Arsenic Contamination Of Paddy Fields Round Some Industry And Mine Zones In Hunan Province And Remediation Of Arsenic Contaminated Paddy Soils By Chemical Extraction

Arsenic (As) is known to be a very toxic element and carcinogen to humans.Owing to polluted irrigation water, paddy fields are prone to be contaminated with As.Non-ferrous metal abounds in Hunan province, and some paddy fields round someindustry and mine zones have been contaminated with As. Arsenic contamination ofpaddy fields can reduce rice yield and cause damage to human health through foodchain, and, at the same time, remediation of As contaminated paddy field is verydifficult. Therefore studies of remediation of As contaminated paddy field are ofimportance. Paddy soils and rice plants were sampled and analyzed to investigate andevaluate As contamination of paddy fields round some industry and mine zones inHunan province, and laboratory batch experiments were conducted to studyremediation of As contaminated paddy field by chemical extraction. The main resultswere summarized as follow:1. Evaluation of soil As contamination showed that 9 zones had beencontaminated with As, among which 4 zones had been contaminated with both As andheavy metals. The main pollutants were As for 2 zones, Zn and Cd for another zone,and Zn for the other zone. Arsenic contaminations of paddy soils were the mostserious in Sanyang town of Pingjiang county (Yueyang city), Xinqiao village ofShifeng county (Zhuzhou city), and Songbai town of Changning county (Hengyangcity), and complex contaminations of As and heavy metal were the most serious inXinqiao village of Shifeng county (Zhuzhou city), and Songbai town of Changningcounty (Hengyang city). Evaluation of brown rice As contamination indicated thatbrown rice produced from all sites had been contaminated with As by nearby industryand mine zones. Arsenic contaminations of brown rice were the most serious inZhongfang town of Linxiang city (Yueyang city), Sanyang town of Pingjiang county(Yueyang city), Dengshiqiao town of Heshan county (Yiyang city), Xinqiao village of Shifeng county (Zhuzhou city), and Songbai town of Changning county (Hengyangcity). However there were only 2 sample sites, 2# sampling site in Dengshiqiao townof Heshan county (Yiyang city) and 2# sampling site in Songbai town of Changningcounty (Hengyang city), failing to meet tolerance limit in grain (0.7 mg/kg).Arsenic contents in various tissues of rice plants were in the order of roots＞straw＞husk＞brown rice, and there existed extremely significant linear relationships betweenAs contents in various tissues and soil As contents. In As contaminated paddy soils,there were differences of ability of As uptake among various rice plants. Althoughsome zones were contaminated with both As and heavy metals, rice yield mainlyrelated to soil As contamination. The single pollution index of As was moreappropriate for evaluating effects of complex pollution on rice yield than the generalpollution index.2. In all extractants applied in batch experiments, phosphoric acid (H₃PO₄),potassium hydroxide (KOH), and potassium dihydrogen phosphate (KH₂PO₄) provedto be effective in removing As from soils. Arsenic removal could reach equilibriumwithin 360, 360, 720 min for H₃PO₄, KOH, and KH₂PO₄, respectively. Elovich modeland first-order model proved to best and worst fit the kinetic data of As removal,respectively, among four models used in kinetic study, indicating that As removalshould not be a simple process.3. Batch experiments of 2 paddy soils showed that H₃PO₄, KH₂PO₄, and KOHwere effective in removing As from the soils, and removal efficiencies from Jinshuisoil were more than those from Dengjiatang soil. In Dengjiatang soil, As removalefficiency of 22.85%, 20.21%, and 27.35% was attained and, in Jinshui soil, removalefficiency of 39.82%, 14.33%, and 36.84% was attained due to using H₃PO₄, KH₂PO₄,and KOH of 200 mmol/L, respectively. Three extractants mainly removed those Asspecies which availabilities were high. Both H₃PO₄ and KH₂PO₄ removed the greatmass of As of Ca-bound from Dengjiatang soil, H₃PO₄ and KH₂PO₄ removed thegreat mass of As of Fe- and Ca-bound and many As of Fe-bound from Jinshui soil,respectively, and KOH removed the great mass of As of Al-and Fe-bound from 2soils. In addition, As removal efficiency could be greatly promoted by twice extractions and combined application of two extractants. Hence chemical extractionwas effective in remedying As contaminated soil.The mechanism of As removal by KH₂PO₄ was that absorbed As was displacedby phosphate ions (PO₄^3-). The mechanism of As removal by KOH was displacementreaction of hydroxyl ions (OH^-) with As. The effectiveness of H₃PO₄ could beattributed to the synergetic function as a donor of PO₄^3- which displace As and as anacid to dissolve metallic components of the soil with which As was associated. PO₄^3-and OH^- displaced As through 2 mechanisms, which were a competition between 2coordination reactions and a competition between precipitation reaction andcoordination reaction.4. Three extractants resulted in change of soil pH values and dissolution of soilcomponents (calcium (Ca), magnesium (Mg), and silicon (Si)). H₃PO₄ decreased soilpH values, KOH increased soil pH values, but KH₂PO₄ hardly led to change of soilpH values. At high concentrations, H₃PO₄ resulted in dissolution of a great deal of Caand some Si and Mg from 2 soils, and dissolution of some A1 and Fe from Jinshui soil.While dissolution of soil components due to using KH₂PO₄ or KOH was much lessthan H₃PO₄. Kinetic study showed that both changes of soil pH values and dissolutionof Ca and Mg due to using 3 extractants were more rapid than As removal from soil.Therefore effects of extractants on soil properties would be inevitable if As wasfurthest removed.5. Chemical extraction inhibited germination and seedling growth of rice plants.H₃PO₄ and KOH decreased germination ratios of rice, seedling heights, root lengthsand fresh weights. At lower concentration (100 mmol/L), however, H₃PO₄ and KOHdid not result in decrease in germination ratios and had only slight effects on seedlingheights, root lengths and fresh weights. KH₂PO₄ proved not to have remarkableeffects on germination ratios, and promoted growth of seedling at the concentration of100 mmol/L. Among these three extractants, KH₂PO₄ proved to be not only effectivein removing As from soil but also environment-friendly.6. Although chemical extraction of As from soils resulted in negative effects onsoil to a certain degree, chemical extraction could rapidly and effectively remove As from soils and permanently alleviate or eliminate As toxicity to crops and damage tohuman health, and was easy to operate. In addition, negative effects of chemicalextraction could be minimized through adoption of some countermeasures, such asapplication of environment-friendly extractants and choice of proper concentrations ofextractants. In a word, chemical extraction could be applied to large-scale paddyfields with serious As contamination due to its effectiveness, rapid kinetics, andoperational easiness.