Abstract

Plant-derived foods are critical sources of essential minerals, yet environmental contamination by trace elements (TEs) can result in their bioaccumulation, posing potential health risks. This study quantified Fe, Al, Zn, Cu, Co, Ni, Cr, and Pb in rice from Daloa, Issia, and Vavoua, Haut-Sassandra region, Côte d’Ivoire, to assess compliance with international safety standards. Thirty samples (10 per locality) representing the varieties Wita 9, Bouaké Amélioré, and V10 were collected from local producers. Following acid digestion, trace elements concentrations were determined by flame atomic absorption spectrometry. Fe ranged from 10.21 to 36.80 mg/kg, Al from 7.76 to 29.02 mg/kg, Zn from 20.05 to 41.07 mg/kg, and Cu from 1.00 to 5.25 mg/kg. Co (0.91–2.05 mg/kg), Ni (0.01–0.30 mg/kg), and Pb (0.002–0.024 mg/kg) levels were low, whereas Cr concentrations (0.25–2.00 mg/kg) exceeded WHO thresholds in some samples. ANOVA (p < 0.05) revealed significant differences between localities. Overall, the rice studied poses no immediate health risk regarding most TEs and retains significant nutritional value. However, the cumulative nature of certain metals, particularly Cr, underscores the need for continuous monitoring and implementation of preventive strategies at the production level. These results highlight the importance of integrating nutritional quality assessment with rigorous food safety surveillance to ensure the production of safe, mineral-rich staple foods.

1. Introduction

Rice (Oryza sativa L.) is a staple food for over half of the global population, particularly in Asia and Africa, ranking first among cereals for direct human consumption, second in global production volume after maize, and second in cultivated area after wheat ^{1, 2}. Global rice production is predominantly concentrated in Asia, with China, India, and Indonesia leading output, while in Africa, demand is rapidly increasing due to population growth and urbanization ². In Côte d’Ivoire, national rice production reached approximately 1.1 million tons in 2021, satisfying only 40% of domestic milled rice needs; the remaining demand is met through imports, estimated at 2.4 million tons annually ³. Supply disruptions, such as the 2022 export restrictions imposed by India, have highlighted the vulnerability of domestic rice security and prompted government initiatives aimed at boosting local production as part of broader agricultural transformation strategies ³.

The intensification of rice cultivation in Côte d’Ivoire increasingly relies on chemical inputs, including fertilizers, herbicides, insecticides, and other agrochemicals ^{4, 5}. While these inputs enhance yields and contribute to food security objectives, their excessive or improper use can lead to the accumulation of trace metal elements (TMEs) in soils. TMEs absorbed by rice plants may enter the human food chain and bioaccumulate across trophic levels, potentially posing acute or chronic health risks ^{6, 10}. Globally, numerous studies have reported trace metal contamination in staple foods, emphasizing concerns for both food safety and public health ^{7, 8, 9}. Despite these risks, data on TME levels in rice produced in Côte d’Ivoire and specifically in the High Sassandra region remain limited, representing a critical knowledge gap for food safety assessment, dietary exposure evaluation, and public health policy.

This study aims to quantify the concentrations of TMEs in rice cultivated across three locations in the High Sassandra region and to assess compliance with established international safety thresholds. We hypothesize that intensified agricultural practices and the widespread use of agrochemicals may lead to elevated TME levels in rice, potentially exceeding safe consumption limits and posing a risk to consumer health. By providing region-specific data, this research contributes to evidence-based strategies for agricultural management, food safety regulation, and public health protection in Côte d’Ivoire.

2. Materials and Methods

The plant material used for this study consisted of three rice varieties (Oryza sativa L.), namely Bouake AM, V 10, and Wita 9 (Figure 1). These rice varieties were chosen because of their abundance in the region. This abundance is thought to be linked to their taste, their cycle length (3 and 4 months), their disease resistance, and their yield (3 to 4 tons per hectare).

Sampling was conducted in accordance with the study design. For the determination of trace elements, a total of thirty (30) rice samples representing different varieties were collected from farmers’ production. Ten samples were collected per locality, including four samples of Wita 9, three of Bouaké Amélioré, and three of V10. Each sample, approximately 500 g, was placed in a labeled container. The collected rice samples were subsequently transported to the SODEMI (Société de Développement Minier) laboratory for trace element analysis.

Trace elements (Fe, Zn, Cu, Co, Ni, Cr, Pb) in rice samples collected from Daloa, Issia, and Vavoua (Haut-Sassandra region, Côte d’Ivoire) were analyzed following the IITA ¹¹ method. For each sample, 0.4 g of dried and ground rice was placed in a 125 mL Erlenmeyer flask pre-washed with perchloric acid and distilled water. A digestion mixture of 4 mL concentrated perchloric acid, 25 mL concentrated nitric acid, and 2 mL concentrated sulfuric acid was added, and the solution was homogenized. Digestion was performed under a fume hood with gradual heating until dense white fumes appeared, followed by brief exposure to high flame. After cooling, 40 mL of distilled water was added, and the mixture was boiled for 30 s. The solution was filtered through Whatman paper and transferred to a 100 mL volumetric flask, adjusted to volume with distilled water, and stored at 4 °C until analysis. Trace element concentrations were determined by flame atomic absorption spectrometry (FAAS) using a SPECTR AA20 Varian spectrometer. Absorbance was measured at element-specific wavelengths corresponding to their maximum absorption. Calibration curves were constructed using certified standard solutions, and concentrations were calculated according to the Beer–Lambert law, with element-specific absorption coefficients ¹¹. This procedure ensured precise and reproducible quantification of trace elements in rice samples from the three study sites.

The concentrations of trace elements in rice samples were analyzed using STATISTICA 7.1. Data are presented as mean ± standard deviation (SD). Analysis of variance (ANOVA) was performed to compare trace element levels across different samples. Two hypotheses were tested:

• H0: Both locality and rice variety affect trace element concentrations.

• H1: Neither locality nor rice variety affects trace element concentrations.

Differences were considered statistically significant when P < 0.05.

3. Results and Discussion

All analyzed rice samples contained detectable levels of iron (Figure 2). The highest iron concentrations were recorded in the Wita 9 variety from Vavoua, Wita 9 from Daloa, and V10 from Vavoua, with respective values of 36.80 ± 0.01 mg/kg, 33.42 ± 0.03 mg/kg, and 32.04 ± 0.06 mg/kg. In contrast, the lowest concentration (10.21 ± 0.01 mg/kg) was observed in the Bouaké AM variety collected in Vavoua. From a regulatory perspective, all measured iron concentrations remained below the maximum permissible limit of 40.7 mg/kg established by international standards ^{12, 36}. Comparatively, the recorded values represent between approximately 25% and 90% of this admissible threshold, confirming that none of the analyzed samples exceeded internationally accepted safety limits. Therefore, with regard to iron content, the rice samples do not present a toxicological risk related to excessive intake. Statistical analysis further revealed significant differences among certain localities and varieties. One-way ANOVA showed that iron concentrations varied significantly between sampling sites (p < 0.05). Post hoc comparisons indicated that samples from Vavoua (particularly Wita 9 and V10) were significantly higher in iron content compared to Bouaké AM from Vavoua and some samples from other localities. These statistically significant differences highlight the influence of geographical origin and varietal characteristics on iron accumulation in rice grains. From a nutritional standpoint, the presence of iron in rice is beneficial, as iron is an essential micronutrient involved in hemoglobin synthesis, oxygen transport, and the prevention of iron-deficiency anemia. In contrast to many trace metals where toxicity is the primary concern, iron more commonly poses deficiency-related public health challenges rather than toxicity issues under normal dietary exposure ¹³. However, chronic excessive intake may induce oxidative stress and metabolic disturbances due to iron’s pro-oxidant activity. The observed variability among samples may be attributed to differences in genetic background of rice varieties, soil physicochemical properties, agricultural practices (fertilizer application, irrigation methods), and environmental conditions across production sites in the High Sassandra region of Côte d’Ivoire. Notably, the region is characterized by ferralitic soils rich in iron and aluminum ^{14, 17}, which may enhance iron bioavailability and subsequent uptake by rice plants. Compared with previous findings, the iron concentrations reported in the present study were markedly higher than those observed by Edem (0.002–0.004 mg/kg) in rice samples ¹⁹. This discrepancy may be explained by differences in analytical methods, soil composition, geographical conditions, or rice cultivars studied. Overall, the iron levels detected in this study indicate that rice produced in these areas contributes meaningfully to dietary iron intake while remaining within internationally accepted safety limits. Thus, from both toxicological and nutritional perspectives, the analyzed samples can be considered safe and potentially beneficial sources of dietary iron

All analyzed rice varieties contained detectable levels of aluminium (Al). The highest concentrations were recorded in Wita 9 from Daloa (29.02 ± 0.03 mg/kg), followed by V10 from Issia (23.97 ± 0.03 mg/kg) and Bouaké AM from Vavoua (21.52 ± 0.03 mg/kg), whereas the lowest concentration was observed in Wita 9 from Issia (7.76 ± 0.03 mg/kg).

One-way analysis of variance (ANOVA) revealed statistically significant differences among certain localities and varieties (p < 0.05), confirming that geographical origin significantly influences aluminium accumulation in rice grains. Post hoc comparisons indicated that samples from Daloa contained significantly higher aluminium concentrations than those from Issia. These differences may be attributed to variations in soil pH, mineralogical composition, cation exchange capacity, and irrigation regimes, all of which modulate aluminium solubility and plant uptake. From a food safety standpoint, all measured aluminium concentrations were substantially below the maximum permissible limit of 60 mg/kg established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) ^{12, 37}. The detected values represent approximately 13-48% of this international limit, indicating a considerable safety margin. Additionally, the Provisional Tolerable Weekly Intake (PTWI) for aluminium has been set at 1 mg/kg body weight/week, suggesting that exposure through rice consumption alone is unlikely to exceed tolerable thresholds under typical dietary patterns. From a food science perspective, aluminium presence in rice grains may originate from both pre-harvest and post-harvest factors. In highly weathered tropical ferralitic soils, aluminium oxides are abundant and may become more bioavailable under acidic conditions, enhancing root uptake ^{34, 37}. Furthermore, processing steps such as milling, polishing, washing, and cooking can influence final aluminium content. Studies have shown that outer grain layers may contain higher metal concentrations, and polishing can reduce total aluminium levels ³⁸. Water quality during parboiling or cooking may also contribute to metal migration. Although aluminium is not a biologically essential element, chronic excessive exposure has been associated with adverse health outcomes, including bone demineralization, anemia, and possible neurotoxicity ¹⁵. However, toxicity is generally linked to cumulative long-term exposure rather than isolated intake from a single staple food. In the present study, aluminium concentrations remained within internationally accepted safety standards, indicating no significant health risk for consumers in the studied region. Overall, despite statistically significant spatial variations (ANOVA, p < 0.05), aluminium levels in the analyzed rice varieties comply with international food safety standards. From both a toxicological and technological standpoint, the rice produced in these localities can be considered safe for human consumption.

Zinc (Zn) was detected in all analyzed rice samples (Figure 4), with concentrations exceeding 20 mg/kg in every case. The highest concentration was recorded in the V10 variety from Issia (41.07 ± 0.11 mg/kg). One-way ANOVA revealed statistically significant differences among samples (p < 0.05), indicating that varietal and geographical factors influence zinc accumulation in rice grains. Post hoc analysis showed that V10 from Issia was significantly higher in Zn content compared to other varieties and localities, suggesting a strong effect of soil composition and agronomic practices on zinc uptake. From a regulatory standpoint, all measured zinc concentrations were below the maximum permissible limit of 50 mg/kg established by the USDA ^{13, 39}, with observed values ranging from approximately 40% to 82% of this threshold. This indicates that the rice samples are within internationally accepted safety limits for human consumption and do not pose a risk of zinc toxicity. From a nutritional and food science perspective, the relatively high zinc content observed is beneficial, as zinc is an essential micronutrient involved in enzymatic reactions, immune function, growth, and cognitive development ³⁹. Compared to previous reports, the zinc concentrations in this study were markedly higher than those reported by Uche (0.1768 mg/kg in corn and 0.2988 mg/kg in millet) in northwestern Nigeria ⁸. Such differences may be explained by variations in soil mineral content, fertilization practices, rice cultivar genetics, and environmental factors. The observed variability in zinc concentrations may also reflect differences in post-harvest processing and grain polishing, which can influence mineral retention. Studies have shown that zinc tends to accumulate in the outer layers of rice grains; therefore, milling and polishing practices can significantly reduce total zinc content ³⁸. Water quality used during parboiling or irrigation may further affect zinc availability in the edible grain fraction. Overall, despite statistically significant inter-locality differences (ANOVA, p < 0.05), the zinc content in rice produced in the High Sassandra region is both nutritionally valuable and safe for human consumption. These results highlight rice as a dietary source of essential micronutrients, contributing to micronutrient intake in populations relying on cereals as staples.

Nickel (Ni) concentrations in all analyzed rice samples were below the established toxicity threshold of 0.5 mg/kg (Figure 5). The highest levels were observed in Bouaké AM and V10 varieties from Vavoua (0.30 ± 0.01 mg/kg), whereas the lowest concentrations were recorded in Wita 9 from Issia and Daloa (0.01 ± 0.00 mg/kg and 0.05 ± 0.01 mg/kg, respectively). One-way ANOVA indicated statistically significant differences among certain varieties and localities (p < 0.05), suggesting that geographical origin and varietal characteristics influence nickel accumulation in rice grains. From a regulatory perspective, all measured nickel concentrations were well below the maximum permissible limit of 0.5 mg/kg established by the World Health Organization (WHO) ¹². The detected values, ranging from 2% to 60% of this threshold, indicate a substantial safety margin, confirming that consumption of these rice varieties is unlikely to result in nickel-related toxic effects. The relatively low nickel content likely reflects its limited natural availability in the soil and surrounding environment, as well as restricted uptake by rice plants. Nickel bioavailability is influenced by soil pH, organic matter content, and competing cations, which may explain the observed variability across localities and varieties ^{37, 38},38]. Although the detected levels are safe, excessive nickel exposure in humans has been linked to adverse health outcomes, including pulmonary inflammation, fibrosis, dermatological reactions, and potential tumor development ³³. Therefore, continuous monitoring of nickel and other trace metals in staple foods is essential to ensure long-term food safety. From a food science standpoint, post-harvest processing such as washing, milling, and cooking may further reduce nickel concentrations in the edible portion of rice grains, contributing to its safety for human consumption. Overall, rice produced in the High Sassandra region appears safe with respect to nickel content, while remaining a staple of nutritional and economic importance.

Copper (Cu) was detected in all analyzed rice samples. The highest concentrations were observed in the V10 variety from Vavoua (5.25 ± 0.00 mg/kg) and Daloa (4.40 ± 0.28 mg/kg), while the lowest levels were measured in Bouaké AM samples from Issia and Daloa (Figure 6). One-way ANOVA indicated statistically significant differences among varieties and localities (p < 0.05), suggesting that geographical origin, soil composition, and varietal characteristics influence copper accumulation in rice grains. From a regulatory standpoint, all measured copper concentrations were well below the maximum permissible limit of 20 mg/kg established by the World Health Organization (WHO) ¹². The observed concentrations, representing approximately 22–26% of the international safety threshold, indicate a substantial safety margin. Consumption of these rice varieties is therefore unlikely to pose health risks associated with copper toxicity, such as cardiovascular or hepatic complications. Copper is an essential micronutrient required for key physiological processes, including enzymatic reactions involved in oxidative metabolism, hemoglobin synthesis, and antioxidant defense ⁴¹. The relatively high copper content observed in V10 from Vavoua may be attributed to natural soil copper levels, the use of copper-containing fertilizers or pesticides, and specific agronomic practices. Environmental factors such as soil leaching, irrigation water composition, and post-harvest washing may also influence copper retention in rice grains ^{37, 38}. From a food science perspective, monitoring copper levels in rice is important not only for food safety but also for maintaining nutritional quality. Rice represents a significant dietary source of trace elements, and optimizing copper availability through agronomic management can contribute to improved micronutrient intake in populations dependent on cereals as staple foods. Overall, despite statistically significant inter-locality differences (ANOVA, p < 0.05), copper concentrations in the analyzed rice varieties remain within internationally accepted safety limits while contributing to dietary micronutrient intake.

Cobalt (Co) was detected in all analyzed rice samples, with the highest concentration observed in the Wita 9 variety from Issia (2.05 ± 0.01 mg/kg; Figure 7). Other samples exhibited concentrations ranging from 0.70 ± 0.01 mg/kg to 1.05 ± 0.01 mg/kg. One-way ANOVA showed statistically significant differences among certain varieties and localities (p < 0.05), indicating that geographical origin and varietal characteristics influence cobalt accumulation in rice grains.

From a regulatory standpoint, all measured cobalt concentrations were below the maximum permissible limit of 3 mg/kg established by the European Food Safety Authority (EFSA) ¹². The observed values represent approximately 23–68% of this threshold, suggesting a substantial safety margin. Consumption of these rice varieties is therefore unlikely to pose health risks associated with cobalt toxicity. From a nutritional perspective, cobalt is an essential component of vitamin B12 and plays a role in erythropoiesis and cellular metabolism. The cobalt content detected in these rice samples is higher than that reported in cabbage from Niamey (0.42–0.59 mg/kg) ^{38, 39}, indicating that rice can contribute to dietary cobalt intake in populations relying on cereals as staple foods. Variability among samples may be influenced by the limited natural availability of cobalt in the soil and by differences in agricultural inputs, such as fertilizers, irrigation water, and soil amendments applied during cultivation ^{38, 39}. Environmental factors, including soil pH, redox potential, and leaching, can also modulate cobalt bioavailability and plant uptake. Overall, despite statistically significant differences among varieties and localities (ANOVA, p < 0.05), cobalt concentrations in rice produced in the High Sassandra region remain within internationally recognized safety limits while contributing to essential micronutrient intake.

Analysis revealed that the Wita 9 variety from Issia exhibited the highest chromium (Cr) concentration (2.02 ± 0.03 mg/kg; Figure 8), exceeding the WHO regulatory threshold of 1 mg/kg ^{14, 42}. All other rice samples contained chromium below this limit, with the lowest value observed in Bouaké AM from Issia (0.25 ± 0.01 mg/kg) ^{14, 42}. One-way ANOVA indicated statistically significant differences among varieties and localities (p < 0.05), confirming that geographical origin and cultivar strongly influence chromium accumulation in rice grains. The elevated chromium level in Wita 9 from Issia is consistent with observations by Tegegne (2020), who reported concentrations up to 2.2 mg/kg in rice cultivated in Ethiopia ²⁸. In contrast, chromium levels in the other samples were higher than those reported in Iranian rice (0.10 mg/kg) ²⁹. This variation highlights the influence of local soil composition, environmental pollution, and agronomic practices on chromium uptake. Chromium contamination in agricultural products may arise from both natural and anthropogenic sources. Natural sources include chromium-bearing rocks and soils, while anthropogenic inputs can derive from industrial emissions, vehicular traffic, and the use of contaminated irrigation water ^{30, 31}. Atmospheric deposition onto paddy fields may also contribute to metal accumulation in grains ³⁷. While most rice samples remained below the WHO toxicity threshold, the elevated chromium in the Wita 9 variety underscores the importance of continuous monitoring of trace metals in staple foods, particularly in areas potentially affected by industrial or environmental pollution. From a food science perspective, chromium retention can also be influenced by post-harvest processing, including washing, milling, and cooking, which may reduce total metal content in the edible fraction. Overall, these results indicate that although rice produced in the High Sassandra region is generally safe regarding chromium, specific varieties in certain localities may accumulate levels exceeding international safety limits, warranting targeted monitoring and mitigation strategies.

Lead (Pb) concentrations in all analyzed rice samples were well below the established toxicity threshold of 0.1 mg/kg (Figure 9) ¹⁶. The highest levels were observed in samples from Daloa, specifically Wita 9 (0.024 ± 0.001 mg/kg) and Bouaké AM (0.023 ± 0.001 mg/kg), while the lowest concentration was measured in the V10 variety from Vavoua (0.002 ± 0.000 mg/kg). One-way ANOVA revealed statistically significant differences among varieties and localities (p < 0.05), indicating that geographical origin affects lead accumulation in rice grains. Among the analyzed rice varieties, lead was the only toxic heavy metal detected; however, all concentrations remained well below safety limits recommended by the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) ⁴¹. Observed values ranged from 2% to 24% of the international safety threshold, reflecting a substantial safety margin. These concentrations are also lower than those reported by Ahmed (2020) in cereals marketed in Egypt ²⁴. From a food science perspective, the low lead content is likely attributable to its limited mobility in soil and its tendency to form insoluble complexes with organic matter, which reduces bioavailability to plants. Lead uptake occurs primarily through roots via mineral and water absorption, but it is largely sequestered in the cuticle and cell walls in an insoluble form ¹⁷. Post-harvest processing, including washing and polishing, may further reduce lead levels in the edible fraction. Overall, these results indicate that rice produced in the High Sassandra region poses minimal risk of lead exposure for consumers. Continuous monitoring of lead and other trace metals remains essential, particularly in areas potentially influenced by industrial or vehicular emissions, to ensure long-term food safety.

4. Conclusion

This study demonstrated that rice produced in the High Sassandra region contains measurable levels of trace metals, with concentrations varying according to both rice variety and production locality. Importantly, all analyzed trace metals iron, aluminium, zinc, nickel, copper, cobalt, chromium, and lead were generally below established international regulatory thresholds (WHO, EFSA, USDA, JECFA), indicating that the rice is currently safe for human consumption. Despite this, the potential for long-term bioaccumulation of certain elements, particularly in populations relying on rice as a staple food, underscores the importance of ongoing surveillance. To mitigate potential health risks, targeted guidance should be provided to farmers regarding the responsible application of agricultural inputs, including fertilizers, pesticides, and irrigation practices, which can influence trace metal accumulation in crops. Moreover, integrating phytoremediation strategies—such as the cultivation of metal-immobilizing or hyperaccumulator plants into rice farming systems could reduce environmental contamination and limit the transfer of metals into the food chain. Post-harvest practices, including proper washing, milling, and storage, can further minimize trace metal levels in the edible portion of rice grains. Continuous monitoring of trace metals in both soils and rice, coupled with public education and awareness programs on food safety, would strengthen preventive measures and support sustainable agricultural practices. Overall, these findings highlight the necessity of proactive management strategies to safeguard human health, maintain nutritional quality, and preserve the ecological integrity of rice production systems in the High Sassandra region.

ACKNOWLEDGMENTS

The authors express their deep gratitude to all the individuals and institutions that contributed to this work. They would particularly like to thank the laboratory managers who enabled the analyses to be carried out, as well as all the collaborators who participated in the collection and processing of the data. They are also grateful to the scientific supervisors for their expert advice and support throughout the study.

References