Assessment of heavy metals with ecological risk of soils in the industrial vicinity of Tangail district, Bangladesh

This study was conducted to assess the ecological risk of heavy metals in soils collected from the industrial vicinity of Tangail district in Bangladesh. In this study, the levels of six heavy metals namely chromium (Cr), nickel (Ni), copper (Cu), arsenic (As), cadmium (Cd), and lead (Pb) in 15 sampling sites around the industrial vicinity of Tangail district in Bangladesh were assessed. The mean concentration of Cr, Ni, Cu, As, Cd and Pb in studied soils were 11.56, 23.92, 37.27, 6.11, 2.01, and 17.46 mg/kg, respectively. Certain indices, including the enrichment factor (EF), contamination factor (Cf), geoaccumulation index (Igeo), pollution load index (PLI), toxic unit analysis, and principal component analysis (PCA) were used to assess the ecological risk. The enrichment factor of all the studied metals for all sampling sites were in the descending order of Cd > Cu > As > Pb >Ni > Cr. The contamination factor values revealed that the studied soils were highly impacted by Cd. The pollution load index (PLI) values of Cd were higher than 1, indicating the progressive deterioration of soil due to Cd contamination. In the context of potential ecological risk (PER), soils from all sampling sites showed moderate to very high potential ecological risk.


Introduction
For the survival of human life on the planet, soil act as a dynamic natural resource and regarded as the key receiver of the persistent pollutants like heavy metals (Luo et al. 2007, Karim et al. 2014, Proshad et al. 2018. Soil pollution by heavy metals is a global problem that is highly predisposed by humaninduced activities (Han et al. 2002, Vare 2006. Nowadays, soil pollution by heavy metals has become an environmental issue in both developed and developing countries all over the world , Sun et al. 2010. Heavy metals such as nickel (Ni), chromium (Cr), copper (Cu), arsenic (As), cadmium (Cd), and lead (Pb) have been considered as the most toxic elements in the environment by the US Environment Protection Agency (EPA) (Luo et al. 2007, Lei et al. 2010, Proshad et al. 2017). In recent decades, there has been a major concern regarding soil pollution by various heavy metals due to rapid industrialization and urbanization, especially in developing countries (Islam et  . Heavy metals are of great concern due to their toxicity, non-biodegradable properties and accumulative behaviors . Heavy metals may initiate in soils around the industrial area from various sources of which are industrial activities, power generation, manufacturing, waste spills, or fossil fuel burning and waste disposal (Luo et   . The accumulation of heavy elements in soils is a great concern due to their potential environmental risk and harmful effects on soil ecosystems , Cui et al. 2004, Li et al. 2009, Yu et al. 2012, Yuan et al. 2014). To assess the ecological risks of heavy metals in soil dif-ferent methods have been widely used, such as enrichment factor (EF), contamination factor (C i f), toxic unit analysis, and geoaccumulation load index (Igeo) , Rashed 2010). The enrichment factor of an area indicates the relative enrichment in any pollutant when compared to pre-industrial soils from the same environment , Sayadi and Sayyed 2011, Hower et al. 2013, Dias et al. 2014). As soil contamination arising from industry, the study area has raised attention due to its environmental pollution which is facing serious threats due to heavy metals pollution originated from the rapid development, congestion, and activities from industries ). Several studies have stated the concentration of heavy metals in the industrial area soils in Bangladesh , Ahmad and Goni 2010, Rahman et al. 2012. Therefore, in this study, the variations of heavy metals in soils of different soil sampling sites were studied. The objective of this study was to assess the ecological risk of heavy metals in soil in the industrial vicinity of Tangail district in Bangladesh.

Study area and sampling
The samples were collected from Tarutia, Tangail Sadar Upazila of Tangail district, Bangladesh ( Figure 1). Tangail district area is 334.26 km² and situated at the middle part in Bangladesh. Tangail Sadar Upazila is highly densely area in Bangladesh and population density is 1,100/km2 in Tangail district. The study area is situated between Tangail Sadar is located at 24.2500°N to 89.9167°E. Tangail as an industrial vicinity of Bangladesh possesses highly vulnerable to environmental pollution nowadays. There are several types of industrial units including garments, packaging industry, dyeing, brick kiln, metal workshops, battery manufacturing industries, tanneries, textile industries, pesticide and fertilizer industries, different food processing industries and other factories produce huge volumes of effluents that contain trace metals. These industries are discharged untreated wastes randomly to river and canals. Then that wastes are mixed with soils and the soil is continuously polluted by toxic elements in the industrial areas of Tangail district in Bangladesh. Soil samples were collected during March-April, 2016. Tarutia was selected for sampling location situated near the industrial area of Tangail district, Bangladesh. Fifteen soil sampling sites were selected in the industrial areas of Tangail district. Agricultural field soil samples (samples were collected from the surface soil up to 10 cm) were taken and three subsamples collected which were used as composite sample by mixing it thoroughly. Samples were kept in air-dried at normal temperature for two weeks, then ground and homogenized. Soil was taken with the help of a percussion hammer corer (50-80 cm in length) for metal analysis and those samples were treated as pre-industrial sample (Schottler and Engstrom 2006). A porcelain mortar and pestle used to crumble all dried soil samples. Then the samples were sieved with 2 mm nylon sieve. The soil samples were stored in a clean Ziploc bag which was airtight and used for chemical analysis. Several researchers also followed the alike procedure for sampling and storing of soil samples (Oliveira et

Sample analysis
Analytical grade reagents were used for sample analysis and Milli-Q (Elix UV5 and MilliQ, Millipore, USA) water was used for the preparation of the solution. 4.5 mL 35% HCl (Kanto Chemical Co, Tokyo, Japan) in a closed Teflon vessel added with 1.5 mL 69% HNO3 (Kanto Chemical Co, Tokyo, Japan) which was mixed with 0.3 g of the soil sample and Microwave Digestion System (Berghof speedwave ®, Eningen, Germany) was used in metal analysis for soil sample. The digested solution was then filtered using a syringe filter (DISMIC®-25HP PTFE, pore size= 0.

Instrumental analysis and quality control
For hazardous elements, samples were analyzed using inductively coupled plasma mass spectrometer (ICP-MS, 7700 series). Multi element Standard XSTC-13 (SpexCertiPrep®, Metuchen, USA) solutions were used to prepare the calibration curve. Multielement solution (Agilent Technologies, USA) 1.0 μg/L was used as tuning solution covering a wide range of masses of elements. All test batches were evaluated by applying internal quality system and validated if they satisfied the defined Internal Quality Controls (IQCs). Where, (CM/CAl) sample is assumed as ratio of hazardous element concentration of (CM) to that of aluminum (CAl) in the soil sample, and (CM/CAl) background is the same reference ratio in the background sample. Enrichment factor value of toxic element is equal to 1 indicate that toxic elements arise due to natural weathering processes in the environment (Zhang and Liu 2002). When enrichment factor is higher than 1.5 resulting of human interference. Enrichment factor effects of metals known as minor, moderate, severe, and very severe modification when enrichment factor value are 1.5-3, 3-5, 5-10 and >10 respectively (Birch and Olmos 2008).

Contamination factor (C i f)
Contamination factor is the ratio of the metal concentration in the soil to that of baseline or background value: The levels of contamination factor may be grouped into four classes ranged from 1 to 6 which are: low degree (C i f <1), moderate degree (1 ≤ C i f < 3), considerable degree (

Geoaccumulation index (Igeo)
Geoaccumulation index (Igeo) is assumed as an impressive tool to determine contamination degree from toxic metals. At present, geoaccumulation index is used globally to assess soil pollution (Santos et al. 2003). The most effective objective to determine geoaccumulation index (Igeo) is to identify pollution level in soil.
Geoaccumulation index (Igeo) may be assessed by applying equation given here by, Where, Cn is the determined element (n) concentration assessed from soil, Bn is the geochemical baseline value of element n in background sample (Yu et al. 2012). For decreasing possible variation in background values of element n, factor 1.5 is used to ascribe lithogenic effects.

Pollution load index (PLI)
Pollution load index is a compound system for determining the quality of soil. Pollution load index can be determined for six toxic metals like chromium, nickel, copper, arsenic, cadmium, and lead (Suresh et al. 2011). Pollution load index may be measured from a formula given here by: Pollution load index is the result of total toxicity level of hazardous metals in soil.

Potential ecological risk (PER)
The degrees of hazardous metals contamination in agricultural soils are determined by PER index. The equations which were used to calculate PER proposed by Guo and are as follows (Guo et al. 2010): Where,

Toxic unit analysis
The sum of toxic units (ΣTUs) is considered as acute toxicity of toxic metals in agricultural soils. Toxic unit analysis is stated as the ratio of the assessed concentration of hazardous elements in soil to probable effect level (PELs) (

Physiochemical properties of soil
The physicochemical properties of soil are presented in Table 1.
The studied soils pH values were ranged from 5.69 to 7.54 indicating that soils were slightly acidic to neutral excluding the S5, S6, S7, S8, S9, S10, and S13 site that were alkaline (Table 1) because of decomposition of organic matter and subsequent formation of carbonic acid. The highest values of soil pH were observed in S7 and S9 sites. Electrical conductivity (EC) value of the soil was non-saline (0-2 dS/m; SRDI soil salinity class) for all sampling sites which mean the salinity effect is negligible. The range of organic carbon (% C) was 0.149 to 3.113, where the highest value was observed in soil collected from the S3 site. According to the United States soil texture classification system, the textural analysis showed that the soil samples were loam, sandy loam, sandy clay loam, and silt loam (Table 1).

Heavy metal contamination in soil
The heavy metals concentrations of (Cr, Ni, Cu, As, Cd, and Pb) in soil samples are presented in (Table 2, Table 3, and Figure 2). The mean concentrations of Cr, Ni, Cu, As, Cd, and Pb in soil were found 11.56, 23.92, 37.27, 6.11, 2.01, and 17.46 mg/kg, respectively ( Table 3). The maximum value of Cr, Ni, Cu, As, Cd, and Pb were observed in soil collected from the S3, S2, S5, and S1 site. Heavy metals in soils were compared with the other studies in Bangladesh and other countries. Cr, Ni, As, and Pb concentrations of the present study were higher than those of the study conducted in Bangladesh, Spain, Turkey, and India ( Table 3). The mean concentrations of Cu were above the Dutch Soil Quality Standard ( Table 3). The mean concentrations of Cd were above the Dutch Soil Quality Standard and Canadian Environmental Quality Guidelines value ( Table 3). The Dutch Soil Quality Standard is considered as the most appropriate guideline indicating all possi-ble exposure pathways for protecting humans, plants, and animals (Chen et al. 2011). The soil is considered clean, if any metal concentration in soil is below its respective Dutch Target Value. The soil is regarded to be slightly to moderately contaminated, if the concentration level lies between the target values and intervention values. In contrast, if the value is above the Dutch Intervention Value, the soil is considered detrimental to humans, plants, and animals. According to Table 3, Cu and Cd were in the worst situation among the studied metals as the mean concentration of Cu and Cd was higher than the Dutch Target Value.

Source analysis of heavy metal in soil
To identify the source of heavy metals in soils of several sampling sites of the industrial area, a principal component analysis (PCA) was conducted, which has been considered to be an effective tool for source identification (  is significant at the 0.05 level (two-tailed). ** = Correlation is significant at the 0.01 level (two-tailed).

Toxic unit analysis
Possible acute toxicity of heavy metals in soil samples can be estimated as the sum of toxic units (ΣTUs), defined as the ratio of the determined concentration of metal in soil to probable effect levels (PELs) ( Figure 7. The sum of toxic units for the studied metals for the sites S7, S2, and S9 was higher than the other sites, which were in the similar trends of metal concentrations in soils. If the sum of toxic units of soils was greater than 4, indicating a moderate to serious toxicity of heavy metals (Bai et al. 2011). In the studied soils, no samples were found which sum of toxic units was higher than 4.

Ecological risk assessment
In this study, the enrichment factor (EF), contamination factor (C i f), geoaccumulation index (Igeo) and pollution load index (PLI) were applied to assess the heavy metals contamination in soils. The enrichment factor values for studied soils are presented in Figure 3. Cd and Cu showed the highest enrichment factor value indicates the soil pollution for all the sampling sites. As a whole, the enrichment factor of all the studied metals for all sampling sites were in the descending order of Cd > Cu > As > Pb >Ni > Cr. Generally, studies have observed that little enrichment values indicate a great contribution for crusted source to the soil, while high enrichment factors indicate a substantial contribution from anthropogenic sources , Rashed 2010, Yadao and Rajamani 2006. Hakanson defines four types of contamination factors (CF) (Håkanson 1980), four types of degree of contamination (Cd), five types of i r E , and four types of PER are presented in Table 7. The contamination factor for individual metal was presented in Figure 4. In the studied area, contamination factor was higher for Cu and Cd. Igeo values of the present study are presented in Figure 5. For all heavy metals in the studied samples for differ-ent sampling sites, the Igeo values indicated the decreasing order of Cd>Cu>Pb>As>Ni>Cr. The mean of Igeo values for all the studied metals for all sampling sites indicating the soils were slowly contaminated with heavy metals. The value of pollution load index (PLI) equal to zero means perfection; a value of 1 indicates the presence of only baseline level of pollutants and values above 1 indicate progressive deterioration of soil by heavy metals , Proshad et al. 2017, Rashed 2010). As per above grade, studied soils were highly contaminated by Cd and it was observed that pollution load index (PLI) values of all others heavy metals for all sampling sites were not more than one ( Figure 6). Combining the potential ecological risk index of individual metals ( i r E ) and the potential ecological risk index of the environment (PER) ( Table 6) with their grade classifications (Table 7), soils from all sampling sites indicate the moderate to very high potential ecological risk in the studied area. PER represents the sensitivity of various biological communities, to toxic substances and illustrates the potential ecological risk caused by the heavy metals . The order of i r E in soils was in the following descending order of Cd> As> Cu> Ni> Pb>Cr.

Conclusions
Contamination of heavy metals (Cr, Ni, Cu, As, Cd, and Pb) was investigated soils in the industrial vicinity of Tangail district in Bangladesh. This study revealed that all soil samples from different sites were heavily contaminated by heavy metals especially Cu and Cd (80% and 60% samples exceed the Dutch Soil Quality Target Value). The enrichment factor (EF), contamination factor (C i f), geoaccumulation index (Igeo), pollution load index (PLI), toxic unit (TU) analysis revealed that soils in this study were highly contaminated by the Cd. Heavy metals in soil for different sampling sites showed moderate to very high degree of contamination. For individual heavy metal, only Cd had very severe ecological risk for most of the sites, whereas, the study area comprises high potential ecological risk according to the ecological risk indexes of heavy metals. However, it is necessary to further study to explain the reasons for the higher potential ecological risk caused mainly by Cd in different industrial area soils of Tangail district in Bangladesh.