Arsenic speciation and biogeochemical cycling in Pakistan's alluvial rice ecosystem: Health and trade implications

This study examines arsenic (As), a toxic environmental metal, with a focus on its health and trade risks, speciation, and biogeochemical cycling in Pakistan's alluvial rice ecosystem. Soil total As levels achieved an average of 26.02 mg/kg (10.29-84.85 mg/kg), surpassing the WHO's guideline of 10 mg/kg for agricultural soils, indicating widespread contamination. Rice As concentrations ranged from 0.0001 to 4.12 mg/kg, with 56.8 % of samples exceeding the FDA's permissible limit of 0.1 mg/kg-the discrepancy between total As and its species concentrations is due to the different As forms present. Inorganic species (AsIII and AsV) ranged from 2.08 to 10.06 μg/kg and 1.23-10.96 μg/kg, while organic species (MMAs and DMAs) ranged from 1.09 to 9.18 μg/kg and 1.05-9.30 μg/kg, respectively, indicating active methylation. Strong correlations were found between total arsenic and both inorganic (R² = 0.607 and 0.602) and organic species (R² = 0.677 and 0.654). This suggests that total As levels in rice reflect both inorganic and organic forms, with transformation processes contributing to the variation in concentrations. The hazard risk model indicates that 69 % of Pakistan's population (approximately 100 million) faces a high risk of inorganic As exposure due to daily rice consumption, exceeding 0.02-0.04 mg/kg/day. Pakistan, the fourth-largest rice exporter globally, distributes 63.33 % of its rice to Asia, 29.2 % to Africa, 4.96 % to Europe, 1.54 % to North America, and 0.98 % to Oceania. The study predicts that the largest importers of rice with elevated As levels are China (138.25 kg/year), Malaysia (50.93 kg/year), Indonesia (49.20 kg/year), Afghanistan (46.76 kg/year), the UAE (46.50 kg/year), and Saudi Arabia (16.06 kg/year). The study assumes that Indonesia (5,953), China (3,555), Malaysia (2,322), Saudi Arabia (1,481), the UAE (1,092), and Afghanistan (635) face the highest health risks per 100,000 people, which has a significant impact on their populations. Countries with robust monitoring systems are better equipped to mitigate risks, while those with weak or inadequate monitoring face higher exposure. The study advocates for stricter agricultural regulations and enhanced monitoring of As levels in both domestic and exported rice.