ARSENIC MOBILIZATION IN DELTAIC SEDIMENT; SOUTH WESTERN BANGLADESH

The Holocene deltaic aquifer of the South-western part of Bangladesh, where almost all (96%) of the existing shallow tube-wells (10-70 m) exceed the Bangladesh drinking water standard for As concentration (50 µg/l ) was investigated to identify the factors that influence the Arsenic (As) mobilization. Higher As concentration (184 µg/l) in the shallow tubewells (20m) is noticed during ICP-MS analysis. Whereas lower As concentration (2 µg/l) is found at the deeper tube-wells (250 m). Similar results are obtained for sediment analysis showing that lower As (5 µg/g) exists both in the near upper surface (1-5 m) and the deeper part (200 m). Higher As (34.4 µg/g) is found in the deeper sediment indicating the close distribution pattern for both of the ground water As and sediment As. Higher pH (8.98) and low Eh (-140 mV) value of the sediment sample claims that the aquifer is very reductive in nature where As3+ is mostly dominating. Multivariate data analysis using PCA and PLS shows that As has a very strong positive relationship with Fe, Al and Ca. Bicarbonate and phosphate also have found positive correlation with As. However, negative relationships are observed with sodium, copper and nitrate. Presence of higher Ca and bicarbonate, gives an indication of possible carbonate dissolution that might have occurred in that aquifer that could be one of the reasons of releasing arsenic at a higher concentration in the ground water. Phosphate can compete with As for searching of sorption sites which again might stimulate the de-sorption of As. Selective sequential leaching shows that As starts to leach at HCl acid step and higher As was obtained during hot HNO3 acid with 30% H2O2 step reflecting that the As is bound in mostly amorphous and orpiment phase. However, easily leachable As is not found. Close leaching pattern for Al and Fe are noticed at different steps illustrating the possible co-existence and co-precipitation of that two metals. XRF analysis of the sediment sample shows that Al and Fe are the two dominating cations. This is further supported by the SEM-EDX mapping. Leaching column test shows that As could start to mobilize to a greater extent under alkaline condition (pH 10). Geochemical code PHREEQC (speciation analysis) shows that Al, Fe and Mn mineral phases might act as As adsorbent surfaces.