Passivation effects of calcium carbonate and chitosan on arsenic pollution in high pH calcareous soil

Abstract: Arsenic (As) is highly toxic in its inorganic form, being one of five major pollution elements released by the Ministry of Ecological Environment in China. National survey of soil pollution in 2014 showed that the soil in China was high 2.7% over the standard rate of arsenic. Arsenic in the polluted soil was easily absorbed by crops and accumulated in the edible parts. This pollution can lead to the decline of crop production, even threaten to human health via the food chain. In order to eliminate toxic effects of arsenic pollution in soil, passivators are often used to reduce arsenic bioavailability. In heavy metal pollution of acid soil, calcium carbonate is normally selected for the passivation and remediation. However, it is lacking research on whether the addition of calcium carbonate to high pH calcareous soil can passivate arsenic pollution, especially on the combination of calcium carbonate and chitosan. Therefore, it is necessary to solve the problem of arsenic pollution in calcareous soil with high pH value, particularly the passivation effect of calcium carbonate combined with chitosan. In this study, a field experiment of arsenic pollution was conducted for the treatment and remediation of arsenic pollution in calcareous soil with high pH value. Four treatments were, control (CK: no exogenous arsenic and passivator), arsenic pollution (As: only exogenous arsenic), arsenic pollution + calcium carbonate (As + Ca: exogenous arsenic and calcium carbonate), arsenic pollution + calcium carbonate + chitosan (As + Ca + C: exogenous arsenic and calcium carbonate and chitosan). An attempt was made to investigate the effects of calcium carbonate and chitosan addition on the enzyme activity of arsenic -polluted calcareous soil, the fractions changes of arsenic in soil and the migration characteristics of arsenic in corn. The results showed that the activities of soil urease, cellulase and catalase significantly increased by 52.35%、74.92% and 8.72%, respectively, after the combined use of calcium carbonate and chitosan (P<0.05), compared with arsenic treatment. Exogenous arsenic mainly distributed in soil in the residual fraction, more than 60% of the total arsenic. Compared with arsenic treatment, the addition of calcium carbonate only, or together with chitosan, can significantly reduce the water-soluble arsenic by 17.15% and 27.03% (P<0.05), respectively. As+Ca+C treatment remarkably increased calcium arsenic, iron arsenic and aluminum arsenic by 13.97%、14.24% and 13.85% (P<0.05), respectively, where the passivation rates of adding Ca and Ca+C to arsenic contamination of calcareous soil were 9.78% and 18.37% (P<0.05), respectively. The accumulation of arsenic in crops parts increased, after maize was planted on arsenic polluted soil. Compared with arsenic treatment, calcium carbonate and chitosan were used together significantly reduce the arsenic content in corn seed, root, stem and leaf by 50%、13.98%、16.51% and 14.94% (P<0.05). It can be concluded that the combination of calcium carbonate and chitosan can play a better role in the passivation of arsenic pollution in calcareous soil with high pH value than the single use of calcium carbonate. Therefore, maize planting combined with the application of the proposed passivator can realize the safe production in high pH calcareous soil polluted by arsenic.