Abiotic transformation of chlorinated organics at the active surface of iron-bearing minerals in soils and sediments

Chlorinated organic compounds are emerging pollutants of widespread concern because of their toxicity,bioaccumulation,persistence,and lack of adequate regulatory measures.Their abiotic transformation,facilitated by iron-bearing minerals,is critical to their natural dissipation in soils and sediments.However,further exploration is needed to understand their underlying mechanisms and potential engineering applications under different redox conditions.This paper reviews the abiotic transformation behaviors and mechanisms of chlorinated organics at the active surface of iron-bearing minerals under anoxic and oxic conditions and summarizes the strategies for enhancing the abiotic transformation efficiency of chlorinated organics.The abiotic transformation rate under oxic conditions can be a few orders of magnitude higher than that under anoxic conditions.Under anoxic conditions,chlorinated organics undergo reductive dechlorination through reductive elimination,hydrogenolysis,dehydrohalogenation,and nucleophilic substitution.A close relationship between the abiotic transformation of chlorinated organics and the production of hydroxyl radicals by iron-bearing minerals under oxic conditions was discovered.Synthetic active iron-bearing minerals,carbonaceous materials,and biological synergy can facilitate abiotic dechlorination under anoxic conditions.Meanwhile,the regulation of redox conditions,the introduction of ligands,and the utilization of coexisting anions are proposed to enhance oxidative degradation.This study is expected to improve the comprehension of the abiotic degradation of chlorinated organics mediated by iron-bearing minerals and provide the theoretical foundation for developing new approaches aimed at addressing chlorinated organic pollution.

Biochar-based composites for removing chlorinated organic pollutants: Applications, mechanisms, and perspectives

Chlorinated organic pollutants constitute a significant category of persistent organic pollutants due to their widespread presence in the environment,which is primarily attributed to the expansion of agricultural and industrial activities.These pollutants are characterized by their persistence,potent toxicity,and capability for long-range dispersion,emphasizing the importance of their eradication to mitigate environmental pollution.While conventional methods for removing chlorinated organic pollutants encompass advanced oxidation,catalytic oxidation,and bioremediation,the utilization of biochar has emerged as a prominent green and efficacious method in recent years.Here we review biochar's role in remediating typical chlorinated organics,including polychlorinated biphenyls(PCBs),triclosan(TCS),trichloroethene(TCE),tetrachloroethylene(PCE),organochlorine pesticides(OCPs),and chlorobenzenes(CBs).We focus on the impact of biochar material properties on the adsorption mechanisms of chlorinated organics.This review highlights the use of biochar as a sustainable and eco-friendly method for removing chlorinated organic pollutants,especially when combined with biological or chemical strategies.Biochar facilitates electron transfer efficiency between microorganisms,promoting the growth of dechlorinating bacteria and mitigating the toxicity of chlorinated organics through adsorption.Furthermore,biochar can activate processes such as advanced oxidation or nano zero-valent iron,generating free radicals to decompose chlorinated organic compounds.We observe a broader application of biochar and bioprocesses for treating chlorinated organic pollutants in soil,reducing environmental impacts.Conversely,for water-based pollutants,integrating biochar with chemical methods proved more effective,leading to superior purification results.This review contributes to the theoretical and practical application of biochar for removing environmental chlorinated organic pollutants.

Residual Levels and New Inputs of Chlorinated POPs in Agricultural Soils from Taihu Lake Region

Selected persistent organochlorine pesticides (OCPs), including 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and its principal metabolites 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) and 1,1-dichloro-2,2-bis(p-chlorophenyl)e- thane (DDD), hexachlorocyclohexane (HCH) and its isomers (α-,β-, γ-, and δ-HCH), hexachlorobenzene (HCB), endo- sulfan, dieldrin, and endrin were quantified to determine current levels of organochlorine pesticides, to assess the eco- toxicological potential, and to distin…

Dechlorination of Chlorinated Aliphatic Compounds by Micro-scale Al-Zn-Mg/Fe Powders as Advanced Zero-valent Iron

Micro-scale Al-Zn-Mg/Fe composite powders (MAF) with high reactivity and good storage properties were prepared by reducing iron onto the surface of Al-Zn-Mg alloy powders. Experimental results show that MAF as advanced zero-valent iron are highly effective for degradation of chlorinated organic compounds. The efficiency of degradation for carbon tetrachloride and perchloroethylene is higher than 99% within a period of 2 h. The efficiency of degradation for trichloroethylene by MAF after storing for one month is equivalent to that by freshly prepared nano-size zero-valent iron particles.

Preparation and application of electro-catalytic material Fe_2O_3-ZnO/C

Electro-catalysts Fe203 compounded by ZnO were prepared by a sol-gel method, which were titled as Fe203-ZnO. Electro-catalysts Fe203-ZnO loading on the bamboo charcoal was titled as Fe203-ZnO/C. The catalytic materials were characterized by X-ray diffraction （XRD） and scanning electron microscope （SEM）. The obtained catalysts were assembled to three-dimensional electrodes to degradation of chlorinated organic in paper wastewater. And the performance tests show that three-dimensional electrodes have high activities for degradation of chlorinated organic in paper wastewater. There are many factors affecting the electro-catalytic performances of the three-dimensional electrodes. And the orthogonal experiment results show that the optimum operating condition is as follows： the calcination time of the catalysts 2 h, the mass ratio of Fe to Zn 4：1, the voltage 12 V, the mass of the catalytic materials 6 g, the value of pH 9, and the treating time 2.5 h. Under these conditions, the optimum removal efficiency of chlorinated organics in paper wastewater is 47.58%.

Surfactant-assisted removal of 2,4-dichlorophenol from soil by zero-valent Fe/Cu activated persulfate

The organic compounds contaminated soil substantially threatens the growth of plants and food safety.In this study,we synthesis zero-valent bimetallic Fe/Cu catalysts for the degradation of 2,4-dichlorophenol(DCP)in soils with persulfate(PS)in combination of organic surfactants and exploring the main environmental impact factors.The kinetic experiments show that the 5%(mass)dosage of Fe/Cu exhibits a higher degradation efficiency(86%)of DCP in soils,and the degradation efficiency of DCP increases with the increase of the initial PS concentration.Acidic conditions are favorable for the DCP degradation in soils.More importantly,the addition of Tween-80,and Triton-100 can obviously desorb DCP from the soil surface,which enhances the degradation efficiency of DCP in soils by Fe/Cu and PS reaction system.Furthermore,the Quenching experiments demonstrate that SO_(4)^(-1)·and·OH are the predominant radicals for the degradation of DCP during the Fe/Cu and PS reaction system as well as non-radical also exist.The findings of this work provide an effective method for remediating DCP from soils.

Chlorinated hydrocarbons in animal tissue samples from the Arctic Area

This study presents our preliminary results of the concentrations of chlorinated hydrocarbon contaminants in muscle, liver of caribou and intestine tissues of seal collected from the Arctic Area by China Arctic Scientific Expedition in April 1995. Hexachlorocyclohexanes (α, β, γ and δ HCH), DDT group (p,P′ DDE, o,P′ DDT, p,P′ DDD and p,P′ DDT) and polychlorinated biphenyls (PCBs) were analyzed. The concentrations of ∑HCH ranged from 3 13 ng/g to 7 02 ng/g and those of ∑DDT ranged from 0 16 ng/g to 1 28 ng/g. 14 individual PCB congeners, each in the range of 0 01 0 12 ng/g, were detected and the concentration of ∑PCB was from 0 14 ng/g to 0 60 ng/g. Also, the results were compared with the data reported previously.

Formation of organic chloride in the treatment of textile dyeing sludge by Fenton system

In the oxidation treatment of textile dyeing sludge,the quantitative and transformation laws of organic chlorine are not clear enough.Thus,this study mainly evaluated the treatment of textile dyeing sludge by Fenton and Fenton-like system from the aspects of the influence of Cl^(-),the removal of polycyclic aromatic hydrocarbons (PAHs) and organic carbon,and the removal and formation mechanism of organic chlorine.The results showed that the organic halogen in sludge was mainly hydrophobic organic chlorine,and the content of adsorbable organic chlorine (AOCl) was 0.30 mg/g (dry sludge).In the Fenton system with pH=3,500 mg/L Cl-,30 mmol/L Fe^(2+)and 30 mmol/L H_(2)O_(2),the removal of phenanthrene was promoted by chlorine radicals (·Cl),and the AOCl in sludge solid phase increased to 0.55 mg/g (dry sludge) at 30 min.According to spectral analysis,it was found that ·Cl could chlorinate aromatic and aliphatic compounds (excluding PAHs) in solid phase at the same time,and eventually led to the accumulation of aromatic chlorides in solid phase.Strengthening the oxidation ability of Fenton system increased the formation of organic chlorines in liquid and solid phases.In weak acidity,the oxidation and desorption of superoxide anion promoted the removal and migration of PAHs and organic carbon in solid phase,and reduced the formation of total organic chlorine.The Fenton-like system dominated by nonhydroxyl radical could realize the mineralization of PAHs,organic carbon and organic chlorines instead of migration.This paper builds a basis for the selection of sludge conditioning methods.

Effective removal of chlorinated organic pollutants by bimetallic iron-nickel sulfide activation of peroxydisulfate

Chlorinated organic pollutants(COPs)have caused serious contaminants in soil and groundwater,hence developing methods to remove these pollutants is necessary and urgent.By a simple hydrothermal method,we synthesized the bimetallic iron-nickel sulfide(FeNiS)particles which exhibited excellent catalytic property of COPs removal.FeNiS was chosen as the peroxydisulfate(PDS)activator to removal COPs including 4-chlorophenol(4-CP),1,4-dichlorophenol(1,4-DCP)and 2,4,6-trichlorophenol(2,4,6-TCP).The results show that FeNiS can efficiently activate PDS to produce sulfate radical(SO4·-)which plays major role in the oxidative dechlorination and degradation due to its strong oxidizing property and the ability of producing hydroxyl radicals(·OH)in the alkaline condition.Meanwhile,the Cl-abscised from COPs during the dechlorination can turn into the chlorine radicals and enhance the degradation and cause further mineralization of intermediate products.This bimetallic FeNiS catalyst is a promising PDS activator for removal of chlorinated organics.

Total organic halogen(TOX) in human urine: A halogen-specific method for human exposure studies

Disinfection by-products（DBPs） are a complex mixture of compounds unintentionally formed as a result of disinfection processes used to treat drinking water. Effects of long-term exposure to DBPs are mostly unknown and were the subject of recent epidemiological studies. However,most bioanalytical methods focus on a select few DBPs. In this study, a new comprehensive bioanalytical method has been developed that can quantify mixtures of organic halogenated compounds, including DBPs, in human urine as total organic chlorine（TOCl）, total organic bromine（TOBr）, and total organic iodine（TOI）. The optimized method consists of urine dilution, adsorption to activated carbon, pyrolysis of activated carbon, absorption of gases in an aqueous solution, and halide analysis with ion chromatography and inductively coupled plasma-mass spectrometry. Spike recoveries for TOCl, TOBr, and TOI measurements ranged between 78% and 99%. Average TOCl, TOBr, and TOI concentrations in five urine samples from volunteers who consumed tap water were 1850, 82, and 21.0 μg/L as X^-, respectively.Volunteers who consumed spring water（control） had TOCl, TOBr, and TOI average concentrations in urine of 1090, 88, and 10.3 μg/L as X^-, respectively. TOCl and TOI in the urine samples from tap water consumers were higher than the control. However, TOBr was slightly lower in tap water urine samples compared to mineral water urine samples, indicating other sources of environmental exposure other than drinking water. A larger sample population that consumes tap water from different cities and mineral water is needed to determine TOCl, TOBr, and TOI exposure from drinking water.

Variation in assimilable organic carbon formation during chlorination of Microcystis aeruginosa extracellular organic matter solutions

This study investigated the chlorination of Microcystis aeruginosa extracellular organic matter（EOM） solutions under different conditions, to determine how the metabolites produced by these organisms affect water safety and the formation of assimilable organic carbon（AOC）. The effects of chlorine dosages, coagulant dosage, reaction time and temperature on the formation of AOC were investigated during the disinfection of M.aeruginosa metabolite solutions. The concentration of AOC followed a decreasing and then increasing pattern with increasing temperature and reaction time. The concentration of AOC decreased and then increased with increasing chlorination dosage, followed by a slight decrease at the highest level of chlorination. However, the concentration of AOC decreased continuously with increasing coagulant dosage. The formation of AOC can be suppressed under appropriate conditions. In this study, chlorination at 4 mg/L, combined with a coagulant dose of 40 mg/L at 20°C over a reaction time of 12 hr, produced the minimum AOC.

The migration and transformation of dissolved organic matter during the freezing processes of water

This study investigated the partitioning behavior of dissolved organic matter（DOM） in liquid and ice phases, as well as the changes in the optical properties and chlorine reactivity of DOM during the freezing processes of water. DOM was rejected from the ice phase and accumulated in the remaining liquid phase during water freezing. Moreover, the decrease in freezing temperature, as well as the increase in dissolved organic carbon（DOC）concentration of feed water, caused an increase in DOM captured in the ice phase. The ultraviolet-absorbing compounds, trihalomethane precursors, as well as fulvic acid- and humic acid-like fluorescent materials, were more liable to be to be rejected from the ice phase and were more easily retained in the unfrozen liquid phase during water freezing, as compared with organics（on average） that comprise DOC. In addition, it was also found a higher accumulation of these organics in the unfrozen liquid phase during water freezing at higher temperature. The freeze/thaw processes altered the quantity, optical properties, and chlorine reactivity of DOM. The decrease in ultraviolet light at 254 nm as well as the production of aromatic protein- and soluble microbial byproduct-like fluorescent materials in DOM due to freeze/thaw were consistently observed. On the other hand, the changes in DOC, trihalomethane formation potential, and fulvic acid- and humic acid-like fluorescence caused by freeze/thaw varied significantly between samples.

Halogen-specific total organic halogen analysis： Assessment by recovery of total bromine

Determination of halogen-specific total organic halogen（TOX） is vital for studies of disinfection of waters containing bromide, since total organic bromine（TOBr） is likely to be more problematic than total organic chlorine. Here, we present further halogen-specific TOX method optimisation and validation, focusing on measurement of TOBr. The optimised halogen-specific TOX method was validated based on the recovery of model compounds covering different classes of disinfection by-products（haloacetic acids, haloacetonitriles,halophenols and halogenated benzenes） and the recovery of total bromine（mass balance of TOBr and bromide concentrations） during disinfection of waters containing dissolved organic matter and bromide. The validation of a halogen-specific TOX method based on the mass balance of total bromine has not previously been reported. Very good recoveries of organic halogen from all model compounds were obtained, indicating high or complete conversion of all organic halogen in the model compound solution through to halide in the absorber solution for ion chromatography analysis. The method was also successfully applied to monitor conversion of bromide to TOBr in a groundwater treatment plant. An excellent recovery（101%）of total bromine was observed from the raw water to the post-chlorination stage. Excellent recoveries of total bromine（92%–95%） were also obtained from chlorination of a synthetic water containing dissolved organic matter and bromide, demonstrating the validity of the halogen-specific TOX method for TOBr measurement. The halogen-specific TOX method is an important tool to monitor and better understand the formation of halogenated organic compounds, in particular brominated organic compounds, in drinking water systems.

Unveiling the secondary pollution in the catalytic elimination of chlorinated organics: The formation of dioxins

Since the discovery of polychlorinated dibenzo-p-dioxins and dibenzofurans(PCDD/Fs)in the process of municipal solid waste incineration(MSWI),a large number of researches have been conducted to reveal their formation mechanisms and emission characteristics.As one of national priority control pollutants,chlorinated organics are inclined to transfer into PCDD/Fs in the heterogeneously catalyzed process,which has been considered to be one of great challenges in environmental catalysis.However,so far direct evidences to support such a conversion process are insufficient,and the reaction mechanisms are lack of exploration.This study investigated the catalytic elimination of chlorobenzene(CBz)over a range of industrially applied active species including Pt,Ru,V,Ce and Mn oxides,and explored their reaction byproducts,chlorine adsorption/desorption behaviors and PCDD/F formations.We found that all of these species could generate the PCDD/Fs,amongst which,Mn species were the most active for PCDD/F formation.Approximately 140 ng I-TEQg-1 PCDD/Fs were detected on the Mn-CNT surface after ageing at250℃for 30 h.Even using the dichloromethane(DCM)as a precursor,significant PCDD/Fs were still detected.The Ru and V species were shown to generate much less polychlorinated byproducts and PCDD/Fs,owning to their sufficiently high abilities in Cl desorption,which were through the semi-Deacon and Br(?)nsted H reactions,respectively.

Catalytic destruction of chlorobenzene over K-OMS-2: Inhibition of high toxic byproducts via phosphate modification

In the process of catalytic destruction of chlorinated volatile organic compounds(CVOCs),the catalyst is prone to chlorine poisoning and produce polychlorinated byproducts with high toxicity and persistence,bringing great risk to atmospheric environment and human health.To solve these problems,this work applied phosphate to modify K-OMS-2 catalysts.The physicochemical properties of catalysts were determined by using X-ray powder diffraction(XRD),scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS),hydrogen temperature programmed reduction(H_(2)-TPR),pyridine adsorption Fouriertransform infrared(Py-IR)and water temperature programmed desorption(H_(2)O-TPD),and chlorobenzene was selected as a model pollutant to explore the catalytic performance and byproduct inhibition function of phosphating.Experimental results revealed that 1 wt.%phosphate modification yielded the best catalytic activity for chlorobenzene destruction,with the 90%conversion(T90)at approximately 247℃.The phosphating significantly decreased the types and yields of polychlorinated byproducts in effluent.After phosphating,we observed significant hydroxyl groups on catalyst surface,and the active centerwas transformed into Mn(IV)-O…H,which promoted the formation of HCl,and enhanced the dechlorination process.Furthermore,the enriched Lewis acid sites by phosphating profoundly enhanced the deep oxidation ability of the catalyst,which promoted a rapid oxidation of reaction intermediates,so as to reduce byproducts generation.This study provided an effective strategy for inhibiting the toxic byproducts for the catalytic destruction of chlorinated organics.

Carbon-based materials for electrochemical dechlorination

Electrochemical dechlorination reaction(EDR)is a promising,environmentally friendly,and economically profitable technology for treating chlorinated organic pollutants.For efficient environmental protection,electrocatalysts with high stability and low cost are of extremely significance to the development of EDR technology.Carbon-based materials have aroused broad interest as electrocatalysts for many electrochemical reactions due to their characteristics including large specific surface area,controllable structure,good conductivity,and chemical stability.For EDR,the carbon-based materials also show many unique superiorities,like strong adsorption capacity to chlorinated organic compounds(COCs),excellent catalytic activity and stability,and environmental compatibility.This review starts with a detailed summary on the mechanisms of electrochemical dechlorination(direct and indirect electron transfer pathway)and factors affecting the effectiveness of EDR.Then the paper comprehensively overviews the current progresses of carbon-based materials for EDR of COCs,following their two major application scenarios,i.e.,directly as electrocatalysts and as advanced supports for other catalysts.Moreover,the formation of different active sites in carbon-based electrocatalysts and their EDR activities are analyzed.Finally,the current challenges and perspectives in this field are discussed.This review will provide an in-depth understanding for the design of advanced carbon-based materials and promote the development of EDR technology.

Unveiling the importance of reactant mass transfer in environmental catalysis:Taking catalytic chlorobenzene oxidation as an example

To date,investigations onto the regulation of reactants mass transfer has been paid much less attention in environmental catalysis.Herein,we demonstrated that by rationally designing the adsorption sites of multi-reactants,the pollutant destruction efficiency,product selectivity,reaction stability and secondary pollution have been all affected in the catalytic chlorobenzene oxidation(CBCO).Experimental results revealed that the co-adsorption of chlo robenzene(CB)and gaseous O_(2)at the oxygen vacancies of CeO_(2)led to remarkably high CO_(2)generation,owning to their short mass transfer distance on the catalyst surface,while their separated adsorptions at Bronsted HZSM-5 and CeO_(2)vacancies resulted in a much lower CO_(2)generation,and produced significant polychlorinated byproducts in the off-gas.Howeve r,this separated adsorption model yielded superior long-term stability for the CeO_(2)/HZSM-5 catalyst,owning to the protection of CeO_(2)oxygen vacancies from Cl poisoning by the preferential adsorption of CB on the Bronsted acidic sites.This work unveils that design of environmental catalysts needs to consider both of the catalyst intrinsic property and reactant mass transfer;investigations of the latter could pave a new way for the development of highly efficient catalysts towards environmental pollution control.

Rapid degradation of hexachlorobenzene by micron Ag/Fe bimetal particles

The feasibility of the rapid degradation of hexachlorobenzene （HCB） by micron-size silver （Ag）/iron （Fe） particles was investigated.Ag/Fe particles with different ratios （0,0.05%,0.09%,0.20%,and 0.45%） were prepared by electroless silver plating on 300 mesh Fe powder,and were used to degrade HCB at different pH values and temperatures.The dechlorination ability of Fe greatly increased with small Ag addition,whereas too much added Ag would cover the Fe surface and reduce the effective reaction surface,thereby decreasing the extent of dechlorination.The optimal Ag/Fe ratio was 0.09%.Tafel polarization curves showed that HCB was rapidly degraded at neutral or acidic pH,whereas low pH levels severely intensified H2 production,which consumed the reducing electrons needed for the HCB degradation.HCB degradation was more sensitive to temperature than pH.The rate constant of HCB dechlorination was 0.452 min-1 at 85℃,50 times higher than that at 31℃.HCB was degraded in a successive dechlorination pathway,yielding the main products 1,2,4,5-tetrachlorobenzene and 1,2,4-trichlorobenzene within 2 hr.

Impact of combining chlorine dioxide and chlorine on DBP formation in simulated indoor swimming pools

The main objective of this study was to assess the combined use of chlorine dioxide（ClO_2）and chlorine（Cl_2） on the speciation and kinetics of disinfection by-product（DBP） formation in swimming pools using synthetic pool waters prepared with a body fluid analog（BFA）and/or fresh natural water. At 1：25（mass ratio） of ClO_2 to Cl_2, there was no significant reduction in the formation of trihalomethanes（THMs） and haloacetic acids（HAAs） for both BFA solution and natural water compared to the application of Cl_2 alone. When the mass ratio of ClO_2 to Cl_2increased to 1：1, substantial decreases in both THMs and HAAs were observed in the natural water, while there was almost no change of DBP formations in the BFA solution. Haloacetonitriles and halonitromethanes levels in both water matrices remained similar. In the presence of bromide, the overall DBP formation increased in both BFA solution and natural water. For the DBP formation kinetics, after 72 hr of contact time,very low formation of THMs and HAAs was observed for the use of ClO_2 only. Compared to Cl_2 control, however, applying the 1：1 mixture of ClO_2/Cl_2 reduced THMs by 〉 60% and HAAs by 〉 50%. Chlorite was maintained below 1.0 mg/L, while the formation of chlorate significantly increased over the reaction time. Finally, in a bench-scale indoor pool experiment, applying ClO_2 and Cl_2simultaneously produced less THMs compared to Cl_2 control and kept chlorite at 〈 0.4 mg/L, while HAAs and chlorate accumulated over 4-week operation period.

Investigation of Stable C and Cl Isotope Effects of Trichloroethene and Tetrachloroethylene during Evaporation at Different Temperatures

There are variations of reported isotope enrichment factors of chlorinated organic contaminants in evaporation processes. Trichloroethene （TCE） and tetrachloroethylene （PCE） were chosen to study carbon and chlorine isotope effects during evaporation at different temperatures. Equilibrium vapor-liquid carbon and chlorine isotope effects experiments were also conducted. In the equilibrium liquid-vapor system, the 13C was enriched but 37Cl was depleted in the vapor phase, being consistent with previous results. For evaporation average carbon isotope enrichment factor εc were ＋0.28‰± 0.01‰ for TCE and ＋0.56‰±0.09‰ for PCE at temperature from 20 to 26 ℃. Meanwhile, average chlorine isotope enrichment factor εCl were -1.33‰±0.21‰ for TCE and -1.00‰±0.00‰ for PCE. The results indicate that during evaporation the equilibrium isotope effect attenuates the magnitude of carbon isotope fractionation whereas enhances the chlorine isotope effect. Isotope fractionation during evaporation is determined by both equilibrium and kinetic factors. Chlorine isotope fractionation is influenced by the evaporation rate which is linked to temperature. When using stable isotope to inves- tigate the behavior of chlorinated organic contaminants in groundwater with slow biodegradation rate, the isotope fractionation resulted from evaporation should be taken into consideration. Furthermore, the environment conditions such as temperature are also factors to be considered.