the N-NO2 flux during triticale vegetation. The research showed that the method used was suitable. Average daily N-NO2 flux ranged from 2.78 to 5.09 mg ha–1 day–1 depending on phenophase and treatment. Statistically significant differences in N-NO2 flux between two monitored treatments (300 kg N ha–1 and 0 kg N ha–1) were not observed, nor between two investigated phenophases.

Magnetic Hydroxyapatite for Batch Adsorption of Heavy Metals

In this work, magnetic hydroxyapatite or hydroxyapatite-iron (III) oxide (HAp-Fe3O4) composite was used as the adsorbent of heavy metals and the performance was evaluated using the batch test. The presence of heavy metals in the effluent from wastewater discharge can be toxic to many organisms and can even lead to eye burns. Therefore, hydroxyapatite synthesized from the chemical precipitation of calcium nitrate tetrahydrate and diammonium hydrogen phosphate solutions is used to remove heavy metal in aqueous media. Magnetic properties of Fe3O4 can help prevent formation of secondary pollutants caused by the loss of adsorbent. The synthesized HAp-Fe3O4 can remove cadmium, zinc and lead effectively, which is up to 90% removal. Reusability study shows that the adsorbent could retain heavy metal ions even after four cycles. The percentage removal of heavy metals maintains at around 80% after four times of usage. The composite of HAp-Fe3O4 demonstrates good performance and stability which is beneficial for heavy metal removal in the future.

Simulation of primary and secondary particles in the streets of Paris using MUNICH.

High particle concentrations are observed in the streets. Regional-scale chemistry-transport models are not able to reproduce these high concentrations, because their spatial resolution is not fine enough. Local-scale models are usually employed to simulate the high concentrations in street networks, but they often adopt substantial simplifications to determine background concentrations and use simplified chemistry. This study presents the new version of the local-scale Model of Urban Network of Intersecting Canyons and Highways (MUNICH) that integrates background concentrations simulated by the regional-scale chemistry-transport model Polair3D, and uses the same complex chemistry module as Polair3D, SSH-aerosol, to represent secondary aerosol formation. Gas and aerosol concentrations in Paris streets are simulated with MUNICH, considering a street-network with more than 3800 street segments, between 3 May and 30 June. Comparisons with PM10 and PM2.5 measurements at several locations of Paris show that the high PM10 and PM2.5 concentrations are well represented. Furthermore, the simulated chemical composition of fine particles corresponds well to a yearly measured composition. To understand the influence of the secondary pollutant formation, several sensitivity simulations are conducted. Simulations with and without gas-phase chemistry show that the influence of gas-phase chemistry on the formation of NO2 is large (37% on average over May and across all modelled streets), but the influence on condensables is lower (less than 2% to 3% on average at noon for inorganics and organics), but may reach more than 20% depending on the street. The assumption used to compute gas/particle mass transfer by condensation/evaporation is important for inorganic and organic compounds of particles, as using the thermodynamic equilibrium assumption leads to an overestimation of the organic concentrations by 4.7% on average (up to 31% at noon depending on the streets). Ammonia emissions from traffic lead to an increase in inorganic concentrations by 3% on average, reaching 26% depending on the street segments. Not taking into account gas-phase chemistry and aerosol dynamics in the modelling leads to an underestimation of organic concentrations by about 11% on average over the streets and time, but this underestimation may reach 51% depending on the streets and the time of the day.

Impacts of chlorine emissions on secondary pollutants in China

The identified components of fine particulate matter with dynamic diameters less than 2.5 μm (PM2.5) suggests the remarkable increase in Cl− in PM2.5 during autumn and winter over North China. Anthropogenic chlorine emissions, including those from biomass burning, coal combustion and biofuel burning, are the main sources of Cl−, which could potentially affect the formation of secondary pollutants in the troposphere. In this study, we resolve the effect of chlorine emissions on PM2.5 and O3 by using the Weather Research and Forecasting (WRF) model coupled with Chemistry (WRF-Chem model) with an updated chemistry mechanism that includes the gaseous chemistry of chlorine, N2O5 heterogeneous reactions and detailed aqueous chemistry. Additionally, the biomass burning emissions of chlorine are derived from the Fire Inventory of the National Center for Atmospheric Research (NCAR) (FINN), which are considered the main contributor to atmospheric chloride over inland areas. The improved model is able to simulate the atmospheric Cl− in PM2.5 with reasonable agreement with the available observations in China. In addition, this model reduces the mean bias of ground-level PM2.5 concentrations from −7.5% to 3.2% over the BTH area and from −11.6% to −0.4% over the Northeast China Plain (NEP). The model also slightly improves O3 simulations by reducing the mean bias from 25.3% to 23.9% over mainland China. The anthropogenic chlorine emissions are estimated to increase the monthly average PM2.5 by 7.5 μg/m3 (9.1%), whereas ozone decreases slightly by up to 2.1 ppbv (4.3%) in November 2014. The influence of ozone was more significant in summer, reaching 2–8 ppbv over the biomass burning regions in 2014. The increase in Cl− and NH4+ accounted for the impacts on PM2.5, and the decline in O3 could be explained by the cycle between chlorine radicals and gases and changes in precursor concentrations. Our results suggest the importance of chlorine emissions and their chemical processes in the formation of PM2.5 and O3 over high-emission regions in China.

Important contributions of alkenes and aromatics to VOCs emissions, chemistry and secondary pollutants formation at an industrial site of central eastern China

Volatile organic compounds (VOCs) play important roles in the tropospheric atmosphere. In this study, VOCs were measured at an urban site of Zibo city in September 2018 using an online gas chromatography-time of flight mass spectrometry instrument (GC-TOFMS). Diurnal profile analysis showed that while the removal of most reactive hydrocarbons such as alkanes, small alkenes, and aromatics was governed by OH chemistry, nighttime removal for several highly reactive alkenes by ozone was also significant. The influence of nighttime removal on emission ratio determinations were further explored by parameterization method based on OH radical chemistry/ozone chemistry and comparison of emission ratios with nearby regions. The results revealed that nighttime removal for several alkenes likely leaded to underestimation of emission ratios in previous studies. The emission ratios of aromatics in Zibo were also unexpectedly higher than those at the receptor site in central eastern China. Detailed source analyses of VOCs using a positive matrix factorization (PMF) method suggested important contributions from both petrochemical emission and vehicle exhaust & LPG usage for measured alkenes and dominant contribution from coal combustion for aromatics. Ozone formation potential (OFP) and secondary organic aerosol potential (SOAP) calculation suggested that alkenes were the dominant contributor to OFP (59.4%); while aromatics could produce large amounts of SOA (1.10 ± 0.88 μg m−3) and naphthalene alone dominated (~43.0%) the total SOA estimated from measured VOCs. The results enable improved mitigation strategies and chemical links between direct emissions and SOA/ozone impacts on climate and health in these regions.

Optimization of a volatile organic compound control strategy in an oil industry center in Canada by evaluating ozone and secondary organic aerosol formation potential

Volatile organic compounds (VOCs) play a vital role in the formation of photochemical smog and haze in large urban environments. Previous source apportionment studies have focused on the contribution of different sources to VOC concentration with a view to pinpointing the major culprits for effective emission mitigation. However, different VOC sources may have different ozone (O3) and secondary organic aerosol (SOA) formation potentials. From a control perspective, it would be more rational to consider the role of individual VOC sources in secondary pollution; therefore, here, we propose a tiered source identification method that considers the formation potentials of O3 and SOA, which we applied in Calgary, Alberta, a site under the influence of multiple competing VOC sources. The pollution characteristics, secondary pollutant formation potential, and geographical origin of VOC sources were investigated over a five-year period. Seven major sources were identified using the positive matrix factorization (PMF) model, among which vehicle exhausts and solid fuel combustion were the dominant VOC sources responsible for O3 (60%) and SOA (63%) formation. Combustion of both liquid fuel (gasoline and diesel) and solid fuel (wood and coal) has exceeded the contribution of oil and gas production and become the top contributor to O3 and aerosol pollution in Calgary. This finding is consistent with the significant reduction (32.2–99.8%) in oil and gas production in Calgary over the period of 2013–2017. The source apportionment results show that the primary VOC source has shifted from conventional oil and gas extraction to a mixture of vehicle exhausts and oil and gas extraction, indicating the effectiveness of emission control measures taken in the energy sectors. Moreover, regionally transported VOCs from combustion sources in southeastern British Columbia have greatly increased the VOC level and secondary pollutant formation in Calgary. To effectively alleviate secondary pollution problems, the performance of joint pollution control measures has been suggested by the governments of both Alberta and British Columbia. These findings reveal that the tiered source identification strategy combining the traditional receptor model with socioeconomic factors, emission inventory, and source region analysis is a robust and promising tool for the interpretation of source apportionment results and optimization of secondary pollution control.

Zwitterionic hybrid aerobeads of binary metal organic frameworks and cellulose nanofibers for removal anionic pollutants

Different adsorbents have been applied to remove various anionic pollutants in water treatment processes. However, the development of economic, sustainable, and high-performance adsorbents is still a challenge. Herein, we introduce a new strategy to produce highly porous (98.96%), very low density (0.015 g/cm3), and cost-effective spherical and nanostructured hybrid aerogels, termed aerobeads, from cellulose nanofibers (CNF) and metal-organic frameworks (MOFs). The zwitterionic MOFs@CNF aerobeads (with 10–50 wt% MOF loading) were synthesized via simple dropping of a cross-linked hydrogel containing CNF and a binary mixture of anionic and cationic MOFs in liquid nitrogen, followed by freeze-drying. The flexible aerobeads (diameter of 2–3 mm) with hierarchical porous structure demonstrated an outstanding adsorption capacity toward both diclofenac (121.20 mg/g) and methyl orange (49.21 mg/g). Moreover, the aerobeads were easily collected after use from the solution without any complicated separation methods or the formation of secondary pollutants.

HONO Budget and Its Role in Nitrate Formation in the Rural North China Plain.

Nitrous acid (HONO) is a major precursor of tropospheric hydroxyl radical (OH) that accelerates the formation of secondary pollutants. The HONO sources, however, are not well understood, especially in polluted areas. Based on a comprehensive winter field campaign conducted at a rural site of the North China Plain, a box model (MCM v3.3.1) was used to simulate the daytime HONO budget and nitrate formation. We found that HONO photolysis acted as the dominant source for primary OH with a contribution of more than 92%. The observed daytime HONO could be well explained by the known sources in the model. The heterogeneous conversion of NO2 on ground surfaces and the homogeneous reaction of NO with OH were the dominant HONO sources with contributions of more than 36 and 34% to daytime HONO, respectively. The contribution from the photolysis of particle nitrate and the reactions of NO2 on aerosol surfaces was found to be negligible in clean periods (2%) and slightly higher during polluted periods (8%). The relatively high OH levels due to fast HONO photolysis at the rural site remarkably accelerated gas-phase reactions, resulting in the fast formation of nitrate as well as other secondary pollutants in the daytime.

Effects of forests on particle number concentrations in near-road environments across three geographic regions

Trees and other vegetation have been advocated as a mitigation measure for urban air pollution mainly due to the fact that they passively filter particles from the air. However, mounting evidence suggests that vegetation may also worsen air quality by slowing the dispersion of pollutants and by producing volatile organic compounds that contribute to formation of ozone and other secondary pollutants. We monitored nanoparticle (>10 nm) counts along distance gradients away from major roads along paired transects across open and forested landscapes in Baltimore (USA), Helsinki (Finland) and Shenyang (China) − i.e. sites in three biomes with different pollution levels − using condensation particle counters. Mean particle number concentrations averaged across all sampling sites were clearly reduced (15%) by the presence of forest cover only in Helsinki. For Baltimore and Shenyang, levels showed no significant difference between the open and forested transects at any of the sampling distances. This suggests that nanoparticle deposition on trees is often counterbalanced by other factors, including differing flow fields and aerosol processes under varying meteorological conditions. Similarly, consistent differences in high frequency data patterns between the transects were detected only in Helsinki. No correlations between nanoparticle concentrations and solar radiation or local wind speed as affecting nanoparticle abundances were found, but they were to some extent associated with canopy closure. These data add to the accumulating evidence according to which trees do not necessarily improve air quality in near-road environments.

Electrocoagulation Treatment of Electroplating Wastewater: A Review

Electrochemical methods such as electrocoagulation (EC), electrooxidation (EO), and electroreduction (ER) are promising technologies to remove organics and heavy metals contained in industrial effluents discharged from numerous industries such as electroplating, textile, and metal processing. EC is widely applicable to treat wastewater that has a wide range of pollutants as well as being applicable to treating organic pollutants having a chemical oxygen demand (COD) range of 1,000–13,000 mg/L, and metals concentration in the range of 10–2,500 mg/L depending on the type of pollutants/metals. Several parameters such as pH, current density, electrode gap, electrolysis time, and agitation speed plays a major role during EC. Various laboratory results show that EC works effectively in the pH range 2–8, current density 20–200 A/m2, electrode gap 1.5–3 cm, electrolysis time 10–150 min, and stirrer speed 30–200 rpm for the reactor size 1–2.5 L depending on the type of wastewater/effluent. Moreover, this process has been proved to be versatile and environmentally friendly. This review mainly focuses on the separation of heavy metals from electroplating effluent (EPE), including other pollutants removal from various industrial effluents such as textile, distillery, paint, and tannery. EC operation is oriented on the supply of direct current in metal electrodes and, consequently, metal ions released into the solution. This results in an increased metal concentration in the solution that finally precipitates as metal hydroxides along with pollutants. Numerous techniques are available to treat heavy metals bearing effluent, which includes precipitation, adsorption, bioadsorption, ion exchange, and membrane separation. These methods have several limitations in term of high operating costs and a large amount of sludge generation, while the EC process has several advantages over conventional technologies such as requirement of less external chemicals, easier installation, lower secondary pollutants formation, odor and color removal, and lower residence time. However, high electricity consumption due to formation of oxide film on the surface of the electrode is a major drawback of this process because oxide film works as an electric insulator. Overall, the EC method has wide potential to treat different kinds of effluents.

Reducing the Formation of Secondary Pollutants in the Process of NonCatalytic Purification of Flue Gases from Nitrogen Oxides

The possibilities of the technology of non-catalytic reduction of nitrogen oxides (SNCR) to reduce the possible formation of secondary pollutants: ammonia, carbon monoxide and nitric oxide (I) are considered. The conditions for the process of reduction of nitrogen oxides by the thermal decomposition products of urea with a minimum formation of the above pollutants are determined.

The pollution levels, variation characteristics, sources and implications of atmospheric carbonyls in a typical rural area of North China Plain during winter

Atmospheric carbonyls were measured at a typical rural area of the North China Plain (NCP) from November 13 to December 24, 2017 to investigate the pollution characteristics, sources and environmental implications. Fifteen carbonyls were detected, and formaldehyde, acetaldehyde and acetone accounted for about 81% at most. The concentration of the total carbonyls in heavily polluted days was twice more than that in clean days. In contrast to other carbonyls, m-tolualdehyde exhibited relatively high concentrations in the clean days in comparison with the polluted days. The ratios of three principal carbonyls to CO showed similar daily variations at different pollution levels with significant daytime peaks. Multiple linear regression analysis revealed that the contributions of background, primary and secondary sources to three principal carbonyls showed similar variation trends from the clean level to the heavily polluted level. The OH formation rate of formaldehyde showed a similar variation trend to its photodegradation rate, reaching the peak value at noon, which is important to maintain relatively high OH levels to initiate the oxidation of various gas-phase pollutants for secondary pollutant formation at the rural site. OH radical consumption rate and ozone formation potential (OFP) calculations showed that formaldehyde and acetaldehyde were the dominant oxidative species among measured carbonyls. As for OH radical consumption, n-butyraldehyde and m-tolualdehyde were important contributors, while for ozone formation potential, n-butyraldehyde and propionaldehyde made significant contributions. In addition, the contribution of carbonyl compounds to secondary organic aerosol (SOA) formation was also important and needs further investigation.

Single source impacts on secondary pollutants using a Lagrangian reactive puff model: Comparison with photochemical grid models

Abstract Recent updates to U.S. regulatory guidelines for modeling impacts of single sources to secondary pollutants, such as ozone (O3) and fine particulate matter (PM2.5), require the use of established photochemical models with a complete treatment of gas-phase and aerosol chemistry as the preferred method for a Tier 2 demonstration approach. Such models include photochemical grid models (PGMs), as well as Lagrangian puff models with the requisite treatment of processes governing the formation of secondary pollutants from their precursors. SCICHEM is an open source reactive puff model that includes treatments for gas, aerosol, and aqueous-phase chemistry that are comparable to those in PGMs. The dispersion component of SCICHEM is based on the state-of-the-science puff model, SCIPUFF (Second-order Closure Integrated Puff). The model is a valuable tool that requires fewer resources and setup cost to run than PGMs to calculate the impacts of a single source or a small number of sources on downwind ozone and PM2.5. This paper describes a model inter-comparison study of secondary pollutant impacts due to emissions from a hypothetical coal-fired power plant stack. The models compared in this study include SCICHEM and the two most commonly used PGMs, CMAQ and CAMx. Stacks are located in 4 separate regions of the U.S. with distinct differences in chemical environments and geographical features. The key inputs (hypothetical stack parameters and emission rates, meteorology) to the three models are based on a common dataset for consistency. The source impacts calculated by the PGMs are based on zero-out simulations (i.e., with and without the source of interest) as well as the source apportionment tools available in CAMx, while SCICHEM directly predicts source impacts in a single simulation. Annual simulations are conducted to determine the frequency distribution of source impacts over the 4 regions. The comparison of model results shows both similarities and differences in the frequencies and magnitudes of peak impacts predicted by the three models. There is good agreement among all models in their predictions of 98th and 99th percentile impacts.

One decade of VOCs measurements in São Paulo megacity: Composition, variability, and emission evaluation in a biofuel usage context

In South America, the observations of atmospheric pollutants are deficient, and few cities have implemented air quality monitoring programs. In addition, Volatile Organic Compounds (VOCs) observations are still missing, and little is known about their contributions to the atmospheric composition and impacts in a large ethanol usage context like Brazil. Here, we present a wide range of VOCs that have been measured for ten years in São Paulo Megacity (SPM) in different campaigns at traffic, urban and background sites. When compared with other cities worldwide, the average VOCs ambient concentrations in SPM were higher by factors of 2 to 10. However, the ambient VOCs distribution among these cities is homogeneous even for ethanol, aldehydes and alkenes species. Emission ratios (ER) were established related to carbon monoxide and acetylene, which did not depict strong seasonal and interannual variability in SPM. When compared with previous studies, ERs showed an enrichment from road-tunnel to background, suggesting the presence of other sources than traffic. A good agreement in ER was found with Los Angeles and Paris; but limited consistencies with Middle East and Asia cities. Our ethanol measurements show that contrasted ER can be obtained depending on the emission process involved, with a strong impact of evaporation on ethanol concentrations. The multiyear acetaldehyde analysis displayed that ER could be a valuable metric to assess the long-term changes in emissions sources. Finally, VOCs emissions were calculated from ER and compared with those estimated by the global emission inventory (Edgar). The total VOC emissions estimated by the global inventory agree very well with those from our observations up to 75%. Nevertheless, the VOCs speciation is misrepresented in the inventory, mainly for oxygenated and heavier alkanes compounds. These inconsistencies will also have an impact on the quantification of secondary atmospheric pollutants formation associated to road transport emissions.

Hydrodynamic cavitation based advanced oxidation processes: Studies on specific effects of inorganic acids on the degradation effectiveness of organic pollutants

Abstract The use of cavitation in advanced oxidation processes (AOPs) to treat acidic effluents and process water has become a promising trend in the area of environmental protection. The pH value of effluents – often acidified using an inorganic acid, is one of the key parameters of optimization process. However, in the majority of cases the effect of kind of inorganic acid on the effectiveness of degradation is not studied. The present study describes the results of investigations on the use of hydrodynamic cavitation (HC) for the treatment of a model effluent containing 20 organic compounds, representing various groups of industrial pollutants. The effluent was acidified using three different mineral acids. It was demonstrated that the kind of acid used strongly affects the effectiveness of radical processes of oxidation of organic contaminants as well as formation of harmful secondary pollutants. One of important examples is a risk of formation of p-nitrotolune. Sulfuric acid was the only chemical used for acidification which caused effective treatment with lack of formation of monitored type of secondary pollutants. The best treatment effectiveness – during a 6-hour cavitation process - in most cases much above 80% along with 90% TOC removal was obtained in the case of sulfuric acid. Nitric acid provided lower effectiveness (above 60% for most of the compounds). The worst performance are reported for hydrochloric acid – below 50% of degradation for most of the compounds.

Minimizing secondary pollutant formation through identification of most influential volatile emissions in gasoline exhausts: Impact of the vehicle powertrain technology

Despite the growing share of hybrid vehicles in worldwide vehicle fleet, the impact of battery/engine switch on secondary pollutant precursors formation remains poorly understood. We herein identify the species to be removed primarily from the exhaust of hybrid vehicles to minimize secondary pollutants formation. Pollutants from a conventional (thermal) and a hybrid vehicle are herein evaluated in a chassis dynamometer bench by using two widely used driving cycles, WLTC and FTP75. Hybrid vehicle exhibits the lowest regulated emissions during all the different tests, except for NOx. The results reveal that low and cold start phases of the cycles contribute mainly to the ozone potential, although methane, butadiene and formaldehyde can be emitted at the tailpipe all along the driving cycle. We evidence for the first time a significant contribution of gas-phase reactions to butadiene and methane production inside the exhaust pipe, especially during warm restarts and accelerations. This study indicates that pollutant traps should target primarily monoaromatics which contribute typically to 40% of photochemical ozone creation and 50% to Secondary Organic Aerosols (SOA). However, the contribution of alkanes to ozone differs markedly for the hybrid vehicle, reflecting a different share of ozone production paths with respect to conventional vehicles.

Airborne microorganisms exacerbate the formation of atmospheric ammonium and sulfate

Haze pollution is inseparable from the transformation of air pollutants especially the ammonium and sulfate. Chemical and physical processes play important roles in this transformation. However, the role of microbial processes has rarely been studied. In this report, we applied the cultivation-independent metagenomic approach to study airborne microorganisms, investigating the potential microbial-catalyzed transformation of ammonium and sulfate in PM2.5 samples. Functional genes predict that airborne microorganisms have the potential to catalyze ammonium formation but not ammonium oxidation since no ammoxidation genes were identified. We also found that the frequency of sulfate-forming genes was 1.56 times of those for sulfate-reducing genes. It was speculated that microbial metabolisms in the atmosphere could contribute to the accumulation of ammonium and sulfate. With the increase of PM2.5 concentration, the frequency of functional genes and the relative abundance of genera which involved in nitrogen and sulfur metabolisms increased. That suggested air pollution was conducive to the microbial-mediated formation of ammonium and sulfate. Overall, our results provided evidence for the possible role of microbial processes in the air pollutant transformation and brought a new perspective for studying the formation of secondary air pollutants.

A review on adsorptive remediation of Cr (VI) by magnetic iron oxides and their modified forms

Cr (VI) is highly toxic, carcinogenic and bio accumulative to living organisms. Its extensive industrial applications contaminate the natural aqueous ecosystems. Different techniques such as photocatalysis, adsorption, reverse osmosis, ion exchange, chemical precipitation, electrochemical reduction, solvent extraction etc. have been investigated to remove hexavalent chromium from contaminated water. Among these, adsorption is deliberated as the most effective and promising process because it is easy to design, simple to operate and suitable to avoid formation of secondary pollutants. Various adsorbents such as clay minerals, biosorbents, industrial wastes and magnetic nanomaterials have been used for the remediation of Cr (VI). The use of magnetic nanomaterials as adsorbents for removal of heavy metals has attracted much attention in recent years because of their large surface area and superparamagnetic characteristics. These materials not only exhibited high adsorption capacity but also could easily be recycled by applying an external magnetic field, making the separation process much easier. In this review, the adsorption behavior of different iron oxides such as maghemites, magnetites, ferrites and their modified forms towards the removal of Cr (VI) was discussed. The present review focuses on adsorption mechanism of Cr (VI) through the evaluation of adsorption capacities. Kinetics, thermodynamics and equilibrium studies for the adsorption process were depicted briefly. Optimum adsorption conditions (initial Cr (VI) concentration, pH, contact time, adsorbent dose and temperature), thermodynamic properties (ΔHo, ΔGo, ΔSo), kinetic model, fitted isotherm, adsorption mechanism and maximum adsorption capacities were listed in the table.

Estimating economic and environmental benefits of urban trees in desert regions.

Trees in urban areas have a significant impact on air quality and other environmental issues. Trees can affect the concentration of air pollutants that we breathe in by directly removing pollutants or avoiding emissions and secondary pollutant formation in the atmosphere. In addition, trees have other benefits including increasing property value, intercepting storm water runoff and saving energy needed for cooling of buildings in hot seasons. In this work, we estimate economic and environmental benefits of three tree species typical for desert regions such as Acacia tortilis, Ziziphus spina-christi and Phoenix dactylifera. The benefits varied by species with Acacia tortilis having the highest overall benefits, mostly because of its large leaf surface area and canopy shape. Tree benefits from carbon reduction reached up to US $14 billion annually. Mature trees tended to be more beneficial than smaller trees for improving environmental conditions. The location of trees had minimal impact on the overall economic value. This assessment provides urban planners, foresters, and developers in desert regions with the information needed to make informed decisions on the economic and environmental benefits of urban tree planting.