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Mobility of Chromium, Copper and Arsenic in Amended Chromated Copper Arsenate Contaminated Soils

Aim: The use of copper-based preservatives such as chromated copper arsenate (CCA) and creosote to prolong the life of lumber present environmental concerns because they contain heavy metals and polycyclic aromatic hydrocarbons which are toxic to humans. The aim of this study was to investigate the effects of sewage sludge biosolid amendment on the distribution and mobility of chromium, copper and arsenic in chromated copper arsenate contaminated soils subjected to phytoremediation using maize (Zea mays L.). Original Research Article Nakiguli et al.; AJACR, 6(4): 33-48, 2020; Article no.AJACR.61001 34 Place and Duration of the Study: Random composite soil samples from Kitetika wood factory, Wakiso, Uganda and sewage sludge biosolid from National Water and Sewerage Corporation plant in Bugolobi, Kampala, Uganda were collected and prepared. Maize grains were obtained from FICA Seeds Limited (Uganda). The pot experiments and analysis of samples were done at Mbarara University of Science and Technology (Mbarara) and Directorate of Government Analytical Laboratory, Kampala (Uganda), respectively. Methodology: The fresh CCA contaminated soils and sewage sludge biosolid were analyzed for physicochemical parameters and heavy metals (chromium, copper and arsenic). Sewage sludge biosolid was added to 1 kg of the contaminated soils at 5-25% (w/w) in 2 L plastic containers, watered and maintained at 25 °C for 14 days to stabilize. Controls were set up with unamended soils. Thereafter, maize was planted in the potted soils for 40 days. The concentrations of the trace metals in the soils were determined after 20 and 40 days of maize growth by atomic absorption spectroscopy. Results: The concentrations of chromium, copper and arsenic in fresh CCA contaminated soils were 365.8 ± 6.18 mg/kg, 109.72 ± 14.04 mg/kg and 28.22 ± 3.8 mg/kg, respectively. Basing on mobility factor, bioavailability of the trace metals followed the chemical sequence copper (8.9%) < chromium (17.1%) < arsenic (30.2%). Conclusion: The maize variety experimented could be used to phytoextract or phytostabilize the trace metals in the CCA contaminated soils without or with 5-25% amendment. Amendment with sewage sludge biosolid improved the phytoremediation potential of maize. Arsenic was the most mobile and bioavailable metal in CCA contaminated soils. Further studies should use other local maize varieties such as Longe series.


INTRODUCTION
The dynamic nature of the building environment in Uganda poised by rapid surge in population, urban agriculture and massive industrialization has led to a rise in the demand for wood [1,2]. Thus, numerous unregulated industries have sprung up in Ugandan wood industry, leaving several environmental footprints [3][4][5]. Because of its susceptibility to attack by insects, wood is often preserved using chemicals [6,7]. Wood preservation have been classified broadly as oil borne or waterborne, contingent on the nature of the chemicals employed in the treatment formulation [7]. Oil borne preservatives include creosote and pentachlorophenol, which are used to treat wood for specific industrial uses. On the other hand, waterborne preservatives such as chromated copper arsenate (CCA) are the most common, and are used for preservation of lumber products used in commercial and residential applications [6].
The inadvertent use of preservatives such as CCA, creosote and other nascent copper-based formulations to prolong the life of lumber present environmental concerns because they contain heavy metals and polycyclic aromatic hydrocarbons which are toxic to humans [7][8][9][10]. Arsenical exposure in humans for example causes vomiting, bloody diarrhoea, oesophageal discomfort, abdominal pain and in chronic cases may cause cancer and dermal changes [11]. Chronic human exposure to hexavalent chromium causes skin irritation and corrosion of mucous membranes of the respiratory system as well as lung cancer [12].
However, the toxicity, mobility, bioavailability and geochemical processes of trace metals in the environment vary, contingent on their geochemical fractions [18,19]. Sequential extraction is the preferred analytical technique for quantification of various binding fractions of heavy metals in environmental matrices [18,20]. Trace metals typically exists as water soluble, exchangeable, carbonates bound, occlude in manganese, iron oxides and hydroxides, bound to organic matter or appear in the residual fraction [21,22]. These forms are proportionate to the salient interaction processes, bioavailability, transfer behaviors as well as toxicity of the metals [23]. For example, wood treated with CCA contain arsenic in the inorganic pentavalent form [24,25] which act as an insecticide, and copper as Cu (II) which protects the wood from fungi [10]. Chromium on the other hand occurs in the trivalent form [26] and fixes copper and arsenic complexes such as chromium (III) arsenate or chromium dimerarsenic clusters [27] into the structures of lignin, cellulose or hemicellulose of the wood [28,29]. In soils however, the distribution of trace metals is influenced by pH, redox potential, ionic strength, biological, redox and complexation reactions.
The use of CCA for treatment of wood has been banned in many countries, though this has not been fully implemented in some developing countries like Uganda. In countries where it has been banned, some structures built using CCA treated wood are still in existence, and leach CCA into the immediate environment, causing heavy metal pollution [30,31]. For this reason, remediation of such contaminated soils are still required to reduce the risk of exposure to the heavy metals by humans. Phytoremediation, one of the popular approaches used for this purpose utilizes higher plants and their associated rhizosphere microorganisms to remediate soils, sediments, surface and ground water contaminated with toxic metals, organics and radionuclides by either removing the pollutants or lowering their bioavailability [32][33][34]. Plants effect this using their roots in which the pollutants are accumulated, and thus their bioavailability can be modulated [35,36]. In continuity of our study [5], we evaluated the effects of sewage sludge biosolid amendment on the potential of maize in cleaning up CCA contaminated soils. Further, the mobility of chromium, copper and arsenic in the different fractions of the amended soils after 20 and 40 days of phytoremediation were investigated.

Brief Description of the Study Area
The study was conducted using soil samples collected from Kitetika Wood Factory located along Gayaza road, Wakiso district of Uganda, about 7 miles from Metropolitan Kampala (coordinates 0.4030814 and 32.585174; Fig. 1). This establishment commenced active operations 20 years ago, producing treated poles used for distribution of hydroelectricity, fencing land and construction. The main chemical used for treatment of poles at this facility is CCA [5].

Sample Collection
Random aggregate soil samples (50 kg in total) were collected from Kitetika Wood Treatment Factory in Kitetika village, Nangabo Sub-county, Wakiso district of Uganda using plastic spades at depths of 0-15 cm. The composite samples were thoroughly mixed to give the final sample which was subsequently packed into air-tight polythene bags. Sewage sludge biosolid (50 kg) was obtained in clean polythene bags from National Water and Sewerage Corporation Plant situated in Bugolobi, Kampala, Uganda.

Sewage Sludge Application and Pot Experiments
Composted sewage sludge biosolid (SSB) were added to 1 kg of CCA contaminated soils at 5, 10, 15, 20 and 25% (w/w) and each was replicated thrice. The mixtures were potted in 2L plastic containers and these were watered with double distilled water to the soil water holding capacity (Fig. 2). Control soils were set up without any amendment. The containers were left at room temperature (25 ℃) for two weeks for the soils to stabilize with intermittent mixing. After two weeks (14 days), soil samples (5 g) were obtained from each container and subjected to physicochemical analysis [5, 37,38].
Maize grains were soaked in water for 5 hours and six viable ones were sown in each container. Watering was done with double distilled water when necessary and hand weeding was used to keep the plants free of weeds. The experiments were maintained in a screen house at 27-28 ℃ to preclude any aerial deposition of heavy metals on the soil surfaces. Soil samples were taken again from the potted soils after 20 and 40 days of maize growth.

Heavy Metal Analysis
Soil samples (1.00 g) were digested using 5 ml of a mixture of aqua regia (3:1 v/v concentrated nitric acid: concentrated hydrochloric) and 1 ml of perchloric acid in 250 ml conical flasks. The samples were digested on a heating digester until white fumes of perchloric acid appeared. The solutions were cooled and subsequently filtered into 50 ml volumetric flasks and made to the mark with distilled water [18]. A 20% blank prepared using 15 ml of 63% nitric acid (Sigma Aldrich) and 5 ml of 98% sulphuric acid was first run to remove any traces of background interferences that would otherwise cause inaccuracy of results. All the samples were analyzed for Cr, Cu and As using AA 6300 Shimadzu double beam atomic absorption spectrophotometer (Shimadzu Corporation, Japan) at Directorate of Government Analytical Laboratory, Kampala, Uganda. Analyses, including five sets of standards for each metal were run in triplicate and the absorbances were used to compute the concentrations of the metals from the standard curves [39]. The results in mg/L from the instrument were converted to the standard unit (mg/kg) for easy comparison with the set international compliance guidelines [14,40].
All the reagents used in this study were of high analytical purity. All the volumetric ware used were soaked in 5% nitric acid overnight and rinsed with double distilled water. Standard solutions were prepared, and these were used for calibration and quality assurance for each of the analytical batch. Quality control was performed with spiked samples analyzed once for every 10 samples. Recovery percentages from the spiked samples ranged from 96.8% to 103%. Method detection limits with reagent blanks were calculated and these were 1.60, 0.50 and 0.90 mg/kg for Cr, Cu and As respectively. Analytical, equipment and filtration blanks were determined throughout the analyses, and subtractions were used to correct the heavy metal concentrations obtained. All samples were analyzed in triplicate.

Sequential Extraction and Assessment of Metal Mobility
The concentration of chromium, copper and arsenic in the soil fractions were determined after 20 and 40 days of planting. The extraction scheme used was based on six operationally defined fractions [41,42] modified from Tessier et al. [18] (Table 1).  The pH of 15 ml of 30% hydrogen peroxide solution in a conical flask was adjusted with concentrated nitric acid to 2.0. The acidic solution was added to the residue from F 4 . The resultant solution was heated for 5.5 hours on a water bath at 80 ℃. After cooling, 5 ml of 3.2M ammonium acetate in 20% nitric acid was added. The resultant solution was shaken for 30 minutes and diluted with 20 ml of double distilled water. F 6 Residual Measured 5 ml of nitric acid and 1 ml of perchloric acid were added to the residue from F 5 in a 250 ml conical flask. The mixture was heated on a hot plate at 60 ℃ for 6 hours. Exactly 1 ml of 2M nitric acid was added.
After each extraction, the supernatant solutions were separated by centrifugation at 1500 rotations per minute for 15 minutes (except for the residual fraction) followed by filtration through Whatman No. 1 filter paper. All the solid phases (residues) were washed with distilled water (10 ml) between the extraction steps. The washings were filtered through Whatman No.1 filter paper and the filtrate were added to the supernatant fraction.
Heavy metal mobility in the soils were assessed on the basis of their absolute and relative content of fractions weakly bound to the soil components. The relative index of the heavy metals mobility were calculated as the "mobility factor" using Equation Where F 1 to F 6 are the fractions defined in Table  1.

Statistical Analysis
All quantitative data, unless otherwise stated, were presented as means with errors represented by standard deviations attached. Significant differences between means of the investigated parameters were determined by one-way ANOVA and separated using Turkey pairwise test. The analyses were ran using Minitab statistical software (Release 17, Minitab Inc., USA) with statistical significance set at p < 0.05.

Physicochemical Characteristics of CCA Contaminated Soils and Sewage Sludge Biosolid
Physicochemical properties of soils influence metal speciation, mobility, bioavailability and toxicity in them. In this study, the pH of both CCA contaminated soils (6.77 ± 0.11) and SSB (6.64 ± 0.50) were near neutral (  [46] recorded pH in the range of 5.90 ± 0.10 to 6.10 ± 0.20 and 5.92 ± 0.10 respectively for soils from CCA contaminated sites. The slight differences in the pH of CCA contaminated soils to those previously reported may be due to natural processes such as the rate of decomposition of organic matter and leaching of cations, and the differences in their cation exchange capacities. Lower CEC of soils are known to lead to increase in soil pH [47]. Thus, the higher pH recorded could be due to soil factors as CCA is often applied as a water-based mixture of 0.6-6.0% (w/w) chromic acid, copper oxide and arsenic acid with pH between 1.6 to 2.

Physicochemical Characteristics of the Unamended and Amended Soils
The properties of the potted control and CCA contaminated soils after 14 days of amendment with SSB are given in Table 3.
The amended soils recorded increment in nearly all the physicochemical parameters unlike the unamended soils. For example, pH first reduced to 6.35 ± 0.02 with 5% amendment and then increased up to 6.99 ± 0.18 at 25% amendment. Soil pH controls the solubility and hydrolysis of metal salts, ion pair formation, surface charge of manganese, iron and aluminum oxides, organic matter and clay [58] Table 4 gives the bioavailability of chromium, copper and arsenic in different fractions of soils after 20 and 40 days of planting (DAP). In unamended CCA contaminated soils, chromium associated with soluble and iron-manganese oxide fractions were low. Most of it and copper were in the organic bound and residual fractions. Arsenic had the highest concentration in the exchangeable and residual fractions probably due to ion exchange. When exchanged for another cation, arsenic is solubilized, becoming bioavailable for plant uptake. In the residual fraction, this could have been due to low pH. Soil parameters such as pH, organic matter, clay and iron oxides affect the redistribution of elements down the soil profile due to differences in their mobility. A low soil redox potential for example, increases mobility and toxicity of arsenic by reducing arsenic (V) to arsenic (III) [61].

Fractionation of Chromium, Copper and Arsenic in Unamended and Amended Soils 20 and 40 Days after Planting
With amendment, chromium concentrations increased but reduced with increase in the number of days. After 20 days, chromium that remained in the soil was more compared to that taken up by maize plants (Table 4). Chromium was low in the soluble and exchangeable fractions after 40 days compared to it was initially because it might have been adsorbed by the amendment. On the other hand, the concentration of copper was lowest in the soluble fraction with 25% amendment after 20 days while residual fraction had the highest concentration of copper. Normally, copper adsorption in the soil occurs in the most avid binding sites and the remainder is distributed among fractions bound with less energy, greater availability and mobility. Copper concentrations 40 DAP increased for all incremental amendments. As a cation, copper can be exchanged for other cations in clay and organic matter. Most copper deposited in the soil is strongly adsorbed to the upper soil layers and is bound to organic matter, carbonate minerals, iron and manganese oxides [61]. It may also get complexed by organic ligands such as carboxylic and phenolic groups, or other organic acids, increasing its mobility in the soil [62].
After 20 and 40 days of planting, arsenic was strongly adsorbed in the exchangeable, organic, and residual fractions (Table 4). Adsorption and desorption of arsenic on oxides has been recognized as a mechanism of its mobility control [63][64][65]. In addition, carbon rich sludge is known to increase arsenic solubility [66]. As expected, phytoremediation had no significant impact on the concentration of arsenic in the soluble, exchangeable and carbonate fractions due to adsorption by organic matter. Previous authors [67,68] reported that addition of organic matter reduced arsenic mobility. Phosphate and arsenic compete directly for sorption sites on soil particles and about 77% displacement of total arsenic in the soil by phosphates was reported in a previous study [69]. Overall, these observations agreed with [42] who used poultry droppings as amendment and more metals were dissolved in the control pots than in pots with amendments.

Mobility of Chromium, Copper and Arsenic in Unamended and Amended CCA Contaminated Soils
Mobility of metals in soils is assessed basing on the absolute and relative content of the fractions weakly bound to the soil components. In this study, six operationally defined fractions (F 1 to F 6 ) representing the forms of the metals in the soils were used. The mean mobility factors were 17.1%, 8.9% and 30.2% respectively for chromium, copper and arsenic (Fig. 3).   under oxidized conditions [70,71]. Mobility of chromium depends on its oxidation state (+3 or +6), presence of organic matter and a divalent ion. Cr 6+ which is more soluble, bioavailable and toxic [72] can readily be converted to oxides of Cr 3+ or precipitated with iron hydrous oxides that have very low mobility and bioavailability in the soil [73]. For example, at a pH of 6.5-11.5, chromium exists as an immobile Cr 3+ species such as Cr (OH) 3 due to interaction with negatively charged ion colloids in the soil [73,74].
The average potential mobility of arsenic at different amendments varied significantly, indicating that it was more mobile than the other metals. The presence of heavy metals in this phase where they can be taken up by plants from the soils is the most hazardous to the ecosystem.

CONCLUSION
The MM3 maize variety experimented could be used to phytoextract or phytostabilize trace metals in the CCA contaminated soils without or with 5-25% sewage sludge amendment. Amendment with sewage sludge biosolid improved the phytoremediation potential of maize. Arsenic is more mobile and bioavailable in CCA contaminated soils than copper and chromium. The overall mobility of the trace metals considering their abundance in the fractions was exchangeable > bound to carbonate > bound to oxides > bound to organics > residual. Further studies should use other local maize varieties such as Longe series which take up to 120 days before maturity.

DISCLAIMER
The products used for this research are commonly and predominantly use products in our area of research and country. There is absolutely no conflict of interest between the authors and producers of the products because we do not intend to use these products as an avenue for any litigation but for the advancement of knowledge. Also, the research was not funded by the producing company rather it was funded by personal efforts of the authors.

ETHICAL APPROVAL
This study was approved by the Chemistry Department, Mbarara University of Science and Technology, Uganda for Caroline Kiwanuka Nakiguli (Approval No. 2010/HD/019).