Dynamics of arsenic-containing compounds sorption on sediments

Abstract

Background

River and lake sediments constitute complex and difficult analytical samples. On the other hand, sediments play a fundamental role in the distribution of toxic compounds in aquatic systems and in the evaluation of the current state and the course of changes taking place in the environment.

Among elements present in the environment in trace concentrations, but having well-elaborated toxic properties, one of most dangerous is arsenic. The element occurs in the environment in several chemical forms, predominant are inorganic forms of As(V) and As(III), and methylated forms such as monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA).

Objectives

Reported herein are experiments, which were undertaken with the aim of examining the dynamics of arsenic sorption in sediments and its dependence on speciation of the element and the pH in the environment. Simultaneously, influence of organic matter content and chemical composition of the sediments on As sorption, were investigated.

Methods

Sediment samples (upper 10 cm layer) were collected from three sites located in the vicinity of Cracow:

• • Rudawa river — 37 km long river, flowing mainly through the suburban area;

• • Vistula river — heavily contaminated, main Polish river;

• • Dobczyce reservoir — artificial reservoir on Raba river, total capacity of 125,000,000 m³, supplying about 60% of drinking water to the city of Cracow.

Using XRD and IR methods, mineral composition of sediments was analysed. Concentrations of iron, manganese, aluminium and arsenic as well as organic matter content in solid samples were analysed.

Sorption experiment

100 g of dry sediment sample was placed in a 400 ml volume glass beaker. 300 ml of arsenic containing solution was poured into the beaker. Every 30 min (total experiment time span equalled 7 to 7.5 hours), 0.5 ml of the suspension’ (the beaker content was constantly mixed) was collected and centrifuged, and the concentration of arsenic was determined in the supernatant.

In the series of experiments, solution contained about 0.05 µg/ml of arsenic in organic (DMAA) and inorganic forms, at different pH values — 3, 5, 7 and 9.

Results and Discussion

Examined sediments of Vistula river and Dobczyce Lake were characterised by relatively high concentrations of arsenic, iron, manganese and aluminium.Rudawa river. At pH 3, the concentration of inorganic As in solution decreased from an initial value of 0.049 µg/ml to 0.012 µg/ml in 7.5 hours time. The same decreasing tendency was found at pH 5 (initial value 0.046 µg/ml, after 7.5 hours - 0.015 µg/ml). Less distinct sorption effects were noted at pH 7 and 9.

Vistula river. At all pH values, a very significant decrease of inorganic As concentration up to 80% of the initial value, as well as a short time of achieving an equilibrium state (1 – 2.5 hours), was found.

Dobczyce lake. Inorganic As was distinctly accumulated in sediments at pH 3 (decrease of As concentration in solution from 0.052 to 0.007 µg/ml after 7.5 hours of the experiment) and at pH 5. At pH 7, about 40% of As was accumulated in sediments (solution As concentration decrease from 0.050 to 0.030). Only 20% of As adsorbed on sediments at pH 9.Very weak accumulation of organic form of As (DMAA) was found in all examined sediments at pH 3 and 5. At higher pH values no sorption was registered.

Conclusions

The following facts were established:

• • inorganic form of arsenic was effectively accumulated in all analysed sediments;

• • inorganic As sorption in sediments strongly depended on pH — for sediments of Rudawa river and Dobczyce lake, the highest sorption was found at pH 3, and a decrease of accumulated As was noted with an increase of pH (at pH 9, the lowest amount of As was immobilised in sediments); in the case of Vistula river sediments, a reverse tendency was found;

• • organic form of arsenic (DMAA) was accumulated only at pH 3 in all sediments and less significantly at pH 5 (Vistula river sediments); no sorption of that form of As was found in the other sediment samples;

• • state of equilibrium (no change in inorganic As concentration in liquid medium) was achieved in 1 hour for Vistula river sediments, after about 2 hours for Rudawa river and after about 4 hours for Dobczyce lake sediments.

Recommendations

The presented method of analysis of As sorption in sediments and results visualization (based on a newly defined coefficient) provides a useful tool for the characterization of sorptive properties of sediments and may be used for other compounds as well. Crucial, functional information — how much of a given element may be accumulated in sediments at a certain pH — is obtained without digestion of the sediment sample, in an experiment taking less than one day.