This is a complex question, given the range of characteristics of the various metals. Some clarifying question: what is your volume of solution? Its pH? Is your goal collection of the metals (individually or in bulk) or purification of the water?

In you aqueous solution heavy metals may be in complexed form with humic acid.First you may attempt evaporation of  the solution.From the residue humic acid with complexed heavy metals can be seperated by extracting with dilute alkali (NaOH).(The humic acid can be precipitated in acid medium and seperated by filtration. But another problem is some heavy metals may be solubilized in acid medium.Heavy metals in  acidic aqueous phase can be estimated by AAS.).From the humic acid heavy metals can be removed by extracting with DTPA solution.The extracted heavy metals can be estimated by using AAS.Hope, other colleagues may  provide better options.

I propose to use acidification to separate metals from humic acids. As mentioned by Dr. Rao humic acids precipate in acidic conditions and heavy metals mostly dissolve as free (uncomplexed ions).

You could either evaporate most of the water away and then add acid and decant the metal solution away from the precipitated organic or separate with a membrane filtration (500-1000 dalton).

The details of the procedure will depend on the scale. Do you need this for analysis, preparative or technical scale?

 I agree with comment  Annangi Rao above

n you aqueous solution heavy metals may be in complexed form with humic acid.First you may attempt evaporation of the solution.From the residue humic acid with complexed heavy metals can be seperated by extracting with dilute alkali (NaOH).(The humic acid can be precipitated in acid medium and seperated by filtration. But another problem is some heavy metals may be solubilized in acid medium.Heavy metals in acidic aqueous phase can be estimated by AAS.).From the humic acid heavy metals can be removed by extracting with DTPA solution.The extracted heavy metals can be estimated by using AAS.Hope, other colleagues may provide better options.

First of all, thanks to all of you for your answers, I really appreciate it! I will try the procedures you guys proposed, if you got any tips regarding volumes used ect., feel free to share them. 

To give a bit more of clarification (done at a pilot scale water treatment plant). The solution is gathered from wastewater via (anion) ion exchange column containing strong base (SBA) resin. When NOM is leaking, the resin will be regenerated with a NaCl solution.

This regenerate is collected and concentrated using nanofiltration (to remove as much salt as possible). When pressure becomes too high, the membranes are cleaned and the solution containing the NOM is collected (around 20L total). 50mL is analyzed and it contains about 23 g/L TOC (we have no way to measure the NOM unfortunately) with a pH of 7,3 and various µg/L of metals (lots of Zn & Ni, then Cu & Cr and traces of the other metals).

Our first goal is to see if it is possible to separate the metals and NOM on small (lab) scale. There are no future goals for the created solutions/products.

I have experience with a similar water with very high organic content that was considered to supply to a biogas plant, but this was not possible due to the concentrations of heavy metals.

We had a good separation of heavy metals by adding 10 mg/L NaS which causes the metals as very dense black crystals. Sulfide of most heavy metals are very insoluble at slightly basic conditions there your organic molecules will remain in solution.

 Mr.Sebastiaan,you may go through the following  publication.Removal of heavy metals is also a part of the procedure.

Colorimetric determination of humic matter with 0.2N NaOH extraction by Tucker ,M.R. In:Reference Soil and Media Diagnostic Procedures for the Southern Region of the United States. Edited by Donohue,S.J.Southern Cooperative Series Bulletin No.374 August !992.Virginia Agricutural Experimental Station,Colleage of Agriculture and Life Sciences ,Virginia Polytechnic Institute &State University.Publication available online.

The removal of heavy metals in the presence of NOM, HA is strongly dependent on solution pH, especially under acidic condition, point of zero charge or salt effect and specific surface area. These are the determining parameters 

Dr.Naveen , good references one can look into.

Dear sebastian

If you want to use natural adsorbent you can follow the paper :

http://biotech-asia.org/pdf/vol12no3/bbra_volume12_Issu03_p2451-2457.pdf

http://biotech-asia.org/pdf/vol12no3/bbra_volume12_Issu03_p2451-2457.pdf

Thank you all for your suggestions! I will propose them to my supervisor to discuss the possibilities for continuing my research.

Dear Sebastiaan,

I just had a problem like yours in December 2016. The water extracted from a deep well has shown the presence of iron and manganese associated to organic matter. It was a difficult tank to remove the three of them to turn this water potable. We had not a one month to solve the problem, so our experiments were very short.  We oxidized the organic matter with KMnO4 and H2O2. In addition we used aluminum polychloride to form tiny flocs that could settle after 30 minutes of rest. Settled water, which was not completely free from organic matter and metals, was then submitted to filtration an activated carbon media to retain organic matter and in sequence to another filtration in a column containing zeolite to retain manganese and iron. The practical results were pretty good! If you have time to optimize my attempt in your study, please keep me informed!

Dear Sebastian,

Recovery is possible but I would want to know the quantity, I am dealing with. Cost economics do not work effectively if you are dealing in huge quantities. Secondly, what is the purpose of your recovery? Do you have any recovery ratios in mind? Prof. Jay Meegoda has worked extensively in this aspect. He might personally assist you. https://civil.njit.edu/people/meegoda.php

Best wishes and do keep me updated.