Metal contamination in soil and vegetables around Savar tannery area, Dhaka, Bangladesh: A preliminary study for risk assessment

Abstract

Monitoring of heavy metal concentrations in soil and their accumulation in vegetables grown in the newly shifted tannery area of Savar, Bangladesh, is crucial for human health. Heavy metals (i.e., Cr, Pb, Cu, Zn, Ni, and Cd) concentrations in soil and vegetable samples were determined by atomic absorption spectrophotometer (AAS). In soil, 3220 mg/kg Cr was observed, which was 32-fold greater than the WHO/FAO recommended limit. Ecological risk indices such as the contamination factor, enrichment factor, pollution load index, and geoaccumulation index showed metal levels as moderately to very highly contaminated. The non-carcinogenic risk (NCR) was found to be higher, and the carcinogenic risk (CR) exceeded the acceptable value 1 × 10⁻⁶ and posed greater risks to children than adults, especially for Cr in soil. The main exposure pathway for soil metals was 97.8–99.9% due to oral ingestion. The concentration of heavy metals especially Cr, Pb, Zn, and Cd, in vegetables was alarming as they crossed the safety limit. The calculated mean hazard index (8.71) for vegetable samples showed elevated levels of potential NCR, while CR for Cr and Cd, exceeded the acceptable limit of 1 × 10⁻⁶, indicating the probability of cancer risk to humans through the consumption of vegetables. This study revealed a to-and-fro analysis of the present scenario of the tannery area, giving importance to human health and the environment.

1. Introduction

Pollution of soil by poisonous metals is the most concerning situation in the soil and plant systems, according to world scientists. In soils, a considerable quantity of heavy metals has been detected due to various anthropogenic as well as natural factors [1]. In developing countries like Bangladesh, heavy metals may originate from uncontrolled manmade activities such as mining, processing, and smelting of synthetic chemicals, cleaning of skins, ship making, and breaking, transportation, battery recycling, agricultural activities etc. [2,3]. Among them, the tannery industry intensifies heavy metal contamination in the Hazaribagh area of Dhaka City, Bangladesh [1,[3], [4], [5], [6]].

In Bangladesh, leather contributes to economic development due to raw material availability and earning foreign currency. Following the relocation of tannery industries from the heart of Dhaka City, Hazaribagh, to Savar Leather Industrial Park, they are encountering some challenges in terms of environmental compliance. The Bangladesh government has currently allotted 200 acres of land for the entire tannery industry, while 17 acres of land have been allotted for the CETP facility, which will treat 25000 m³ of wastewater daily [7]. Besides, according to the experts, everyday tannery activities produce 80–100 metric tons of solid and liquid waste, which is a major concern for tannery waste treatment. At the time of leather treating, several inorganic and organic chemicals such as chromium sulfates, pigments, dyes, different acids, calcium hydroxide, sodium chloride, ammonium chloride, etc. are used [3,8,9]. The chemicals used for leather treatment may contain various toxic metals such as Zn, Cd, Cr, Cu, Ni, Pb, etc., in the tannery effluents and contaminate the soil, water, and other environmental compartments of the surrounding area [7,10,11].

In Bangladesh, farmers use excessive amounts of pesticides, fertilizers, and chemicals on crop field, which contaminate the soil-plant system and intensify long-term hazards to humans [12]. After entering the human body, heavy metals may cause cardiovascular diseases, fracture of bones to various extents, kidney failure, cancers, and DNA damage [13,14]. In recent decades, heavy metal contamination in soil-plant systems has been reported in different countries like; Pakistan [15,16], Italy [17], China [18], India [19,20], and Nigeria [21]. To the best of our knowledge, no detailed study regarding risk assessment has been conducted so far around the recent relocation of the tannery industrial area of Savar, Bangladesh. The present investigation was, therefore, aimed firstly, at investigating the level of heavy metal contamination in surface soil and vegetables and assessing the health risks associated with these metal contaminants from vegetables grown in the soil around the Savar leather industrial park area in Bangladesh. Subsequently, it may be in future scientific interests to find out the current picture of heavy metal contamination in soil and plants to unveil the risks associated with heavy metals to humans.

2. Methodology

2.1. Study area

The research area is located at 90° 13′ 57.2″E to 90° 14′ 42.5″E and 23° 46′ 0.5″N to 23° 47′ 49.5″N in the western part of Dhaka. The region consists of a wet, hot, humid, and monsoon climate with an annual mean temperature of 25 °C, and an average precipitation of 1854 mm. Runoff is common in this region during the monsoon season due to its elevation of 26.5 feet above sea level [22]. Garment manufacturing, textiles, dyeing, knitting, leather processing, etc. have been practiced in the Savar area for several years. Tannery industries situated in Savar are one of the dominant industries in the selected study area. The main drainage systems of the tannery industry estate in the study area are connected with the Dhaleshwari River, which receives huge amounts of untreated waste from surrounding industries in Dhaka City.

2.2. Pretreatment of collected ample

A total of 45 surface soil and 27 most abundant plants and vegetable samples were collected around the tannery industry and within 2 km² adjacent areas. The sampling area has been split into 4 zones named as Zone 1: Jomjom residential area close to tannery industry which is separated by a boundary wall of the south-eastern part of the CETP; Zone 2: downstream and opposite of Dhaleshwari riverside from tannery industry zone (TIZ) and opposite to TIZ; Zone 3: tannery industry areas and CETP; and Zone 4: upstream and opposite site of Dhaleshwari River from TIZ for collecting soil and vegetable samples (Fig. S1). The Global Positioning System (GPS) was applied to record the coordinates of collected soil and vegetable samples. The soil sample was collected from 0 to 25 cm by a steel auger from three different places around 1.0 m and mixed thoroughly and placed in a zipper polyethylene bag. Then the samples were oven dried maintaining 105 ± 3 °C for 16 h, and then samples were ground by mortar and pestle to create fine powder sample. From the point of soil sample, mostly consumable vegetable samples were also picked and washed with tap water to remove any soil particles. Following collection, clean, fresh vegetable samples were placed in an oven set to 80 °C overnight and ground into a fine powder.

2.3. Soil sample digestion

Soil samples were digested following APHA standard method 3050 B [23]. In a Pyrex glass beaker, 10 mL of HNO₃ (65% w/v) was added to 1.0 g of dried, ground, finely powdered soil. The solution was heated to 95 ± 5 °C facilitating vapor recovery by covering the beaker with a watch glass, refluxed for 12–15 min, and cooled. Again, the solution was allowed to reflux for 30 min with the addition of 5 mL of HNO₃, and repeated additions of HNO₃ were made until brown fumes were completely eliminated. The solution was heated at 95 ± 5 °C for 2 h to reduce the final sample volume to approximately 5 mL. It was then cooled, and 2 mL of water was added, followed by 3 mL of 30% (w/v) H₂O₂. The solution was warmed to initiate the peroxide reaction and H₂O₂ was added incrementally (1 mL) up to a total of 10 mL until the color of the solution was observed unchanged or a less effervescence was noticed. Finally, 5 mL of samples were prepared by heating again for 2 h at 95 ± 5 °C under watch glass. After allowing the solution to cool, 10 mL of concentrated HCl was added. Then the final volume of the extracted sample was made up to 30 mL with distilled water. To obtain clean soil extract, 0.45 m filter paper was used, and the samples were stored in polypropylene bottles for trace element analysis using an atomic absorption spectrophotometer (AAS) (Shimadzu AA 7000).

2.4. Digestion procedure of plant sample

In a digestion beaker, 0.5 g ground vegetable sample was taken, 5 mL of concentrated HNO₃ was added, and the mixture was kept overnight covered with a watch glass. The next day, the sample was heated for 1 h at 125 °C on a hot plate and allowed to cool. The digestion process was then heated continuously with 1–2 mL of 30% (w/v) H₂O₂ and 5 mL of HNO₃ (65% w/v). After that, 30% H₂O₂ was added repeatedly until a clear solution was noticed. Then heat was applied by reducing the temperature to 80 °C without a watch glass until the solution was clear of residue and nearing dryness. Finally, the sample volume was made 20 mL with the addition of 1:2 diluted HNO₃ and deionized water. The solution was kept in clean plastic bottles for AAS analysis followed by filtration through 0.45 μm size nylon filter paper.

2.5. Metals analysis

Cu, Zn, Cr, Ni, Pb, and Cd were measured from soil and plant extracts using an air acetylene flame atomic absorption spectrophotometer (FAAS) of the Shimadzu AA7000. Three standards and a blank were run before starting sample analysis for the calibration of the AAS instrument. Each of the samples was tested three times and the average with standard deviation was reported. In addition, X-ray fluorescence (XRF) analysis of a soil sample from four zones was obtained with the Shimadzu Lab Center XRF-1800 for measuring the major elements present in the soil. Small amounts of every sample were taken to thoroughly mix to make a sample for each zone for XRF analysis. Chemicals used in this study were analytical grade reagents and 18 MΩ water was used to prepare the standards required for sample preparation and extraction.

2.6. Statistical analysis

2.6.1. Contamination factor (CFⁱ)

CF estimates heavy metal contamination in soil and is determined by equation (1).

$$C{F}^i=\frac{C^i}{B^i}$$ (1)

where Cⁱ is regarded as the soil metal concentration and Bⁱ is the elemental (i-th) standard background concentration. The standard background heavy metal concentration has been calculated here in this study by considering three widely accepted background standards, which are (i) the average concentration in shale, (ii) the average of Earth's crustal content, and (iii) the worldwide mean concentration of metal in soils [[24], [25], [26]]. There are four categories to explain the contamination factor: CFⁱ < 1 indicates low soil contamination, 1 ≤ CFⁱ < 3 indicates moderate contamination, 3 ≤ CFⁱ < 6 indicates considerable contamination, and 6 ≤ CFⁱ indicates very high soil contamination [27].

2.6.2. Enrichment factor (EF_s)

The enrichment factor quantifies the abundance of heavy metals in soil sample compared to the metal in control soil. The calculation of EFs is expressed in equation (2).

$$E{F}_s=\frac{Metalconcentrationinsoilsample}{Metalsincontrolsoilsample}$$ (2)

EF values ≤ 1 signifies enrichment via natural sources and higher than 1 indicates manmade sources. Furthermore, enrichment factor is classified as follows: EF_s˂ 2, indicates minimum enrichment; 2 ≤ EF_s < 5, indicates moderate enrichment; 5≤ EF_s < 20, indicates significant enrichment; 20 ≤ EF_s ≤ 40, indicates very high enrichment; 40 ≤ EF_s, indicates extreme enrichment of metals [26].

2.6.3. Pollution load index (PLI)

The PLI is used to assess the agglomeration of pollutants especially heavy metals in an area [28]. It is determined by equation (3).

$$PLI=\sqrt[n]{C{F}^1\times C{F}^2\times C{F}^3\times \cdot \cdot \cdot \times C{F}^n}$$ (3)

where, CF stands for contamination factor and n indicates the number of metals. An area is indicated polluted when the PLI is greater than 1.

2.6.4. Geoaccumulation index (I_geo)

The I_geo determines contamination status of an area in terms of heavy metal concentration compared to background value of the metals [29] and assessed by the following equation:

$$I_{geo}={\log }_2\left[\frac{C^n}{1.5\times B^n}\right]$$ (4)

in equation (4), Cⁿ is the concentration of metal in the collected soil and Bⁿ is the background concentration of element n. Due to lithological variation in background concentration, the numerical factor 1.5 is multiplied with the standard background concentration. I_geo is classified into seven groups: I_geo ≤ 0 signifies unpolluted; 0 <I_geo ≤ 1 signifies slightly polluted, 1 <I_geo ≤ 2 signifies moderately polluted, 2 <I_geo ≤ 3 signifies moderately to severely polluted, 3 <I_geo ≤ 4 signifies severely polluted, 4 <I_geo ≤ 5 signifies severely to extremely polluted and 5 <I_geo ≤ 6 signifies extremely polluted by metals [29].

2.6.5. Ecological risk factor (Erⁱ)

The Erⁱ determines the ecological risk posed by heavy metals and signifies the impact of environmental contamination on the ecology [26,30,31] and is assessed by the following equation:

$$E{r}^i=T^i\times C{F}^i$$ (5)

in equation (5), Tⁱ represents toxic response factor of soil metal and CFⁱ is the contamination factor. The Tⁱ value of Cr, Pb, Cu, Cd, Ni, and Zn are respectively 2, 5, 5, 30, 5, and 1 as proposed by Hakanson [27]. Further, the risk factor has been divided into five categories: low potential Er: $E{r}^i$ < 40, moderate potential Er: 40 ≤ $E{r}^i$ < 80, considerable potential Er: 80 ≤ $E{r}^i$ < 160, high potential Er: 160 ≤ $E{r}^i$ < 320, and very high Er: ${\text{Er}}^i$ ≥ 320.

2.6.6. Index for ecological risk (RI)

The RI denotes the totality of the risk factors (Erⁱ) for a certain location where the heavy metal toxicity and their environmental responses are studied [32]. The RI for Zn, Cr, Cu, Pb, and Ni is calculated with equation (6).

$$RI=\sum _{i=1}^{\infty }E{r}^i$$ (6)

where low Er: RI < 150, moderate Er: 150 ≤ RI < 300, considerable Er: 300 ≤ RI < 600 and very high Er: RI ≥ 600 [32].

2.7. Human exposure assessment

2.7.1. Risk for non-carcinogenic

Non carcinogenic risk analysis is a multi-step process where chronic daily intake (CDI), hazard quotient (HQ), hazard index (HI), and risk characterization has to be determined [33]. Human heavy metal exposures from contaminated soil could take place via different pathways. Oral ingestion (CDI_ing), dermal absorption through adhered exposed skin (CDI_dermal), and inhalation of soil particles (CDI_inh) are all important potential areas [34]. Other researchers indicated the principal human exposure pathway of Pb, Cu, Cr, Ni, and Cd from the soil is ingestion [35,36]. This study analyzed non-carcinogenic human health risks (child and adult) following equations (7) - (11) suggested by USEPA [[37], [38], [39], [40], [41]] and USDOE [33].

$$CD{I}_{ing}=\frac{CS\times IngR\times EF\times ED}{BW\times AT}\times CF$$ (7)

$$CD{I}_{dermal}=\frac{CS\times SA\times AF\times ABSd\times IngR\times EF\times ED}{BW\times AT}\times CF$$ (8)

$$CD{I}_{inh}=\frac{CS\times InhR\times EF\times ED}{BW\times AT}\times CF$$ (9)

$$HQ=\frac{CDI}{RfD}$$ (10)

$$HI=\sum HQ=H{Q}_{ing}+H{Q}_{dermal}+H{Q}_{inh}$$

$$HI=\sum HQ=H{Q}_{ing}+H{Q}_{dermal}+H{Q}_{inh}=\frac{CD{I}_{ing}}{Rf{D}_{ing}}+\frac{CD{I}_{dermal}}{Rf{D}_{dermal}}+\frac{CD{I}_{inh}}{Rf{D}_{inh}}$$ (11)

where CS designates the concentration of heavy metals in soils. All other parameters, standard values, and units, used to evaluate non-carcinogenic risks are shown in Table S1. The HQ was calculated to divide CDI by corresponding metal (Cr, Cu, and Pb) reference doses (RfD). The value of HQ lower than 1 indicates no adverse health risk and an HQ greater than 1 signifies potential serious human health risks. The cumulative non-cancer hazard (HI) was determined by adding the values of HQ from dermal, ingestion, and inhalation. HI exceeding unity indicates extreme human health risks, and the rising tendency of HI increases the chances of non-carcinogenic risk occurrences accordingly [42].

2.7.2. Risks for carcinogen (CR)

The likelihood of causing cancer in the human body due to heavy metal exposure could be estimated by the equations from 12 to 15 as developed by USEPA [[37], [38], [39], [40], [41]] and USDOE [33].

$$C{R}_{ing}=\frac{CS\times AF\times IngR\times EF\times ED\times CF\times CS{F}_{ing}}{BW\times AT}$$ (12)

$$C{R}_{dermal}=\frac{CS\times SA\times AF\times AB{S}_d\times EF\times ED\times CF\times CS{F}_{ing}\times AB{S}_{GI}}{BW\times AT}$$ (13)

$$C{R}_{inh}=\frac{CS\times ET\times EF\times ED\times IUR}{PEF\times 24\times AT}\times {10}^3$$ (14)

$$Totalcarcinogenrisk\left(C{R}_{total}\right)=\sum Risk=C{R}_{ing}+C{R}_{dermal}+C{R}_{inh}$$ (15)

where, CR_ing, CR_dermal and CR_inh indicate carcinogenic risks induced by ingestion, cancer risk through dermal contact and inhalation of contaminated soil successively. Carcinogenic risks assessment values are shown in Table S1. As suggested by equation (15), the cumulative cancer risks posed by soil heavy metals have synergistic evidence from various exposure pathways. Total cancer risk values lower than 1 × 10⁻⁶ (1 possibility of developing cancer from 10,00,000 cases) dictates negligible risk, while CR_total > 10⁻⁴ is to be unacceptable for human health perspectives [34,37]. Some of the researchers have considered the acceptable values of carcinogenic risk as 1 × 10⁻⁶ < CR ≤ 1 × 10⁻⁴ [43].

2.8. Data analysis

2.8.1. Daily metals intake from vegetables

Daily consumption of studied vegetable by a human being developed by USEPA [37] could be attained by equation (16).

$$AD{I}_{veg}=\frac{MC\times I{R}_{veg}\times CF\times EF\times ED}{BW\times LS}$$ (16)

where BW, LS MC, IR_veg, CF, EF, and ED indicate body weight, average life span of exposure, metals in examined vegetables, relative ingestion rate, factor for conversion, frequency of exposure, and duration of exposure, respectively. All other values of standard parameters are demonstrated in Table S2.

2.8.2. Cumulative potential risk assessment of vegetables

A daily diet of heavy metal contaminated vegetables is considered the main exposure pathway for heavy metals in the human body [26,44]. The USEPA first recommended calculating the target hazard quotient (THQ) for determining potential health hazards from consuming heavy metals-containing vegetables, which is calculated by dividing average daily metal-containing vegetable intake by its oral reference dose (ORfD) (Table S2) of the certain contaminants [14,41]. THQ of six studied metals in the collected vegetables was calculated using equation (17), and hazard index (HI) was calculated using equation (18) for risk assessment as a cumulative potential.

$$THQ=\frac{AD{I}_{veg}}{ORfD}$$ (17)

$$HI=TH{Q}_{total}=\sum THQ$$ (18)

2.8.3. Target carcinogenic risk (TCR)

Potential risks of carcinogenic which could be the consequence of exposure of carcinogenic agents and chemicals are determined by TCR for a definite life span of exposed human. Equation (19) was used to assess TCR.

$$TCR=\frac{MC\times EF\times ED\times I{R}_{veg}\times OS{F}_{CR}}{BW\times LS}\times {10}^{-3}$$ (19)

where MC, EF, IR_veg, ED, OSF_CR, LS, and BW represent the collected vegetable concentration, exposure frequency, ingestion rate of vegetables, exposure duration, oral slope factor, average life span, and body weight of individuals, respectively. All other values of standard parameters for TCR determination are given in Table S2.

3. Results and discussion

3.1. Heavy metal content in the soil

Average concentrations of six determined heavy metals in soils were summarized in Table 1. The mean Cr content of four zones was 3220 mg/kg, which was 10-fold higher than the maximum permissible concentration (MPC) in Finland and China and 32-times higher than the WHO/FAO safety limits. The mean Cr concentration ranged from 50 mg/kg to a maximum of 10200 mg/kg (Table 1). In zone 3 (actual leather processing site), the highest average concentration of Cr was found at 10200 mg/kg. Out of the 45 soil samples, SS12, SS13, SS24, SS25, SS28-SS35, and SS36 showed an extremely high concentration of Cr, especially in zone 3 (Fig. 1A), (Table S3). Extremely high Cr concentrations in the soil could be the result of the use of tanning agents enriched with Cr: generally, basic chromium sulfate (BCS) and chrome syntan, as in chrome-tanned leather. BCS is used in the tanning and re-tanning processes at a 4–16% of the skin weight according to the final product desired. The investigated result of zone 1 for Cr showed that the soil samples at the CETP vicinity contain a higher concentration than other samples for zone 1 which might be due to the intrusion of Cr-containing wastes caused by the accidental breakage of the boundary wall between Jomjom residential area and the CETP previously (Fig. 1A). On the other hand, zones 2 and 4 were housed with a lower quantity of Cr compared to the others (Table 1), (Table S3). However, as the results reveal, all the zones exceeded the background and control sample Cr concentrations, signifying certain human induced interventions in Cr deposition in the sampling zones.

Table 1.

Soil pH and heavy metal concentration (mg/kg) in different zones of Savar tannery area and its nearby (Sn = soil samples collected in a zone).

Samples			pH	Cr	Pb	Cu	Zn	Ni	Cd
This study	Zone 1 (Sn = 14)	Mean	6.4	2620	7.25	26.2	205	18.9	3.35
		Min	6.2	0.2	0.00	4.17	37.3	0.00	0.00
		Max	6.7	35700	16.9	48.7	354	54.9	10.2
	Zone 2 (Sn = 06)	Mean	6.8	54.2	4.22	25.5	243	26.0	2.23
		Min	6.4	24.3	0.00	23.1	206	24.1	1.76
		Max	6.9	188	9.42	28.4	334	29.5	3.12
	Zone 3 (Sn = 16)	Mean	7.3	10200	6.11	18.5	247	9.21	5.11
		Min	6.8	38.9	0.00	4.02	49.6	5.10	0.00
		Max	7.8	53400	18.0	51.5	1370	12.5	13.0
	Zone 4 (Sn = 09)	Mean	6.5	50.1	41.5	58.7	286	16.1	0.87
		Min	6.2	38.1	13.9	42.0	158	6.40	0.00
		Max	6.7	57.2	58.6	103	417	28.8	2.37
Mean of four zones			6.8	3220	14.8	32.2	245	17.6	2.89
Control			6.2	15.4	40.1	65.0	45.2	9.31	0.45
Background value [26]			–	19.7	90.7	75.7	38.0	35.3	1.56
China limit [45]			–	200	120	100	250	100	0.60
Finland limit [46]			–	100	60	100	200	50	1
WHO/FAO [47]			–	100	100	100	300	50	3
India limit [48]			–	–	250–500	135–270	300–600	75–150	3–6

Fig. 1.

Distribution of Cr (A) and Cd (B) in the collected soil samples.

Cu, Ni and Pb in the examined soil exhibited lower concentrations than those reported in the international organizations. Cd showed the highest concentration in zone 3 (5.11 mg/kg) indicating a value far beyond the recommended value suggested by Finland, China, and the WHO. In addition, the concentration of Cd was greater than 1 mg/kg in 75% of soil samples, which could be the consequence of using Cd-containing fertilizer in the sampling of agricultural areas (Fig. 1B).

In this study, the ranges of SiO₂, Al₂O₃, Fe₂O₃, Cr₂O₃, ZnO, and NiO were found to be 60–64, 11–12, 8–10, 0.03–0.1, 0.02–0.04, and 0.02%, respectively, where SiO₂ was found to have the highest composition (Table S4). It was revealed that 18 types of different organic and inorganic matters were present in all of the samples and their compositions were nearly identical among the four different zones except for P₂O₅, SiO₂, Fe₂O₃, Cr₂O_3, and SO₃.

3.2. Contamination assessment in soil

The contamination status and degree of contamination were assessed using various factors such as contamination and enrichment factor and compared to a control sample from a non-industrial area of Bangladesh (Table 2). Extreme enrichment of Cr and Cd and moderate endowment of Zn and Ni were observed in zones 1 and 3 while the metals Pb and Cu showed minimal enrichment. On the contrary, in zones 2 and 4, significant enrichment of Zn and moderate enrichment of Cr and Ni were noticed, whereas minimum enrichment was found for Cu, Cd, and Pb (Table 2). Therefore, it is concluded with respect to the revealed result that the area has been altered by human induced activities that lead to Cd, Cr, Ni and Zn enrichment. However, other possible causes of this metal enrichment include the use of Cr and Zn as tanning materials in leather processing and Cd-containing fertilizer for vegetable cultivation and agricultural use. A similar study about Cd pollution in soil and vegetables has been reported earlier elsewhere [10]. Except for Cd, Zn, and Cr, other metals, e.g., Cu, Ni, and Pb, were found to range from 0.24 to 0.78, 0.26–0.73, and 0.05–0.46, respectively.

Table 2.

Enrichment and contamination factor of heavy metals in the soils (n = no of collected soil sample).

	Sampling Zone	Cr	Pb	Cu	Zn	Ni	Cd
EF	Zone 1 (n = 14)	170	0.18	0.40	4.52	2.03	7.44
	Zone 2 (n = 6)	3.52	0.11	0.39	5.37	2.79	1.43
	Zone 3 (n = 16)	659	0.15	0.28	5.46	0.99	8.51
	Zone 4 (n = 9)	3.25	1.04	0.90	6.32	1.73	0.87
CF	Zone 1 (n = 14)	133***	0.08	0.35	5.38**	0.54	2.15*
	Zone 2 (n = 6)	2.76*	0.05	0.34	6.39***	0.73	1.43*
	Zone 3 (n = 16)	517***	0.07	0.24	6.50***	0.26	3.27**
	Zone 4 (n = 9)	2.55*	0.46	0.78	7.52***	0.46	0.56

EFs˂ 2, minimum; 2 ≤ EFs < 5, moderate; 5≤ EFs < 20, significant; 20 ≤ EFs ≤ 40, very high enrichment; and 40 ≤ EFs, extreme enrichment. *1 ≤ CFⁱ < 3, moderate, **3 ≤ CFⁱ < 6, considerable and ***6 ≤ CFⁱ, very high contamination.

In view of the contamination factor, high contamination of Cr was noticed at high industrial Cr deposition zones 1 (CF = 133) and 3 (CF = 517). Moderately contaminated by Cr at zone 2 where CF was observed as 2.76 and zone 4 where CF was 2.55. As stated earlier, zone 3 is the tanning zone where production of Cr-tanned leather is practiced. Nevertheless, during the construction process, initial production, and transitional period of the tannery from Hazaribagh to Savar, Dhaka, the overflow of drain and heavy rainfall could spread out the Cr containing effluents to the sampling area. The breaking of the boundary wall between the dumping yard (where Cr-containing and other solid waste are being dumped) and residential area could be the cause of the high Cr contamination in samples of zone 1. The CF of metal Zn showed very high contamination in zones 2, 3, and 4, but only moderate contamination in zone 1 (Table 2). Furthermore, zone 1 showed moderate contamination, and zone 3 showed considerable contamination for Cd.

Fig. 2(a–c) depicts the calculated I_geo, PLI and C_deg results of heavy metals in soils. Within agreement with the contamination factor and ecological risk, the soils were unpolluted with Cu, Ni, and Pb. This implies that the level of Cu, Ni, and Pb in the soil did not exceed the standard background concentration. In line with the previous calculation, zones 1 and 3 were extremely polluted by the metal Cr whereas zones 2 and 4 indicated slight pollution. Zones 2, 3, and 4 were moderately to severely polluted by the metal Zn, while zone 1 was moderately polluted by Zn (Fig. 2a). Only zone 1 was selected for slightly polluted by Cd, according to the I_geo. These categories of pollution with elevated I_geo values signify anthropogenic sources of heavy metals in the soils.

Fig. 2.

(a) Geoaccumulation index (I_geo), (b) pollution load index (PLI), and (c) degree of contamination (C_deg) of heavy metals contaminated soil area.

The PLI value of greater than 1 denotes an area that is polluted, whereas less than 1 is considered unpolluted. The PLI of the studied area ranged from 0.72 to 1.90, indicating that it was moderately to severely polluted (Fig. 2b). On the contrary, the degree of contamination (C_deg) in all zones ranges from 11 to 527 in soils (Fig. 2c). C_deg was found at 11.7 for zone 2 and 12.3 for zone 4 falls in the moderate class (8 ≤ C_deg< 16), whereas zones 1 and 3 showed a very high degree of contamination, as the C_deg value was observed as 142 and 527, respectively. As a result, the investigated moderate to very high degree of contamination should be considered for decontamination.

3.3. Ecological risk assessment

The potential Er and risk indices are delineated in Table S5. The risk of ecology was indicated as low to very high, while the Er values were reported between 0 and 1030. However, zone 1 holds a high potential ecological risk (Er = 267) and zone 3 exhibited a very high risk (Er = 1030) for Cr. This high potential risk is the cause of the elevated amount of Cr in zone 1 and zone 3 soils. Furthermore, metal Cd indicated a moderately and significantly increased risk for sampling zones 1 (Er values of 53) and 3 (Er values of 98). Other metals, with the exception of Cr and Cd, had lower electrochemical potential. In addition, zones 2 and 4 were observed to have low Er values, indicating low ecological risk. Considering all studied metals, the cumulative value of Er indicated that zone 1 exhibited considerable risk (RI = 330) and zone 3 exhibited very high risk (RI = 1140). Therefore, from the calculated results, it could be inferred that the soil ecology of zones 1 and 3 is seriously threatened by toxic metals which needs to be risk-free.

3.4. Human exposure assessment

3.4.1. Chronic daily intakes of heavy metals

Oral ingestion (CDI_ing), direct dermal absorption (CDI_dermal), and inhalation (CDI_inh) were studied for assessing chronic intakes of heavy metals in children and adults population human. Intrusions of heavy metal via ingestion are delineated in Table S6. For Cr, daily intakes ranged from 7.4 × 10⁻⁵ to 1.3 × 10⁻¹ mg/d. CDI_ing for adults was observed as 3.6 × 10⁻³ and 1.4 × 10⁻² mg/d in zones 1 and 3 and for children CDI_ing was 3.4 × 10⁻² and 1.3 × 10⁻¹ mg/d in zones 1 and 3, respectively, which were far beyond the USEPA’s recommended value (3 × 10⁻³ mg/d) of oral ingestion. For both the population groups, 10 times greater risk was observed for the children than for the adults. Daily intake of other metals like Cu, Cd, Pb, Ni, and Zn was below the value of reference (Table S6).

For Cr, the minimum and maximum dermal absorption value for adult (3.0 × 10⁻⁴ and 5.7 × 10⁻² mg/d) and children (2.2 × 10⁻⁴ and 4.2 × 10⁻² mg/d), respectively, (Table S7), which exceeded the reference dose value (6.0 × 10⁻⁵ mg/d) set by USEPA [40]. This implies that there might be a probability for Cr toxicity in the human body in the study area. Except for Cr, other metals were below the reference daily dermal intake value that indicates that other metals have no influence on the daily intakes of metals from the soil.

Zone 3 had a higher quantity of Cr intake via inhalation 2.8 × 10⁻³ and 1.3 × 10⁻² mg/d for the children and adult populations, respectively; (Table S8), which was greater than the inhalation guideline value of 3.0 × 10⁻³ mg/d. Compared to ingestion and dermal contact pathways, children inhale more metals from contaminated soil than adults. Nonetheless, metal Cr exceeded the standard guideline value for inhalation of daily intake for children only in Zone 1.

3.4.2. Non-carcinogenic risk

HQ_ing is represented in Table S9, which revealed that zones 1 and 3 indicated a higher value of HQ than 1, which signifies the probability of adverse non-carcinogenic consequences for both children and adults from metal Cr. The child population was discovered to be more vulnerable to non-cancer risks than adults and accounts for ten times the susceptibility to health risks as an adult. None of the other metals exhibited any non-carcinogenic effects on the population of the sampling area as the concentration of metal was below the safe limit (i.e., HQ < 1).

Considering the dermal contact (HQ_dermal) pathway, non-cancer effects via the metals are depicted in Table S10. The HQ_dermal of Cr was greater than one (1) for child and adult populations. Notably, zone 1 (HQ_dermal = 239) and zone 3 (HQ_dermal = 926) for the metal Cr manifested a very high HQ_dermal value for the adults. Similarly, children in zones 1 and 3 posed probable non-carcinogenic health risks, with HQ_dermal values of 179 and 693, respectively. Again, Cd presented a similar trend of hazards was observed for zones 1 and 3. Cu, Pb, Ni and Zn were under the reference limit of HQ (Table S10). HQ_inh for Cr was lower than 1 except for the child population of zone 1 which indicates no potential adverse health effects for the adults and children of other zones (Table S11).

An adverse non-carcinogenic effect was found, while the mean target hazard index (HI) values for Cr in adults and children were 295 and 235 (Table 3). The highest HI was observed (931 for Cr) in case of the adults. Further, HI values of Cd in zones 1 and 3 were noticed to be greater than 1 and the values were 1.17 and 2.16 for the children and 1.52 and 2.8 for adults, respectively which indicate Cd induced risks. The study's findings show that Cr is the primary health risk resulting from tannery activities, and that it can cause serious health problems if proper steps to mitigate, control, and remediate are not taken on time. Furthermore, Cd might be solicitous for the possible health risks in sampling zones 1 and 3. To prevent cadmium pollution in the study area, careful attention should be paid to controlling the use of cadmium-containing fertilizer.

Table 3.

Target Hazard Index (HI) and Total Carcinogenic risk (TCR) of heavy metals through ingestion, dermal absorption, and inhalation of soil.

	Sampling zone	Cr		Pb		Cu		Ni		Cd		Zn		Total Adult	Total child
		Adult	Child	Adult	Child	Adult	Child	Adult	Child	Adult	Child	Adult	Child
Target HI	Zone 1 (n = 14)	240	191	0.08	0.09	0.01	0.02	0.02	0.03	1.52	1.17	0.02	0.03	242	192
	Zone 2 (n = 6)	4.97	3.95	0.05	0.05	0.01	0.02	0.03	0.04	0.00	0.00	0.02	0.03	5.08	4.09
	Zone 3 (n = 16)	931	741	0.07	0.07	0.01	0.01	0.01	0.01	2.80	2.16	0.02	0.03	934	743
	Zone 4 (n = 9)	4.59	3.65	0.45	0.49	0.03	0.04	0.02	0.02	0.00	0.00	0.03	0.04	5.12	4.24
	Mean	295	235	0.16	0.17	0.02	0.02	0.02	0.03	1.58	1.22	0.02	0.03	297	236
Total Carcinogenic Risk (TCR)	Zone 1 (n = 14)	1.1E-03	2.8E-02	6.3E-09	1.6E-07	nc	nc	1.5E-06	4.1E-05	1.7E-06	4.5E-05	nc	nc	1.1E-03	2.8E-02
	Zone 2 (n = 6)	2.2E-05	5.7E-04	3.7E-09	9.2E-08	nc	nc	2.1E-06	5.6E-05	0.0 E+00	0.0 E+00	nc	nc	2.4E-05	6.3E-04
	Zone 3 (n = 16)	4.1E-03	1.1E-01	5.3E-09	1.3E-07	nc	nc	7.5E-07	2.0E-05	3.1E-06	8.2E-05	nc	nc	4.1E-03	1.1E-01
	Zone 4 (n = 9)	2.0E-05	5.3E-04	3.6E-08	9.1E-07	nc	nc	1.3E-06	3.5E-05	0.0 E+00	0.0 E+00	nc	nc	2.1E-05	5.6E-04
	Mean TCR	1.3E-03	3.4E-02	1.3E-08	3.2E-07	nc	nc	1.4E-06	3.8E-05	1.2E-06	3.2E-05	nc	nc	1.3E-03	3.4E-02
	Ingestion (%)	97.8	99.9	93.7	99.2	nc	nc	99.5	100	99.6	100	nc	nc	97.8	99.9
	Dermal (%)	0.07	0.01	5.34	0.79	nc	nc	0.23	0.03	0.14	0.02	nc	nc	0.07	0.01
	Inhalation (%)	2.11	0.08	0.97	0.04	nc	nc	0.23	0.01	0.21	0.01	nc	nc	2.10	0.08

nc-not calculated.

3.4.3. Carcinogenic risk

Metals that show carcinogenic risks (CR) to human health, such as Pb, Cd, Cr, and Ni have been taken into account for the CR analysis in this study. Cd, Cr, and Ni are well known human carcinogens and Pb is considered a plausible human carcinogen [34,40,41,49]. For three exposure pathways, combined metal designated values were depicted in Tables S12–S14; CR_ing (3.38 × 10⁻²) » CR_inh (2.73 × 10⁻⁵) > CR_dermal (3.53 × 10⁻⁶) for children and CR_ing (1.27 × 10⁻³) » CR_inh (2.73 × 10⁻⁵) for adults. This calculation implies that the CR values of children exceeded the guideline value of 1 × 10⁻⁶ while CR_dermal poses no potential carcinogenic risks to adults, suggesting children were more susceptible to cancer risk via all types of exposure pathways. Nevertheless, CR_ing by Cr manifested the highest toxicity indicating potential human carcinogen. CR_ing of Cr for child ranges 5.25 × 10⁻⁴ – 1.07 × 10⁻¹ (mean = 3.38 × 10⁻²) and for adult ranges 2.13 × 10⁻⁵ – 4.0 × 10⁻³ (mean = 1.27 × 10⁻³) that reveals childs are 26-fold more vulnerable than adults to cancer risk (Table S12). In addition, Cd and Ni metals showed CR_ing of 3.18 × 10⁻⁵ and 3.77 × 10⁻⁵ for the child, respectively. Risk through skin absorption was insignificant by Cr for children (mean = 3.51 × 10⁻⁶) and there were no metals posing CR_dermal (Table S13). CR_inh of soil for both adults and children were 8.6 × 10⁻⁵ (zone 3) and 2.22 × 10⁻⁵ (zone 1) for Cr only, which indicates a lower degree of carcinogenic risk (Table S14).

The sum of the carcinogenic risk (TCR) posed by collected soil samples through various exposure pathways is shown in Table 3. The mean value of TCR for the metals (except Pb) exhibited CR > 1 × 10⁻⁶ indicating cancer risks. However, the sequential distribution of the metals responsible for cancer risk indication could be as follows: Cr (mean = 3.38 × 10⁻²) » Ni (mean = 3.8 × 10⁻⁵) > Cd (mean = 3.2 × 10⁻⁵) for the children, and Cr (mean = 1.29 × 10⁻³) » Ni (mean = 1.42 × 10⁻⁶) > Cd (mean = 1.2 × 10⁻⁶) for adults (Table 3). The order of the exposure pathways, contributing to the total carcinogenic risks for different metals was ingestion (97.8–99.9%) »> inhalation (0.08–2.10%) > dermal absorption (0.01–0.07%). It is evident from the result that ingestion is the maximum contributor to CR posed by the exposure pathways which has been reported by other researchers earlier elsewhere [26]. In terms of contribution from metals, Cr appeared 99.8% and others showed only 0.20%, which designates the metal Cr as the highest contributor to the TCR (Table 3). However, the total CR of metal Cr crossed the threshold limit (1 × 10⁻⁶) therefore; it is a critical prerequisite to managing the Cr already entered and present in the soil for sustainability.

3.5. Heavy metals in vegetables

Concentrations of Cu, Cr, Pb, Zn, Cd, and Ni in vegetable samples are presented in Table 4. The revealed heavy metal concentrations have been compared with the FAO/WHO, and Chinese guideline values (Table 4). The obtained data report that the majority of metals crossed the guideline values set by WHO/FAO, and China, especially for Pb, Cr, and Cd. Again, metal concentrations were found to be higher in the vegetables of the tannery area, particularly in zone 3 samples, where leather is processed using metal complex chromium sulfate, primarily Cr.

Table 4.

Heavy metal concentration (mg/kg) of collected vegetable samples in Savar tannery area and nearby area.

Zone	Vegetables	S. ID	Cr	Pb	Cu	Zn	Ni	Cd
Zone 1	Stem amaranth (Amaranthus lividus)	VS1	0.36	1.16	4.80	86.6	0.24	1.06
	Bottle gourd (Lagenaria siceraria)	VS2	0.47	0.00	4.47	75.8	0.30	0.00
	Vine spinach (Basella alba)	VS3	1.39	0.25	5.41	209	0.32	1.88
	Jute leaf (Corchorus olitorius)	VS4	0.00	1.82	4.65	82.2	0.64	1.72
	Sweet pumpkin (Cucurbita moschata)	VS5	1.28	2.23	3.83	75.5	0.22	0.07
	Kalmishak (Ipomoea aquatica)	VS6	2.87	4.21	1.75	59.0	0.07	0.77
	Mean of zone 1		1.06	1.61	4.15	98.1	0.30	0.92
Zone 2	Broccoli (Brassica oleracea)	VS7	1.25	2.15	3.64	70.4	0.34	0.84
	Red amaranth (Amaranthus gangeticus)	VS8	0.74	0.83	3.55	70.3	0.22	1.01
	Stem amaranth (Amaranthus lividus)	VS9	1.10	0.83	3.80	73.6	0.17	1.47
	Chilli (Capsicum species)	VS10	3.04	0.00	3.13	77.4	0.09	1.32
	Spinach (Spinacia oleracea)	VS11	2.57	2.31	10.7	333	0.56	0.74
	Vine spinach (Basella alba)	VS12	3.86	3.8	2.77	80.6	0.12	1.04
	Mean of zone 2		2.09	1.65	4.59	118	0.25	1.07
Zone 3	Red amaranth (Amaranthus gangeticus)	VS13	5.53	0.64	4.49	124	0.14	1.91
	Sweet pumpkin (Cucurbita moschata)	VS14	11.1	0.64	3.06	33.9	0.00	0.89
	Lady's Finger (Abelmoschus esculentus)	VS15	15.7	0.75	3.19	34.5	0.34	1.53
	Eggplant (Solanum melongena)	VS16	13.2	1.08	3.30	76.9	0.23	1.14
	Chilli (Capsicum species)	VS17	2.75	0.00	1.75	42.7	0.00	0.95
	Vine spinach (Basella alba)	VS18	3.04	0.43	1.15	28.3	0.00	0.85
	Kalmishak (Ipomoea aquatica)	VS19	4.79	0.00	4.14	107	0.80	0.71
	Jute leaf (Corchorus olitorius)	VS20	6.39	0.00	2.76	48.4	0.63	0.74
	Lady's Finger (Abelmoschus esculentus)	VS21	11.5	0.21	2.16	38.1	0.46	1.76
	Bottle gourd (Lagenaria siceraria)	VS22	27.9	0.54	3.49	25.7	0.84	1.20
	Red amaranth (Amaranthus gangeticus)	VS23	11.8	1.08	1.99	66.9	0.18	1.76
	Vine spinach (Basella alba)	VS24	14.2	0.00	2.06	73.6	0.60	1.14
	Mean of zone 3		10.7	0.45	2.79	58.3	0.35	1.21
Zone 4	Lady's Finger (Abelmoschus esculentus)	VS25	0.98	1.01	4.03	71.5	0.21	1.10
	Snake gourd (Trichosanthes anguina)	VS26	1.08	1.15	5.12	95.1	0.13	0.45
	Stem amaranth (Amaranthus lividus)	VS27	1.55	2.18	3.46	25.3	0.26	0.19
	Mean of zone 4		1.20	1.45	4.20	64.0	0.20	0.58
	FAO/WHO limit [50]		2.3	0.1	40	9.4	10	0.05
	Chinese standards [51]		0.5	0.1–0.3	10	–	10	0.1–0.2

Cr has been got attention for its high accumulation in the collected vegetables as indicated earlier as deposited in soils especially in zone 3. Metal Cr in vegetables were 2.75–27.9 (Mean = 10.7), 0.74–3.86 (Mean = 2.09), 0.98–1.55 (Mean = 1.20) and 0–2.87 (Mean = 1.06) mg/kg in zones 3, 2, 4, and 1, respectively (Table 4). This indicates that the average concentration of Cr in the collected vegetable samples in zone 3 was almost 21 and 5-folds higher than the standard allowable limit recommended by China and WHO/FAO, respectively. Additionally, excluding zone 4, some of the samples from zones 1 and 2 exceeded the daily heavy metal intake standards regulated by WHO/FAO. The high Cr accumulation in vegetables might be due to the high initial Cr concentration in the soil where vegetables were cultivated and grown up as reported in Fig. 1, Table 1, and Fig. S1. However, the primary definite source of Cr in the collected vegetables is the use of Cr-containing tanning salts in leather processing, which could come from the daily discharge of Cr-rich effluents.

Metal Pb was observed maximum of 4.21 mg/kg in Kalmishak (Ipomoea aquatica) in zone 1 followed by vine spinach in zone 2 (3.80 mg/kg). The concentration of Pb (mg/kg) in zones 1, 2, 3, and 4 distributed as 0–4.21 (Mean = 1.61), 0–3.80 (Mean = 1.65), 0–1.08 (Mean = 1.08) and 1.08–2.18 (Mean = 1.45), respectively (Table 4). This implies that there was not as much industrial discharge of Pb that occurred as there was of Cr. However, the transport system, recycling of batteries, and dumping Pb batteries in the study zones might be responsible causes for this increased Pb concentration.

The concentration of Cu in the collected vegetables is depicted in Table 4 where the minimum and maximum concentrations of Cu (mg/kg) in samples were 1.75 and 5.41 in zone 1, 2.77 and 10.66 in zone 2, 1.15 and 4.49 in zone 3, 5.12, and 3.46 in zone 4. This result indicates that all of the collected vegetable samples are safe in terms of Cu and met the WHO/FAO recommended limits.

In zones 1, 2, 3, and 4, the average concentration of metal Zn in vegetables was 98, 118, 58, and 64 mg/kg, respectively. In this study, Zn content exceeded the allowable concentration suggested by the WHO/FAO (9.4 mg/kg) standards, which indicates anthropogenic application of Zn has occurred. Nevertheless, Zn containing pigments are used in leather finishing, which might intensify the increase of Zn metal in the vegetables through accumulation in the soil. The Ni concentration in vegetable samples was observed within the all acceptable limit highlighted in this research earlier which indicates no Ni deposition in the vegetables. The maximum Ni concentration was 0.84 mg/kg, and the minimum value was 0.0 mg/kg (Table 4).

Table 4's calculated Cd result showed a countable variation in concentration in vegetables and for location. The edible vegetables and plants were observed as notable value of Cd compared to the Chinese national standards (0.1–0.2 mg/kg) and WHO/FAO (0.05 mg/kg). Cd (mg/kg) levels in vegetables collected from zones 3, 2, 1, and 4 were 1.21 (Extent = 0.71–1.91), 1.07 (Extent = 0.74–1.47), 0.92 (Extent = 0–1.88), and 0.58 (Extent = 0.19–10.1), respectively. The analysis indicates that the vegetables exhibited a high accumulation of Cd that might have occurred due to the uncontrolled use of various fungicides, insecticides, and phosphate-containing fertilizers during the cultivation of vegetables containing Cd metal [10].

3.6. Health risk of metals from vegetables

Here, six heavy metals present in the vegetables were selected for evaluating ADI_veg, THQ, and HI as having non-carcinogenic effects on human health through vegetable consumption, and data are presented in Table S15 and Table S16. The highest value of ADI_veg was observed for Zn (0.336 mg/day), while the lowest was for Ni (0.001 mg/day) (Table S15). The mean THQ in vegetables for Cr, Cd, Pb, Zn, Cu, and Ni was 3.57, 2.70, 1.05, 0.80, 0.56, and 0.02, respectively (Table S16).

From vegetable consumption, Pb, Cr and Cd exceeded the value of 1, manifesting considerable health hazards. However, the extent of THQ for Cr ranged from 1.01 to 10.2 for all four sampling zones studied, with zone 3 in particular having a very high THQ value. Uncontrolled discharge of Cr-containing effluent and the solid wastes from tanneries to the soil might lead to this high accumulation of Cr in the soils which continues to be accumulated in the vegetables and contributes 46% to the total HI (Table S16). Furthermore, the mean HI was found to be 8.71 with a maximum of 15 and a minimum of 5.21, corresponding to sampling zones 3 and 4, respectively (Table S16). Contributions to THQ from individual vegetables of Cr, Pb, Cu, Zn, Ni, and Cd are shown in Fig. 3(A-F). Therefore, eating these metal-rich vegetables cultivated in any of the sampling zones might pose adverse risks, especially non-cancer risks to humans.

Fig. 3.

Individual contribution of vegetables (%) to target hazard quotient (THQ) of metal Chromium (A), Lead (B), Copper (C), Zinc (D), Nickel (E), and Cadmium (F).

The target risk for carcinogenicity from metal-contaminated vegetables is demonstrated in Fig. S2. The metals Pb, Cr, Cd, and Ni are well known for their human cancer risk, and Ni, Cd, and Cr, are designated as probable human carcinogen (group A) and Pb in probable human carcinogens (group B) plotted by USEPA and WHO [41,49]. Metal Cd and Cr showed cancer risk in all of the zones which ranges 4.33 × 10⁻² to 1.81 × 10⁻¹ for Cd and 1.18 × 10⁻² to 1.19 × 10⁻¹ for Cr which is far beyond the normal acceptable range (1 × 10⁻⁶ - 1 × 10⁻⁴) indicating prone to potential CR by consuming vegetable grown in the study. The total CR as well as the mean value of CR (9.06 × 10⁻²) also exceeded the acceptable guideline range. From Fig. S2, it is obvious that for all zones, none of the vegetables were risk-free for cancer risks, particularly in zone 3. Therefore, only people's awareness along with pollution control could pave the way to escape the risk. Also, there should be a policy to remediate the contaminated area before planting vegetation and trees.

4. Conclusions

Heavy metal pollution in the tannery area's soils and vegetables was assessed, and it was observed that samples were highly contaminated with Cr and Cd. The calculated value of heavy metals through different indexes depicted that the study area was contaminated with heavy metals. Contaminated soil posed moderate to very high ecological risks, and some of the zones with the value of the hazard index greater than unity showed high potential non-cancer risks. TCR also expressed adverse cancer risks to soil exposure where ingestion was the major pathway and accounted for more than 99%. Children were the most vulnerable group of the population for metal exposure from soil, leading to a potential risk for their health. The levels of Cr, Zn, and Cd in vegetables were higher than the permissible limit, indicating high risks to humans. From the results, it can be concluded that the soils and vegetables in the tannery area are enriched with a high amount of heavy metals, especially Cr. However, Cr-free leather processing could be an alternative for controlling the Cr contamination of the soil as well as vegetables. The study recommended creating awareness among the local people to stop the cultivation of vegetables around the tannery area in Bangladesh.

Declarations

All authors have read, understood, and have complied as applicable with the statement on “Ethical responsibilities of Authors” as found in the Instructions for Authors and are aware that with minor exceptions, no changes can be made to authorship once the paper is submitted.

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Authors can confirm that all relevant data are included in the article and its supplementary information files.

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All of the authors have read and approved the paper and it has not been published previously nor is it being considered by any other peer-reviewed journal. Also, this manuscript has not been submitted to any preprint server before the submission.

Author contribution

Al Mizan: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper. Mohammad Arif Hasan Mamun: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data; Wrote the paper. Md. Saiful Islam: Analyzed and interpreted the data; Wrote the paper.

Declaration of competing interest

The authors have no relevant financial or non-financial interests to disclose.

Appendix A. Supplementary data

The following is the Supplementary data to this article: