Nitrilotriacetic acid in sludge-amended soil: mobility and effects on metal solubility

Environmental Pollution (Series B) 12 (1986) 145-162

Nitrilotriacetic Acid in Sludge-amended Soil: Mobility and Effects on Metal Solubility

Kathryni Garnett, Peter W. W. Kirk, Roger Perry & John N. Lester

Public Health Engineering Laboratory, Imperial College, London SW7 2BU, Great Britain

ABSTRACT

Soil column studies were undertaken to determine the mobility of nitrilotriacetic acid ( NTA ) in agricultural topsoil and its effect on the solubility of Cd, Cu, Mn, Ni, Pb and Zn following land application of sludge containing NTA. The total weight of NTA retained on the columns was > 99 % of that applied, with the levels of leached NTA becoming undetectable after 56 days. The presence of NTA increased the solubility of all six metals studied, although the degree of solubilisation varied with the individual metal, NTA dose and sludge application rate. Such an effect may result in greater metal mobility in sludge-amended soils, with possible changes in availability to plants and/or groundwater contamination.

INTRODUCTION

According to recently published figures, 475 800 tonnes of sewage sludge, expressed as dry solids, are applied to agricultural land within the UK each year, equivalent to 39 % of annual sludge production (Matthews et al . , 1984). Fears that the beneficial effects of sludge application on soil fertility might be outweighed by the long-term risks of heavy metal contamination have resulted in investigations into the mobility of metals within sludge-amended soils. It has been demonstrated that metals introduced by sludge application tend to remain within the layer of incorporation (Emmerich et al., 1982; Baxter et al., 1983; Chang et al.,

145

Environ. Pollut. Ser. B. 0143-148X/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

146 Kathryn Garnett, Peter W. W. Kirk, Roger Perry, John N. Lester

1984). However, a change in the chemical form of a metal might result in movement from the solid phase to the liquid phase, with the increased possibility of metals leaching out of the sludge/soil layer. Such a change could be effected by the formation of soluble metal complexes as a result of chelation with organic ligands. Controlled experiments using soil have demonstrated increased solubility of Ni (Bowman et al., 1981), Cd (Elliott & Denneny, 1982) and Zn (Elrashidi & O'Connor, 1982) in the presence of chelating agents.

Nitrilotriacetic acid (NTA) is being considered as a partial replacement detergent builder in an attempt to alleviate eutrophication, to which sodium tripolyphosphate is considered to be a contributory factor (Hartig & Horvath, 1982) and has been introduced to a limited extent in several countries, including Canada, Switzerland~ and The Netherlands (Perry et al., 1984). In Canada, NTA constitutes approximately 15 ~ by weight of detergents (Perry et al., 1984) and an equivalent formulation in Europe could result in NTAIconcentrations~in raw sewage of up to 30 mg litre- 1 (all NTA concentrations are expressed as the anion NTA 3 -), and possibly as high as 40 mg litre - 1 in certain hard water areas (Stoveland et al., 1980). Rossin et al. (1982a) demonstrated that the adsorption of NTA onto sludge solids was the major removal mechanism during primary sedimentation, so increasing the load on subsequent sludge treatment processes. Currently, about 60 ~o of UK sludge is stabilised by heated anaerobic digestion (Mosey, 1981). As the extent of NTA removal during anaerobic digestion is dependent upon sludge type (Stephenson et al., 1983) it can be predicted that incorporating NTA into detergents would result in the production and ultimate disposal of sewage sludge contaminated with NTA.

The chelating capacity of NTA is widely recognised and its effect on metal solubility has been investigated in a variety of matrices. Allen & Boonlayangoor (1978) demonstrated the mobilisation ofCd, Cu, Fe, Ni, Pb and Zn from river sediments by NTA. In a similar system the distribution of trace metals between sediments and the solution phase was affected by NTA concentrations as low as 0.2 mg litre-~ (Salomons & Pagee, 1981). F6rstner et al. (1983) studied the effect of NTA on heavy metal sorption on a variety of solid phases and calculated that 3 mg litre- ~ of NTA in wastewater would decrease Zn adsorption by 50 ~ and Cd adsorption by 5-20 ~o. The presence of NTA can cause a reduction in heavy metal removal during biological wastewater treatment; increased concentrations of Cr, Cu, Pb, Ni and Zn in the effluent of an activated

Metal solubility and NTA in soil columns 147

sludge pilot plant were associated with high effluent NTA concentrations (Rossin et al. , 1982b; Van t 'Hof et al. , 1984).

The aim of the investigation reported in this paper was to employ soil column studies as a means of assessing the fate and behaviour of NTA in agricultural topsoil, with particular reference to its impact on metal solubility, following the land application of sludge contaminated with NTA.

MATERIALS AND METHODS

Soil columns

Thirty perspex soil columns (19-4 cm internal diameter) were constructed to the specifications outlined by Kirk et al. (1983). To facilitate the even distribution of leachate from each column into two polypropylene collection flasks, a polypropylene Y-piece connector was inserted beneath the collecting hopper and capillary silicone rubber tubing (Jencons Ltd) attached. The columns were supported by wooden frames and stored in an air-conditioned dark room at 15 + 2 °C.

Soil properties

Topsoil (0-20 cm) of a clay loam (Windsor series) with no previous history of sludge application, was obtained from Shonk's Mill, Harold Hill in Essex. Moisture, organic content and pH were determined employing the methods of Avery & Bascomb (1974) and cation exchange capacity by sodium saturation (Chapman, 1965). Total Cd, Cu, Mn, Ni, Pb and Zn were determined by flameless atomic absorption spectrophotometry following HNOa-H202 digestion as described by Krishnamurty et al. (1976). Each column was packed with 6.5 kg of soil that had passed a 6 mm sieve to a depth of 20 cm.

Sludge properties

Primary digested sludge from a works treating a mixed industrial domestic sewage was collected in vented polyethylene containers. Prior to sampling the sludge was thoroughly stirred with an aluminium rod. Total solids content and volatile matter on ignition were determined as

148 Kathryn Garnett, Peter 14I. W. Kirk, Roger Perry, John N. Lester

recommended by the Department of the Environment (1972) and a rapid flameless atomic absorption spectrophotometric method (Sterritt & Lester, 1980) was employed for the determination of total Cd, Cu, Mn, Ni, Pb and Zn.

Sampling and analysis of soil column leachate

All equipment to be used for collection and storage of samples was cleaned by soaking in 5 ~ Decon 90 (Decon Laboratories Ltd, UK) for 24 h and then leached in 10 ~ Analar nitric acid for a further 24 h. Soil column leachate required for NTA analysis was collected and preserved in 0.5 ~o (v/v) formaldehyde and filtered through a 0.45/~m microporous filter (Amicon Ltd) prior to analysis. Concentrations of NTA were determined using a rapid differential pulse polarographic method as described by Kirk et al. (1982). Samples for metal analysis were collected in 1 ~ Aristar nitric acid (BDH Ltd, UK) and filtered through 0.45/~m microporous filters that had been leached for 2 h in 1 ~o Aristar nitric acid and rinsed with distilled water. The first 2-3 ml of filtrate was discarded prior to collection of the remainder (Kirk et al. , in press). Metal concentrations were determined using flameless atomic absorption spectrophotometry. Leachate was collected for analysis every two days, with later samples being amalgamated to provide composite values. Following each 48 h sampling period, 200 ml of tapwater was applied gradually and evenly over the soil surface. This represents an irrigation rate within the range of the long-term annual average rainfall for the UK (Department of the Environment, 1977).

RESULTS

Soil properties

Soil moisture content was 10 ~o (w/w) and organic matter content 6 (w/w). The soil had a pH of 5.8 in distilled water and a pH of 5.5 in 0.01M CaC12 while its cation exchange capacity was 22.3 meq per 100 g of soil. Total concentrations ofCd, Cu, Mn, Ni, Pb and Zn were 0-2, 7.8,246.0, 10.7, 34.7 and 23.0 mg per kilogram dry weight, respectively, which fall within the typical range of values for unpolluted soils (Berrow & Reaves, 1984).


Sludge properties and application rates

Two sludges were utilised during this study, both collected from the same works at different times and designated A and B. The total solids content of sludge A was 0.0841 kg litre- 1 with volatile solids of 44 ~o (w/w) while sludge B had a total solids content of 0.02 kg litre- 1 and volatile solids of 35 ~o (w/w). Total metal concentrations of both sludges are shown in Table 1.

The quantities of sludge solids applied to the soil columns were determined in accordance with guidelines prepared by the Department of the Environment & National Water Council (1977) which recommend maximum additions over a period of 30 years, although one-fifth of the total may be applied in any one year. Rates of application for Cd, Cr, Pb, Mo and the Zn equivalent were calculated from the analytical data presented in Table 1 by employing the following equation (Department of the Environment & National Water Council, 1977):

Application limit (tonne ha- 1) = recommended maximum addition of metal (kgha-1) x 1000

concentration of metal in sludge (mgkg -~)

Zinc equivalent was limiting in both sludges with a recommended 30 year addition of 212 tonne ha - 1 for sludge A and 200 tonne ha - 1 for sludge B.

Sludge A was used for the initial application, with half the soil columns receiving 1.5 litres to represent a '6-year' application (the maximum

TABLE 1 Total Metal Content of Sewage Sludges Applied to Soil Columns

Metal Concentration (mg kg- ~ dry solids)

Sludge A Sludge B

Cd 16.3 5.0 Cr 100.6 122.5 Cu 350.5 443-5 Pb 275'6 211.5 Mn 183.2 168.5 Mo 3.6 7.5 Ni 115'6 75'0 Zn 1 009.9 1 308.5


permissible under the guidelines and equivalent to 5 cm depth) and the remainder 0.5 litres, or a '2-year' application. A 4 g litre-1 stock solution of NTA was prepared from the 'Gold Label' trisodium salt of NTA (Aldrich Chemical Co.) and added to the sludge prior to column application so that each set of fifteen columns was comprised of five controls receiving no NTA, five receiving 20 mg litre- 1 and five receiving 40 mg litre- 1 NTA. After 152 days of routine monitoring a further ' l- year' application of sludge B (1 litre; 3.5 cm depth) was added to those columns which had previously received '2-year' applications.

Mobility of NTA in the soil columns

Leachates from all the NTA-treated soil columns exhibited wide variation in NTA content, although concentrations were generally very low and dropped below the detection limit of 20#g litre -1 after 46 days.

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Determination of NTA in the leachate from columns treated with the lower sludge application and a 20 mg litre-x NTA addition was only possible for the initial 10 days. Figure 1 shows the mean values and ranges of NTA concentrations detected in the leachate from the '6-year' application up to day 46.

The second sludge application resulted in higher initial NTA concentrations in the leachate with mean values after 4 days of 206-7 and 202-7/~g litre-1 for a 20 mg litre-1 and a 40 mg litre-1 NTA addition, respectively. However, these decreased to levels that were generally below detectable concentrations after only 10 days.

Effect of NTA on metal solubility in soil columns

Mean relative standard deviations (RSD) of leachate metal content ranged from 29 ~o to 57 %, with the exception of Pb in the leachate from '6-year' application columns which had an RSD of up to 80 %.

The columns that had received a '6-year' sludge application began to exhibit signs of waterlogging after 96 days of irrigation and therefore no further samples were taken. Generally, NTA increased the mean solubility of Ni (Fig. 2), Mn and Cd, up to days 96, 14 and 18, respectively, and Pb between days 14 and 46, with an addition of 20 mg litre- 1 NTA having the greatest effect in all cases. Substantial increases in mean Mn levels in both NTA-treated and control columns occurred towards the end of the 96-day period, probably in response to the changes in redox potential caused by waterlogging. No mobilisation was apparent for Cu (Fig. 2) and Zn, with the former actually showing decreased solubility in the presence of NTA.

Overall, the '2-year' application columns displayed less scatter in the results of metal determinations and this, plus the absence ofwaterlogging, enabled trends to be discerned more clearly than for the '6-year' application. The mean leachate concentrations of all six metals under investigation increased in the presence of NTA to varying degrees (for example, Cu and Mn in Fig. 3). Maximum differences between NTA- treated and control columns occurred during the initial stages of monitoring and then decreased, with solubilisation generally becoming insignificant by day 46 (Fig. 4). Columns receiving 40 mg litre-1 NTA demonstrated a greater effect on mean metal solubility than those receiving 20 mg litre-1, with the exception of Pb which appeared to be more soluble in the presence of 20 mg litre- 1 NTA (Fig. 4).


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All the columns, whether NTA-treated or controls, displayed an overall decrease in leached Cd, Ni, Mn and Zn with time, probably reflecting the initial removal of less strongly bound metal forms. Less of an effect was observed for Pb and Cu, with their levels becoming constant by day 46. This was presumably attributable to the limited solubility of Pb in sludge-amended soils (Lake et al., 1984) and the small but significant Cu concentration (mean of 0.16 mg litre- 1) of the irrigation water.

In order to reduce variation between replicates the influence of daily fluctuations in leachate volume was minimised by expressing the results in terms of the weight of metal leached, as shown in Fig. 5 for Cu, Mn, Ni and Zn. Presenting the data in this way confirmed the trends observed for

TABLE 2 Cumulative Amounts of Metals and NTA Leached During the First 46 Days Following a

Two-Year Sludge Application

Amount of metal leached (pmol)

Cd Cu Mn Ni Pb Zn

Amount of NTA leached

Otmol)

Control 0.12 1 .26 25.21 25.58 0-39 13.90 - - With 40 mg 0.14 1 .96 35.66 35.38 0.42 19-63 0.337

litre- 1 NTA Increase 0-02 0.70 10.45 9.80 0.03 5.73 0.337

mean metal concentration and demonstrated the extent of solubilisation by NTA. In the presence of 40 mg litre- 1 NTA the mean weight of Cu, Mn, Ni and Zn leached after 46 days had increased above the control value by 56 ~o, 41 ~o, 38 ~o and 41 ~o, respectively. Cumulative quantities of metals and NTA leached during the first 46 days following a 2-year sludge application with and without 40 mg litre- 1 NTA are included in Table 2.

Elevated quantities ofCd, Cu, Mn, Ni, Pb and Zn were detected in the leachate of all the '2-year' columns following a second sludge application. This increased leaching of metals was maintained for four days, after which there was a rapid decline to original concentrations. There was also solubilisation of these metals in the presence of NTA (Fig. 6), with those columns treated with 20 mg litre- 1 NTA displaying a greater effect than those receiving a 40 mg litre-1 addition.


DISCUSSION

Introducing NTA into detergent formulations would inevitably result in its presence in sewage sludge. As 65 ~ of sludge produced in the UK is disposed of to land (Matthews et al., 1984) it is important that the fate and effects of NTA in the soil profile are understood. The results presented in Fig. 1 demonstrate that the majority of NTA applied to the soil columns was retained, with the total weight leached being < 1 ~ of the amount added. This is consistent with a previous observation that there is near complete retention of NTA applied to unsaturated soil columns in weak, synthetic sewage (Dunlap et al., 1972). Similarly, about 95 ~o of NTA was removed in aerobic soil columns with feed concentrations of 40 and 100 mg litre - 1 (Klein, 1974). Kirk et al. (1983) reported that up to 9.5 ~ of added NTA would pass through topsoil after surface spreading of sludge, although, if application was followed by ploughing, this could be reduced to between 1~o and 2~o. NTA removal can be due to biological degradation or adsorption. Biodegradation becomes established following an acclimatisation period that can vary between 5 and 10 days (Dunlap et al., 1972; Tabatabai & Bremner, 1975) and 6 to 8 weeks (Hrubec & Van Delft, 1981), during which time adsorption is the most important removal mechanism. Dunlap et al. (1972) reported that adsorption could be sufficient to retard the movement of NTA through a soil until the NTA-degrading bacteria were established. From the results reported here it would seem likely that bacterial activity was fully established by day 56, as NTA could no longer be detected in the column leachate. Ponding of certain columns following the second sludge application might account for the observed breakthrough of NTA at this time, as previous studies have reported a lack of NTA degradation in anaerobic environments (Dunlap et al., 1972; Tiedje & Mason, 1974).

One consequence of the application of sewage sludge to land is an overall increase in heavy metal content, as metal concentrations in sludge may exceed those in soil by two orders of magnitude or more (Lake et al., 1984). Metals added in this way tend to remain within the depth of sludge incorporation. In a previous study nearly 100 ~0 of the metals added to soil in sewage sludge were recovered from the sludge/soil layers following 25 months of irrigation, with no indication of increased metal concentrations below the sludge/soil interface (Emmerich et al., 1982). Similarly, Chang et al. (1984) demonstrated that there was no statistically significant increase in heavy metal content below the surface 30 cm of the soil profile following 6 years of annual sludge applications, with over 90 '~/o


of deposited Cd, Cr, Cu, Ni, Pb and Zn remaining in the 0-15cm layer. However, during this study there was increased leaching of Cd, Cu, Mn, Ni, Pb and Zn from soil columns treated with sludge containing NTA, although the extent of mobilisation varied depending on the individual cation, NTA dose and sludge:soil ratio. This would indicate that interactions between heavy metals and NTA in sludge following disposal to land could alter the physico-chemical forms of those metals and may, as a result, increase their mobility in the soil profile. Elevated concentrations of soluble metal could be maintained for up to 7 weeks after an initial application of NTA-contaminated sludge (Fig. 3), although it would seem that increases following subsequent applications would only persist for a few days (Fig. 6). Dunlap et al. (1972) irrigated metal-enriched sand columns with synthetic sewage containing 49 mg litre- ~ NTA and concluded that NTA was able to transport Cd, Cr, Pb and Zn through soils, although the lack of an adequate control reduces the validity of these results. More recently, it was suggested that competition between soil exchange sites and the added organic ligand could account for the observed reduction in soil adsorption of Cd when NTA was present during batch adsorption experiments (Elliott & Denneny, 1982). However, a similar investigation into Cu adsorption produced the reverse effect, with Cu solubility decreasing in the presence of NTA (Elliott & Huang, 1979).

The results of this study strongly suggest that increased metal solubility can occur in the presence of NTA in sludge-amended soil, although this cannot be statistically validated with the experimental protocol adopted. It is apparent from Table 2 that a 1 : 1 metal-NTA complex would not explain the quantities of metal leached. Possible explanations for this phenomenon include the decomposition of NTA to an intermediate with metal chelating capacity which may not be determined by the polarographic technique, e.g. iminodiacetic acid (Tiedje et al., 1973), or more complex interactions between NTA adsorbed within the soil column and metal complexes in the sludge-soil mixture. This uncertainty indicates the need for more fundamental studies on the changes in metal speciation involved during mobilisation.

CONCLUSIONS

The results of this study indicate that, under certain conditions, the co- existence of metals and NTA in sewage sludge applied to land could


enhance the mobility of metals such as Cd, Cu, Mn, Ni, Pb and Zn. Increased leaching of heavy metals from the layer of sludge incorporation would carry the associated risks of greater availability to plants and possible groundwater contamination.

A C K N O W L E D G E M E N T S

The authors acknowledge financial support for this study from the Centre Europ6en d'Etudes des Polyphosphates EV. One of us (K.G.) is the recipient of a Science and Engineering Research Council Studentship.

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162 Kathryn Garnett, Peter I4I. 14I. Kirk, Roger Perry, John N. Lester

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Nitrilotriacetic acid in sludge-amended soil: mobility and effects on metal solubility