Studies Of Effectiveness Of Several Amendments On Arsenic Speciation And Bioavailabilility

The environmental risk of Arsenic (As) accumulation in the agricultural soils has been paid more attention recently. It is because As can be transferred to human by the food chain beside of the water supply. For the soil remediation As contaminated, the chemical immobilization technology has received more attention as it is applied easily, economically and effectively. However, generally speaking, less amendment with the high capacity of As immobilization is found, and little information on the relatively mechanisms is available. The laboratory simulation and pot experiments are carried out to investigate the effectiveness of seven chosen amendments including the natural or synthetic materials in regulation of risks. The chemical form changes of soil As and their mechanisms by applying the amendments are studied as well. The main results are as follows:1. In As (V) solution, the As adsorption capacity of amendments from high to is in order of Layered Double Oxide (LDO), modified Red Mud 2, modified Red Mud 1, Red Mud (RM) or Layered Double Hydroxide (LDH), iron phosphate and carbonized straw. The maximum adsorption capacity of As by LDO is 50.12 mg·g^-1. And the As adsorption capacity of modified materials are more efficiently than that of raw materials.2. When the amendments are applied into soils, the available As contents are gradually decreased with the time extending. In the red soils, the As immobilization capacity of amendments from high to low is order of modified RM 1, RM, LDH, carbonized straw, modified RM 2, iron phosphate and LDO, and the maximum immobilization rate of available As by modified RM 2 is 38.39%. While in fluvo-aquic soil from high to low it is in order of, LDO, LDH or modified RM 2 or RM or modified RM 1, iron phosphate or carbonized straw, and the maximum immobilization rate of available As by LDO is 52.19% (low As) and 55.24% (high As).3. By the simulation experiments with different amendments'dosage, the results shows that dosage could reduce diffluent arsenic. When dosage reaches at 5%, the immobilization rate of diffluent arsenic is the highest. The concentration of Al-bound arsenic is in the negative correlation to modified RM dosage. The concentration of Fe-bound arsenic is positively related to modified RM dosage, but negatively to LDO dosage. The amendments could increase Ca-bound arsenic, the concentration will reach the highest when the amendments adds by 5%. Amendments could affect Offal-arsenic in red soil. LDO could increase Offal-arsenic and modified RM could reduce Offal-arsenic. In fluvo-aquic soil, modified RM could increase Offal-arsenic, while LDO could reduce Offal-arsenic.4. For the effectiveness of Amendment influenced arsenic bioavailability, the iron phosphate could reduce the As concentration in small rape effectively in red soil and fluvo-aquic soil with high As, the reducing rate is 30.50% and 54.31%, respectively. LDO could immobilize the arsenic in red soil effectively (43.34%) and improve rape yield (15.22%). 5. The mechanisms of soil arsenic immobilized by amendments include the ion exchange, surface complexation, precipitation, coprecipitation and surface precipitation. LDH and LDO are mainly through exchange layer anion and coprecipitation to immobilize arsenic, while the RMs amendments are mainly through the surface complexation and coprecipitation to immobilize arsenic. The Iron phosphate uses Fe³⁺ to fixed arsenic, while carbonized straw fixed arsenic through precipitation reaction with arsenic as it contains calcium magnesium oxide.