Scholarly article on topic 'In Situ-mobilization of Arsenic in Groundwater – an Innovative Remediation Approach?'

In Situ-mobilization of Arsenic in Groundwater – an Innovative Remediation Approach? Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Martin V. Maier, Margot Isenbeck-Schröter, Lukas B. Klose, Simon M. Ritter, Christian Scholz

Abstract In order to evaluate an inefficient pump & treat remediation at a contaminated site in Germany, processes of arsenic release into groundwater were studied. Groundwater and aquifer material sampling indicated elevated mobility of arsenic and a differentiated spatial distribution of As(V) and As(III). An artificial increase of arsenic mobilization in addition to the ongoing pump & treat remediation was supposed to be a suitable and sustainable remediation method for the contaminated site. Typically, arsenic mobility is increased under iron-reducing conditions, yet a number of water ingredients trigger arsenic mobility, e.g phosphate. The affinity of phosphate to sorb onto mineral phases leads to a competitive surface-complexation of phosphate and arsenic. In column tests, the release of arsenic from the contaminated aquifer material was significantly enhanced by the addition of phosphate. Under oxic and anoxic redox conditions the geochemical processes led to elevated release of mobile arsenic fractions up to a total of 50–60% and almost 100% in a period of 90 days, respectively. Eventually, the amendment of phosphate is able to enhance the efficiency of a pump & treat remediation and is currently investigated on field scale on-site.

Academic research paper on topic "In Situ-mobilization of Arsenic in Groundwater – an Innovative Remediation Approach?"

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Procedia Earth and Planetary Science 17 (2017) 452 - 455

15th Water-Rock Interaction International Symposium, WRI-15

In situ-mobilization of arsenic in groundwater - an innovative

remediation approach?

Martin V. MaierM, Margot Isenbeck-Schrotera, Lukas B. Klosea, Simon M. Rittera,

Christian Scholza

aInstitute for Earth Schience, University of Heidelberg, 69120 Germany

Abstract

In order to evaluate an inefficient pump & treat remediation at a contaminated site in Germany, processes of arsenic release into groundwater were studied. Groundwater and aquifer material sampling indicated elevated mobility of arsenic and a differentiated spatial distribution of As(V) and As(III). An artificial increase of arsenic mobilization in addition to the ongoing pump & treat remediation was supposed to be a suitable and sustainable remediation method for the contaminated site. Typically, arsenic mobility is increased under iron-reducing conditions, yet a number of water ingredients trigger arsenic mobility, e.g phosphate. The affinity of phosphate to sorb onto mineral phases leads to a competitive surface-complexation of phosphate and arsenic. In column tests, the release of arsenic from the contaminated aquifer material was significantly enhanced by the addition of phosphate. Under oxic and anoxic redox conditions the geochemical processes led to elevated release of mobile arsenic fractions up to a total of 50-60 % and almost 100 % in a period of 90 days, respectively. Eventually, the amendment of phosphate is able to enhance the efficiency of a pump & treat remediation and is currently investigated on field scale on-site. © 2017Published byElsevier B.V. Thisisanopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 Keywords: arsenic; groundwater; mobilization; remediation approach

1. Introduction

At an arsenic (As) contaminated site in Germany a pump & treat remediation of As has been performed without measurable success for more than ten years. In order to improve remediation, the University of Heidelberg was involved and focused on the interactions of aquifer material and groundwater, the species distribution of As, the geochemical conditions of the aquifer and the influence of sorption and desorption of arsenic1. Geochemical

* Corresponding author. Tel.: +49 6221 546004; fax: +49 6221 545503 E-mail address: martin.maier@geow.uni-heidelberg.de

1878-5220 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of WRI-15

doi:10.1016/j.proeps.2016.12.114

conditions had to be thoroughly considered, in order to assess the risk for groundwater pollution, the total duration of the remediation and to provide alternatives to the ongoing remediation including latest scientific approaches. As(III)-species, mainly the neutral H3AsO3, is much more mobile than As(V)-species, which is anionic. As(III)-species dominates under anoxic, iron reducing conditions. The retention of As in groundwater is mainly based on the adsorption of As(V) onto mineral surfaces and low desorption lead to typically high retention rates. Surface complexes formed by As(V) onto mineral surfaces are much weaker than mineral-bound arsenic2-5. Phosphorous has a stronger affinity to commit surface complexes and competitively substitutes As in mineral surface complexes6. This effect can be used to desorb the fractions of As(V) from the surfaces.

Generally, different remediation options are available at As contaminated sites, but those approaches in science and practice are still rare7. The increase of As mobility by artificially induced iron reducing conditions might be very effective. In the technical realization, it could be difficult to maintain favorable conditions on field scale. More promising is the usage of natural water compounds that inhibit or avoid the formation of As complexes, such as silicate, hydrogen carbonate, phosphate or organic acids8. Especially the effects of phosphate on the As mobility and

9 11 12 13

biological access are well known - . Additionally, biological processes might enhance arsenic mobility , . This study presents laboratory results of column tests applying phosphate amendment to mobilize As from contaminated aquifer material with the objective of an innovative remediation approach for arsenic contaminated aquifers.

2. Investigation concept & Methods

Pairs of sediment and groundwater samples of different depths were taken via a network of direct push samplings nearby the source of contamination. To identify the maximum soluble fractions of As of the aquifer material that potentially endanger groundwater quality, aqueous elution steps corresponding to14 were carried out (Tab. 1). Based on the profound knowledge of the contaminant distribution, the As-species as well as the geochemical setting, the artificial increase of arsenic mobility was studied as a potential remediation approach.

Table 1. Extraction and elution methods corresponding to14.

Elution arsenic fraction As-species

Water elution Mobile As fraction in water soluble As(III) and As(V)

Phosphate elution Onto surfaces adsorbed As fraction, easy to As(III) and As(V)

mobilize

HCl-elution Acid soluble fraction, bound e.g on carbonates As(tot)

and clay minerals, difficult to mobilize

Agua regia Total As included in minerals (besides As(tot)

silicates)

According to the hydrogen carbonate dominated system at the contaminated site we considered the addition of phosphate as the most promising approach and conducted column tests with contaminated aquifer material. Two tests, one under oxic and one under anoxic conditions were carried out with contaminated sediment, which was seeped through by synthetic groundwater. For anoxic tests the synthetic groundwater was previously degassed by flushing with argon. In both approaches, oxic and anoxic, we added phosphate using stepwise increasing concentrations from 0.5 to 10 mM to a set of two columns each. Two further columns, one oxic and one anoxic, were run as an untreated references. The release of arsenic in the outflow of the columns as well as the corresponding arsenic species and geochemical conditions were studied in detail.

3. Results

Arsenic concentrations of distilled water elutions of sediments from the upper parts of the aquifer (6 -20 m) agree with the magnitude of observed As concentrations in groundwater. However these As concentrations represent only a small portion of the total As. At depths of 20 m to 30 m predominantly As(III) species was found in groundwater, indicating a high mobility of arsenic.

In the groundwater samples, the distribution of As(III) increases with depth and distance. This correlates with the results of the elutions, where the mobile fractions increase with distance to the contaminant source. A total amount

of 14 t As was estimated to be adsorbed within the contamination plume. Sequential elution (Tab.1) of the contaminated aquifer material yields, that a range of 30 to 50 % of total As can be desorbed and might endanger the groundwater.

Regarding the column tests, an enhanced release of arsenic by phosphate amendment was found in both columns under oxic as well as anoxic redox conditions.

In figure 1 the release of As under anoxic conditions is demonstrated without (a) and with (b) phosphate addition. Almost the entire arsenic of the mobile fractions desorbed (96%) within 90 days of the test. During the column tests a remarkable dependency of arsenic mobility on redox conditions was confirmed. The detected processes were very fast and reversible, i.e. adsorbed arsenic under oxic conditions was mobilized under anoxic conditions and vice versa.

IIIIIIIIIIIIIIIIMIIIIIIIIIM

o s ■.■) is :: 'y- il. i- M is so fifi

Fig. 1: (a) anoxic column test without substrate addition (reference); (b) anoxic column test with phosphate addition.

4. Discussion

The realization of all kinds of arsenic remediation approaches requires a profound knowledge of the ongoing processes on-site. Under favourable conditions, a technical monitoring or immobilization might suffice. Since all methods of immobilization cause the remaining of contaminants and an accompanying long lasting monitoring of the aquifer system, the only sustainable remediation approach is the total elimination of mobile arsenic fractions in the aquifer. However, a diligent handling and the concern of the toxicity of the contaminant is essential. The results of our laboratory tests indicate that the method of As mobilization using phosphate might significantly improve the

efficiency of active or passive hydraulic remediation. Several processes were identified causing accelerated As release in the conducted column tests. While low phosphate amendments most likely support biological processes, a high dosage causes an enhancement of the arsenic mobility by geochemical processes. For remediation application the predominant processes need to be quantified and the amendment of phosphate must be adjusted to the minimum required limit. In hydrogen carbonate dominated systems, hydroxylapatite precipitation has to be considered and avoided. According to recent results, there is no technical limitation by precipitation on the investigated site. The column tests show that the approach suits for oxic as well as anoxic conditions. Professional application is necessary to avoid disturbances or changes in the geochemical conditions in the aquifer. Unexpected co-reactions, such as heavy metal release are not probable. For a final statement, further investigations are required. The laboratory results are currently checked at the site in large-scale column experiments. At the same time, the infiltration of phosphate into the aquifer is tested and accompanied by intensive geochemical monitoring. First results are promising and indicate that the laboratory tests can principally be adapted on field-scale. Hence, phospahte amandment into groundwater holds the potential to be a sustainable and fast remediation approach in order to effectively remove the source of arsenic contamination.

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