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Title: Arsenic speciation and concentration in paddy rice : consequences for dietary exposure
Author: Williams, P. N.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2007
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Chapter 1. Introduction General summary covering the toxicity and environmental distribution of arsenic, rice cultivation in Bangladesh including irrigations practices, previous knowledge of arsenic concentration and speciation in rice and the risk posed to the human food chain. Chapter 2. Variation in arsenic speciation and concentration in paddy rice related to dietary exposure Arsenic speciation was surveyed in different rice varieties from different parts of the globe to understand the contribution of rice to arsenic exposure. Pot experiments were utilised to ascertain whether growing rice on arsenic contaminated soil affected speciation and whether genetic variation accounted for uptake and speciation. ICP-MS was used to determine total arsenic concentration and a HPLC system with an anion-exchange chromatography column was employed to separate arsenic species. USA long grain rice had the highest mean arsenic concentration in the grain at 0.26 jag As g'1 (n = 7), and the highest grain arsenic value of the survey at 0.40 |ig As g'1. The main arsenic species detected in the rice extract were As(III), DMA(V) and As(5). In European, Bangladeshi and Indian rice 64 1 % (n=7), 80 3 % (n = 11) and 81 4 % (n = 15) of the recovered arsenic was found to be inorganic. In contrast DMA(V) was the predominant species in rice from the USA, with only 42 5 % (n = 12) of the arsenic being inorganic. Pot experiments show that the proportions of DMA(V) in the grain are significantly dependent on rice cultivar (p = 0.026) and that plant nutrient status is effected by arsenic exposure. Chapter 3. Increase in rice grain arsenic for regions of Bangladesh irrigating paddies with elevated arsenic in groundwaters. Three hundred and thirty of aman and boro rice, ninety-four vegetable and fifty pulse and spice samples were analysed for total organic and inorganic arsenic, using HPLC -ICP-MS. Conc. HNO3 was used for total digestion and 2M TFA for extraction. The districts with the highest mean arsenic rice grain concentrations were all from southwestern Bangladesh: Faridpur (boro) 0.51 > Satkhira (boro) 0.38 > Satkhira (aman) 0.36 > Chuadanga (boro) 0.32 > Meherpur (boro) 0.29 |ig As g"1. Predicted inorganic arsenic intake from rice is modelled with the equivalent intake from drinking water for a typical Bangladesh diet. Daily consumption of rice with a total arsenic level of 0.08 jag As g1 would be equivalent to a drinking water arsenic level of 10 |ig L'1. Chapter 4. Market basket survey shows elevated levels of As in South Central US processed rice compared to California: Consequences for human dietary exposure. One Hundred and thirty four samples of rice were collected from numerous large supermarkets in the towns and cities in the US. An (ORS)-ICP-MS was used to measure the elements Ti (m/z 49), Mn (m/z 55), Co (m/z 59), Cu (m/z 63), Zn (m/z 64, 66, 67, 68), As (m/z 75), Se (m/z 77, 78, 82) and In (m/z 115). Our findings show differences in transitional metal concentrations between polished and unpolished rice and geographical variation in As and selenium between rice processed in California and the South Central US. Modelling arsenic intake for the US population based on this survey shows that for certain groups (namely Hispanics, Asians, sufferers of Celiac disease and infants) dietary exposure to inorganic As from elevated concentrations in rice potentially exceeds the maximum intake of As from drinking water (based on consumption of 1 L of 0.01 mg L'1 In. As) and Californian state exposure limits. Chapter 5. Greatly enhanced arsenic shoot assimilation in rice leads to elevated grain levels compared to wheat barley. Paired grain, shoot and soil of one hundred and seventy three individual sample sets of commercially farmed temperate rice, wheat and barley were surveyed to investigate variation in the assimilation and translocation of As. Rice samples were obtained from the Carmargue (France), Donana (Spain), Cadiz (Spain), California (USA), and Arkansas (USA). Wheat and barley were collected from Cornwall and Devon (England) and the east coast of Scotland. An ICP-MS, was used to measure the elements Ti (m/z 49), As (m/z 75), Se (m/z 77, 78, 82) and In (m/z 115). Transfer of As from soil to grain was an order of magnitude greater in rice than for wheat and barley, despite lower rates of shoot to grain transfer. In rice, grain export of arsenic from the shoot appears to be under tight physiological control as grain/shoot ratio decreases by an order of magnitude (from ~0.3 to 0.003 mg/kg) as arsenic concentrations in shoot increase from 1 to 20 mg/kg.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available