Assessment of The Dioxins Leaching Potentials In Selected Soils Through Breakthrough Curves (BTCs) Model

The current study was to investigate the leaching and groundwater contamination potential of selected Dioxins, in local soil series. Solute transport was modelled through Breakthrough curve (BTC) plots, based on distribution coe�cient (K d ), Retardation factor and Dispersivity, under normal velocity (20 cm day -1 ) and preferential or steady �ow (50 cm day -1 ). In case of Dibenzo -p- Dioxin (DD), distribution coe�cient values were found in order of Charsadda > Peshawar > Sultanpur series, while for 2 Chloro- p-Dioxin (2Cl-DD), the order was Charsadda > Sultanpur > Peshawar. However, the overall sorption was low. Under the normal velocity both of selected Dioxins (DD & 2Cl-DD), BTC plots relatively took longer time to reach the point of saturation as compared to high seepage velocity. However, the overall solute transport was found to be rapid. This behaviour showed that sorption of the Dioxins selected soil series is low and there is potential for leaching and groundwater contamination.


Introduction
Among the various components of Environment, soil forms the most complex system, being comprised of different chemical and biological species (Turner, 2021).Soils perform both as provisionary as well as regulatory roles; they provide food and other resources as well as regulate various processes (nutrients recycling, waste assimilation and degradation etc), (Schröder and Schulte, 2016).However, these capacities can be greatly affected by the addition of numerous contaminants such as heavy metals (HMs), biological pathogens and persistent organics which are released through multiple sources including natural inputs as well anthropogenic sources.Major activities responsible for soil degradation may include intensive use of agro chemicals, mining, solid waste burning, fossil fuel burning, oil spillages and industrial growth multifold increase and accumulation of contaminants in soil strata has been reported (Sharma et al. 2021;Holík et al. 2019).Contaminants from these degraded soils can either entered the human body directly through ingestion, or may become the part of food and water we consume.
One of such contaminants are Dioxins, which is a group of 75 members with similar chemical and structural properties (USEPA, 2017).Persistence, toxicity and strong sorption in the soil matrixes form Dioxins as notorious contaminants and have been included in dirty dozen.Dioxins have been reported to be carcinogenic; while the non-carcinogenic health implications include hepatic, neurological, immunological, reproductive, endocrine, and developmental effects through binding to the Ah receptor (Hulin et al. 2020;Loganathan and Masunaga, 2020).
Another important aspect about Dioxins is their unintentional generation as by product during various activities like land lling, waste burning and bleaching as well during burning process of forest (Pan et

Sample collection
The major soil series selected for this study were Charsadda series, Peshawar series, and Sultanpur series; each series representing a speci c textural and physical properties.Representative soil series were collected respectively from three districts, Charsadda, Peshawar and Swabi, of Khyber Pakhtunkhwa (KP), Province of Pakistan.Each sample was collected at a depth of 30 cm, as a single sample and stored in polyethylene bag.Charsadda series belongs to Swat river alluvium, silty loam, non-calcareous, being dark grey in color with well drained drainage (Table 1).Peshawar series is brown/dark brown, course granular, silty clay loam and strongly calcareous in nature and falls in Udic Haplustalfs category under USDA classi cation.Peshawar series belongs to Piedmont alluvium deposits and possess well drainage.Similarly, Sultanpur series is strongly calcareous, silty loam alluvium deposited soils (Table 1).
Soil texture was determined using a hydrometer method of Ryan et al. (2001), and OM was determined by loss on ignition (Schulte and Hopkins, 1996); organic carbon (OC) was further calculated from OM values by dividing OM with 1.7 (Heaton et al. 2016).pH meter of Consort Electrochemical Analyzer (C931) was used for pH measurement in soil water solution (1 g: 25 mL).Bulk density (ρ) and porosity (θ) were determined using methods of Blake and Hartge (1986) and Schoper (1982), respectively.Soil properties are summarized in Table 1.

Chemicals
All the chemicals used were of analytical grade.Dioxins were purchased from AccuStandard, Inc. (New Haven, CT, USA).Hexane was purchased from Sigma-Aldrich with 99.9% purity.

Batch Sorption
Distribution coe cient values (K d ) for the selected soil series were taken from batch sorption data of Gul et al. (2018).
During their batch sorption analysis, mass to volume (M/V) ratio of soil and solution was kept 1g: 25 mL.Sorbate and sorbent in tubes were then mixed end-over-end for 72 hr to attain equilibrium.After shaking, the tubes were centrifuged for 30 mins at 3000 rpm.After centrifuge 20 mL of solution was extracted with 2 mL hexane for dioxin.Final 1 mL of Hexane extract was stored in GC vials.Desorption phase was carried out by decent and re ll method.Glass tubes containing soil and solutions were weighted at different points.For desorption the same background solution of 0.005M CaCl 2 was used.Rest of the procedure was same as for sorption phase.Details of the method are also being given by Li et al. (2003).
The analysis of Dioxins was performed on Agilent 6890 GC, connected to an Agilent 5975B Mass Selective Detector.
Column used was an Agilent VF-5 ms 30 m × 0.25 mm × 0.25 µm lm thickness with 10-m EZ-guard (part no.CP9013).Initial oven temperature was 40°C and rose up to 350°C, with sample run time of 9.75 min.The injection temperature was 270°C, and helium was used as a carrier gas at the ow rate of 1 min −1 .
Soil water distribution coe cient, K d (LKg −1 ), was calculated from measured sorbed concentration Cs (mgKg −1 ) and the concentration remaining in solution after equilibration Ce (mgL −1 ) according to the following relationship:

Retardation factor and dispersion coe cient
Retardation factor describes the attenuation capacity of soils and is dependent on sorption of solutes and their nature.
Besides that, soil properties such as porosity, bulk density and soil textures also determine the retardation capacity.Retardation factor for the BTCs were calculated as; Where "ρ" denotes bulk density, "θ" represents porosity, and K d (LKg −1 ) is the distribution coe cient value obtained from the batch sorption analysis.Dispersion coe cient "D" (cm 2 day −1 ) was calculated using the following equation: Where "α L " is the longitudinal dispersivity (cm) of the porous media in the direction of transport, and "ν" denotes to average linear velocity (cm day −1 ).Longitudinal dispersivity values for different textures were taken from literature (Kang et al. 2008) and used to calculate dispersion coe cient for selected soil series.

Estimation of BTCs
Convection dispersion solute transport was modeled through in CXTFIT 2.1 in the STANMOD software package to simulate solute transport (Simunek et al., 2003).Time and solute concentration was assumed to be dimensionless.

Physical properties of selected soil series
Table 1.Summarizes the physical properties of the three soil series, which shows vary little variation among data values.All the three soil series were of well drained drainage with porosity ranging between 52 to 60 %.Organic Matter content varied from 1.4 to 2.5 %, highest content being found for Charsadda series.pH of the series ranged from 7.6-8.0,bulk density varied from 1.36-1.50g cm −3 .Highest clay (20 %) and sand (39%) contents were found for Charsadda and Sultanpur series, respectively.2).This behaviour could be attributed to high OM (2.5%) content of the Charsadda series.Another possible reason behind the high distribution value could be the neutral pH of the soil series, as highly alkaline soils are reported to have low sorption tendency for Dioxins.Peshawar and Sultanpur showed little variations among them in case of DD, due to similar physical properties like clay content and OM (Table 1).In case of 2Cl-DD, distribution coe cient values were found in order of Charsadda > Sultanpur > Peshawar.Charsadda series again found with high value of K d values among the three series, but the overall sorption was low in comparison with DD.The possible reason for this behaviour could be the higher molecular size of chlorinated Dioxins molecules, due to which interlayer sorption is hindered.Sultanpur series showed signi cant sorption towards 2Cl-DD, in comparison with the DD.
Low sorption a nities in the two soil series (Peshawar and Sultanpur series) could be due to the clay minerals, as hydration of exchangeable cations is a major determinant of sorption a nities of the clay.Large hydration sphere (Ca + , Na + ), have been reported to prohibit the interlayer interactions and absorption of the Dioxins (Liu et al., 2015).
From the sorption a nities of the studied series indicate the SOM is generally the dominant partitioning phase in soils for hydrophobic compounds as expected, while soil clay fractions may also contribute signi cantly in sorption (Chiou, 2003).According to Li et al. (2003), high Ca +2 content hinders the sorption of non-polar compounds such as Dioxins and PCBs.

Conclusion
Breakthrough curves obtained in the study showed little variations among the plots of three selected series, despite of difference in their K d , retardation factor and dispersivity.This behaviour indicates that the local series due to their low OM content, and calcareous nature have low Dioxins sorption potential for both DD and 2Cl-DD.From the results it is also concluded that clay can also be the controlling factor along with soil OM.Leaching potential and transport of both DD and 2Cl-DD were predicted in all soil series under both seepage velocities (20 & 50 cm day -1 ), which indicates that risk of groundwater resources contamination with Dioxins.Such contamination could bring severe health implications such as Cancer.Therefore, there is need of further research and resource management to minimize severe consequences.

Declarations Disclosure Statement
No potential con ict of interest was reported by the author(s).

Source of funding is Higher Education Commission of Pakistan
al. 2021; Gul et al. 2018; Akele and Tarekegn, 2017).After generation, Dioxins may get deposit in the soil matrixes through absorption by clay and Organic matter (OM), from where there is possibility of Dioxins leaching under preferential ow through macro pores or through movement along loosely bound sediments and y ash (Tao et al. 2021; Müller, 2017; Xu et al. 2017; Ghafoor et al. 2013).Various transport models have been suggested to monitor the transport and leaching of Dioxins from soils, sediments and y ash solute transport, based on soil properties such as bulk density, texture, porosity, OM content, and distribution coe cient (K d ), (Freire et al. 2015; Badea et al. 2013; Yasuhara and Katami, 2007; Frankki et al. 2006).One of such study was conducted by Gul et al. (2018), where the local soils series from Pakistan were selected and assessed for Dioxin leaching through Breakthrough curves (BTCs).BTCs were modeled at normal and fast seepage velocities.Rapid solute transport was predicted from the obtained BTCs under preferential ow conditions.The general assumptions for BTCs calculation were 1) Steady solute ow through column 2) continuous injection of solute and 3) measurement of injecting a chemical solution of a known solute concentration (Ci) into the column inlet as a function of time.Based on the ndings of Gul et al. (2018), the current study was designed for other three major soil series (Charsadda, Peshawar and Sultanpur series) collected from the respective districts (Charsadda, Peshawar and Swabi).The basic aim was to asses Dioxins leaching and transport potential under normal and preferential ow with respect to groundwater contamination risk.Selected Dioxins for the current study include Dibenzo-p-Dioxin (DD) and 2-Chloro Dibenzo-p-Dioxin (2 Cl-DD).The K d values measured from batch sorption experiments were used to calculate soil retardation factor (R) and to model the solute transport in selected soils.

3. 3 .
Figure1a and bshow the BTCs for DD and 2 Cl-DD, under 20 cm day −1 linear velocity.For DD, among the three soil series, BTC for Charsadda series originated at concentration around 0.2 and took relatively longer timer to reach the equilibrium (Fig.1a).Peshawar series BTC reached the end point earlier than the rest of two due to low K d and R values and high dispersivity (480 cm 2 day −1 ).Curve for the Sultanpur reached equilibrium point slowly.The solute

Figure
Figure2aand b, plots of BTCs under high seepage velocity of 50 cm day −1 .For DD, all the plots obtained were identical.All the curves originating at lowest concentration and reaching the point of exhaustion quickly.The same behaviour of the series was also observed for the 2Cl-DD, under the same velocity.It can be concluded from such behavior that under steady ow conditions like preferential ow, rapid solute transport could take place leaving the sorption bed earlier with minimum interaction with soil media(Chowdhury et al. 2013), irrespective of their distribution coe cient and retardation factors.Such behaviour of the rapid solute transport and low sorption indicate groundwater contamination chances, along with severe health implications.

Figures Figure 1 a
Figures

Table . 1
Gul et al. (2018)ristics of selected soil series Details about the batch sorption, dispersion and retardation factor for studied soil series have been summarized in table.2.As mentioned earlier, distribution coe cient values were taken from the study ofGul et al. (2018).Distribution coe cient values were found in order of Charsadda > Peshawar > Sultanpur series.Charsadda series, was observed with high K d value (1583 LKg −1 ), for DD in comparison with other two series (Table 3.2.Soil portioning DD and 2Cl-DD on selected soils

Table 2
Dispersion coe cient at pore water velocity at 20 & 50 cm day −1 , respectively K d values taken from the batch sorption data of Gul et al., 2018 Imbrie and Park (1995))pur reached equilibrium point slowly.The solute transport behavior via BTCs was identical to the plots obtained byGul et al. (2018), for Dioxins, but different than that ofChowdhury et al. (2013).In case of 2Cl-DD, BTC for Charsadda series was recorded to start above concentration of 0.4, with no plot at low concentrations and reaching the equilibrium point quickly.It can be concluded from the Both, Peshawar and Sultanpur series showed similar BCTs.Again the plots obtained were similar to the BTCs, reported byGul et al. (2013), but were different from the plots of other studies.For example, BTCs obtained for pharmaceuticals byKiecak et al. (2019), were some projectile shaped.Similarly in another study conducted byImbrie and Park (1995), curve obtained were "S" shaped, indicating gradual raise in the solute concentration towards the saturation point.It can be concluded that due to little variations in the physical properties of the soil series, collected during the both studies, BCTs obtained were